The Pumpkin
a library of selected
writings of
Douglas A. Kerr
Where
did the "P.E." go?
you are at
the
Index page
The name of this site, "The Pumpkin", alludes to the hollowed-out pumpkin that figured in the infamous Whittaker Chambers/Alger Hiss espionage case in the late 1940's as a storage spot for "special" documents. I hope these documents are equally (but differently) "special".
The current and planned topic areas are as follows; click on a topic area to jump to that section of the index:
Table of Contents
Optics, photography, and imaging
Telecommunication [to come]
Mathematics, science, and engineering
Music and electronic music [to come]
Information systems and computers
The documents require Adobe Acrobat Reader or Adobe Acrobat to view in your browser or to view after download.
For information about the author, click here.
Readers are urged to comment on these articles. To contact the author by e-mail, click here:
Click on a topic area or article title to go to that item
Optics, photography, and imaging
Adding Vertical Tilt Positions to the Canon Speedlite 550EX Flash unit
Annotation Metadata in Digital Image Files
APEX - The Additive System of Photographic Exposure
Astigmatism in Camera Lenses—Meriodional and Sagittal MTF Response
Autofocus Accuracy Specifications in Canon EOS Digital SLR Cameras
Average Scene Reflectance in Photographic Exposure Metering
Canon EOS 20D and other similar cameras - Behavior of Custom Function 4 New
Canon EOS 20D—Replacement of the Shutter Release Switch
Characterizing the Output of Photographic Flash Units Recently revised
Chromaticity and Chrominance in Color Definition
Chrominance Subsampling in Digital Images
The CMOS APS Digital Camera Sensor
Depth of Field Calculator - Excel spreadsheet
Depth of Field in Film and Digital Cameras
Derivation of the "Cosine Fourth" Law for Falloff of Illuminance Across a Camera Image
Describing the Strength of Visible Light
The Effect of
Inserting a Flat Glass Plate into the
Optical Path Downstream from a Lens
Exposure Control in the Canon Wireless Flash System
Exposure Metering Modes in the Canon EOS-300D (Digital Rebel) Digital Camera
Format Size in Digital Photography
The HSV and HSL Color Models and the Infamous Hexcones New
The ISO Definition of the Dynamic Range of a Digital Still Camera Newly revised
Improved Beamwidth Control in Canon Speedlite Flash Units New
JPEG Compression of Still Images
The La Crosse BC-900 Battery Charger/Tester/Conditioner
Lens Principal and Nodal Points
M, X, and V on Camera Shutters
A Metric for Chromaticity Difference New
Mosaic Sensor Arrays in Digital Photography
New Measures of the Sensitivity of a Digital Camera
On the Term “Field of View Crop Factor”
Parallax
Suppression in a Target Rifle Aperture Sight
The next article is a companion
to this one
Parallax Suppression with a Target Rifle Aperture Sight—An Optical Demonstration
Pixel Resolution in Digital Photography
Photographic Exposure Metering and the Infamous “18% Calibration”
Photographic Photometry 101 Recently revised
Polarization of Light and Polarizers
Programmed Automatic Exposure in Canon EOS Digital Cameras
The Proper Pivot Point for Panoramic Photography Recently revised and expanded
Replacing the Focusing Screen on the Canon EOS 20D Digital SLR Camera
Sampling, Aliasing, and the Blur Filter
Setting Camera Exposure in Terms of Ev
Shift of the Plane of Best Focus With Shift In Film Position
Some Principles of Photographic Optics
Vision Correction in Camera Viewfinders
White Balance Diffusers in Digital Photography
Mathematics, science, and engineering
Rail Signal Aspects and Indications
Information systems and computers
The ASCII Character "Octatherp"
ASCII and "ANSI" Character Sets
Doug and Carla - an Urban Legend
Windows users: Click on the title to view the article, or right-click and make the appropriate selection to download the article.
Mac users: Beats me; if you don't recognize how to do the corresponding actions, perhaps there is some show on NPR that will explain it.
Note that the sizes of the PDF files are given in kilobytes (kB). A kilobyte is 1000 bytes. In parentheses, we show the size in kibibytes (KiB). A kibibyte is 1024 bytes, a multiple of common use in the computer field for stating memory size or requirement. Often the prefix name kilo or the symbol k is incorrectly used to refer to a multiple of 1024; sometimes the symbol "K" is used to imply a multiple of 1024 (that was formerly the practice here). All those usages are now deprecated by the applicable international standard for units.
Optics, photography, and imaging
Adding Vertical Tilt Positions to the Canon Speedlite 550EX Flash unit
The Canon Speedlite 550EX flash unit provides vertical head tilt for bounce flash operation. Detented positions are only provided for angles of -7°, 0°, 60°, 75°, and 90°. Other tilt angles are useful for various work. This article describes the modification of the 550EX to add further positions to the detent.
Issue 1, 2004.10.04. 5 pages, 1270 words, 5 photographs, PDF format, 184 kB (180 KiB)
Annotation Metadata in Digital Image Files
Many digital image files accommodate metadata items we may describe as annotation, human-oriented information about the image or its circumstances. In this article we describe three classes of such annotation data items. We also discuss the way several image-manipulation software packages allow us to view, add, or change annotation data.
Issue 1, 2004.05.22. 16 pages, 3300 words. PDF format, 122 kB (119 KiB)
APEX - The Additive System of Photographic Exposure
The Additive System of Photographic Exposure (APEX) provides for stating several factors involved in photographic exposure in logarithmic form. In this way, calculation of the “proper exposure” for a given situation may be done manually using only addition. Although the importance of that has largely faded since the time the system was developed, the scheme is still widely used in technical work relating to photographic exposure, especially the quantity “exposure value” (EV). This article explains the APEX system, and gives cautions about irregularities in its usage that are often encountered.
Issue 7, 2007.08.04. 16 pages, 3700 words. PDF format, 153 kB (149 KiB)
Astigmatism in Camera Lenses—Meriodional and Sagittal MTF Response
When presenting lens performance data in the form of a modulation transfer function (MTF), we often see separate curves for meridional and sagittal response. This primarily relates to a lens aberration called astigmatism. In this article, we discuss astigmatism and the significance of the terms meridional and sagittal.
Issue 1, 2005.04.04. 16 pages, 4700 words. PDF format, 129 kB (126 KiB)
Autofocus Accuracy Specifications in Canon EOS Digital SLR Cameras
Canon, Inc. typically expresses the accuracy tolerance of the autofocus system in their EOS digital SLR cameras as a fraction of the depth of focus. Of interest in relating this specification to its impact on actual photographic work is how this relates to depth of field. In this article, we describe that relationship, as well as the basic significance of the specification, and of depth of focus itself.
Issue 1, 2005.06.06. 9 pages, 2900 words. PDF format, 114 kB (111 KiB)
Average Scene Reflectance in Photographic Exposure Metering
The quantity “assumed average scene reflectance” is widely mentioned in connection with the calibration of “reflected light” photographic meters. Understanding of its significance is elusive. In this paper, we examine the actual significance of this quantity and how it plays a role in deciding upon a calibration constant for a reflected light exposure meter. We also examine the significance of various oft-mentioned values of assumed average scene reflectance, such as 18% and 12.5%, and finally discuss the use of a gray card of known reflectance to perform “incident light” metering using a reflected light meter.
Issue 1, 2005.01.30. 19 pages, 6200 words. PDF format, 159 kB (155 KiB)
Canon EOS 20D and other similar cameras - Behavior of Custom Function 4 New
The Canon EOS 20D, 30D, 5D, and certain 1-series cameras have a Custom Function (Custom Function 4, "C.Fn-04" or "CF04") that allows customization of the effects of half press of the shutter release button and press of the "*" button on the execution of automatic exposure (AE) and automatic focus (AF) functions. Four settings are available. The entire scheme is complex, with the implications varying with the combination of metering, autofocus, and drive modes in effect. This document features a chart that shows the implications of the four settings for those various combinations. The chart itself is accompanied by a narrative synopsis of the implications of the four settings and a brief discussion of where and how each might be useful.
Issue 4, 2007, 2007.11.26. 4 pages, 971 words (synopsis). PDF format, 104 kB (102 KiB)
Canon EOS 20D—Replacement of the Shutter Release Switch
The shutter release switch on the Canon 20D digital SLR camera can fail completely or misbehave. In this article, we describe how to replace the switch. The required disassembly procedure is described in detail, with illustrations.
Issue 1, 2007.08.12. 12 pages, 2780 words, 12 illustrations. PDF format, 424 kB (414 KiB)
Characterizing the Output of Photographic Flash Units Recently revised
We are often interested in quantifying the maximum “output” of a photographic flash unit. We often see descriptions in terms of guide number, beam candlepower seconds (BCPS), and watt-seconds. In this article we explain the different properties we may wish to describe and the various metrics and units that apply to them.
Issue 2, 2008.04.10. 8 pages, 2240 words. PDF format, 114 kB (111 KiB)
Chromaticity and Chrominance in Color Definition
In connection with the definition of color in such fields as computer graphics, television systems, and digital still photography, we encounter the two similar-looking, and often-confused, terms chromaticity and chrominance. In this article we illuminate the distinction between these terms.
Issue 3, 2003.08.09. 4 pages, 1000 words. PDF format, 68 kB (66 KiB)
Chrominance Subsampling in Digital Images
The JPEG and TIFF digital still image formats, along with various digital video formats, have provision for recording the chrominance information (which conveys in a special way what the lay person would describe as the “color” of the pixels) in a resolution lower than that of the image being encoded. There are various standard “patterns” for performing this “chrominance subsampling”, and a curious and confusing notation for indicating them. In this article we discuss the concept of chrominance subsampling and describe this system of notation.
Issue 1, 2005.11.02. 10 pages, 2900 words. PDF format, 188 kB (184 KiB)
The CMOS APS Digital Camera Sensor
Many digital still cameras today use a “CMOS” sensor array. The name comes from the fact that this type of sensor uses the same construction, and can be fabricated with much the same technique, as the familiar CMOS (complementary metal-oxide-semiconductor) integrated circuit chip. However, the designation also implies an architecture and readout technique dramatically different from that of the other popular sensor type, the CCD (charge-coupled device). In this article we discuss the principles and operation of an important form of the CMOS sensor, the active pixel sensor (APS) form. We also describe a specific application of the design in the sensor used by Canon, Inc. in various of its digital SLR cameras.
Issue 2, 2006.06.08. 14 pages, 1965 words, 2 figures. PDF format, 279 kB (272 KiB)
The concept of color and color models (coordinate systems for defining a specific color); color model families: tristimulus family, luminance-chromaticity family, luminance-chrominance family; gamma precompensation; details of specific color models for computer graphics, for TV transmission, for still and moving images, including the RGB family, YIQ, YUV, YCbCr, CIE L*a*b* ("CIELAB"; no, "LAB" is not short for "laboratory"), and many others; the CIE chromaticity diagram; luma and chroma; chromaticity vs. chrominance.
Issue 8, 2005.11.08. 40 pages, 13,200 words. PDF format, 242 kB (236 KiB)
Color temperature is a concept in which the chromaticity ("color") of a specific flavor of white light is described by reference to the chromaticity of the light emitted by a blackbody radiator at a certain temperature.
This article explains the concept and gives cautions regarding some widely-held misconceptions in this area.
Issue 4, 2005.11.08. 10 pages, 2700 words. PDF format, 976 kB (954 KiB)
This Excel spreadsheet allows the user to calculate the near and far limits of the depth of field for focus at any specified distance; the hyperfocal distance; and the near limit of the depth of field for focus at the hyperfocal distance. Separate sheets are provided allowing distances to be stated in meters, millimeters, feet, or inches.
The format allows for the particulars of several "setups" to be entered on separate lines so that the results can be easily compared.
Issue 11, 2006.05.13. XLS format, 52.7 kB (51.5 KiB)
Depth of Field in Film and Digital Cameras
Although the image of an object created by a camera is only “perfectly focused” when the object is at the precise distance to which the camera has been focused, objects at other distances (over a certain range) will have images of what we consider “acceptable sharpness”, an honor for which we must adopt some quantitative, if arbitrary, definition. The range of object distances for which this occurs is spoken of as the depth of field of the camera. This article discusses the traditional concept by which depth of field is defined, quantified, and calculated, and describes the rationales of two outlooks often used to develop a criterion of “acceptable sharpness”. It also discusses the way in which the film frame or format size of a camera influences depth of field. The related topics of depth of focus and out of focus blur performance are also discussed.
Issue 10, 2006.05.15. 27 pages, 7600 words. PDF format, 191 kB (187 KiB)
Derivation of the "Cosine Fourth" Law for Falloff of Illuminance Across a Camera Image
In a photographic system, for a given object luminance (brightness), the image illuminance on the film or equivalent declines as we move outward from the center of the image as a result of the geometric optics involved. The result is a relative darkening of the image toward its borders. If we consider a lens having certain ideal properties, it can be shown that the decline in relative illuminance goes very nearly as the fourth power of the cosine of the angle by which the object point is off the camera axis. Here we derive this relationship. We also discuss differing results given by other authors.
Issue 4, 2007.05.01. 12 pages, 3170 words. PDF format, 146 kB (142 KiB)
Describing the Strength of Visible Light
In many types of technical work it is necessary to describe the "strength" of visible light. The matter is complicated by the fact that there are four distinct circumstances in which the strength of light is a consideration, each having its own dimensionality and unit of measure. In this article we describe these four circumstances and the way in which the strength of light is described for each.
Issue 2, 2003.08.31. 11 pages, 2850 words. PDF format, 113 kB (110 KiB)
The Effect of Inserting a Flat
Glass Plate into the
Optical Path Downstream from a Lens
If a flat glass plate is inserted, perpendicular to the optical axis, into the path of rays from a lens heading to form an image, its effect is to shift the point of convergence of the rays (the point at which an image is formed) away from the lens by an amount depending on the thickness of the plate and its index of refraction. In this article, we derive the expression for the amount of this shift.
Issue 1, 2007.07.13. 4 pages, 920 words, two figures. PDF format, 107 kB (104 KiB)
Exposure Control in the Canon Wireless Flash System
The Canon Wireless Flash System allows freestanding Canon Speedlite flash units remote from a Canon EOS-type SLR camera to be controlled and triggered by optical signals transmitted from a master Speedlite flash unit or Speedlite flash control transmitter located at the camera. The system includes flexible provisions for adjusting the relative contribution to exposure of different flash units, including the topic often spoken of as “flash ratio setting”. In this article, we describe the system and its exposure control provisions.
Issue 1, 2006.06.12, 8 pages, PDF format, 80 kB (78 KiB)
We often hear that “a standard photographic exposure meter [or automatic exposure system] is calibrated to a reflectance of 18% [or maybe 13%, or some other nearby number].” Sometimes the word “gray” appears in the description. What does this mean, and why the large variation in the numerical value? In this article, we look at several relevant ISO standards and see how a “standard exposure meter calibration” is implied by their interaction. In an appendix, we look into the calibration situation for Canon digital SLR cameras, as inferred from a test recommended by the manufacturer. We also discuss the related issue of incident light metering, including by way of the use of a “gray card”. A summary is included.
Issue 1, 2006.05.06. 23 pages, 6100 words. PDF format, 162 kB (159 KiB)
Exposure Metering Modes in the Canon EOS-300D (Digital Rebel) Digital Camera
The Canon EOS-300D (Digital Rebel) digital camera can employ a number of different exposure metering modes for both ambient and (where applicable) flash components of exposure. The user has no free choice of the mode in effect. Rather, it is chosen based on the user's choice of shooting mode and other operational options. This chart shows which modes come into play under what circumstances.
Issue 6, 2004.09.17 1 page, PDF format, 40 kB (39 KiB
The field of view of a camera refers to the region in three dimensional space which is taken in by the camera’s view. It is basically an angular property. There are a number of ways in which its extent can be numerically stated. In this article we discuss the significance of field of view, various approaches to its quantification, and of terms used in that connection.
Issue 2, 2006.02.02 11 pages, PDF format, 114 kB (111 KiB)
Format Size in Digital Photography
In photography, the term “format size” describes the actual physical size of the image captured by the film frame, digital sensor, or equivalent. The widespread popularity of digital photography has brought to the user community a plethora of different format sizes, most of them unique to digital photography. In this article, we review the effect of format size on a number of camera behavior and performance issues. We also debunk various misconceptions that circulate in this area, and discuss terminology used to identify a significant numerical factor.
Issue 2, 2005.09.08 17 pages, 7400 words. PDF format, 133 kB (130 KiB)
A synopsis of this article is available here:
Format Size in Digital Photography - Synopsis
This is a synopsis of the principal topics covered by the tutorial article, Format Size in Digital Photography. Readers should be aware that, of necessity, this synopsis overlooks many important “if’s, and’s, and but’s”, and I have taken certain liberties with technical precision in the interest of conciseness. Readers wishing additional, more detailed, or more rigorous information on these topics should consult the article proper.
Issue 1, 2005.09.08 3 pages, 1160 words. PDF format, 63 kB (62 KiB.)
The HSV and HSL Color Models and the Infamous Hexcones New
The color models known as HSV and HSL (and specific color spaces based on them) are intended to provide ways of describing color that have a broad relationship to the easily‑grasped color attributes hue, saturation, and relative luminance. Almost invariably when an HSV color space is described, it is mentioned that “the color space can be described by a hexcone (a homey synonym for hexagonal pyramid)”. Similarly, we hear that the HSL color space can be “described by a bi‑hexcone” (meaning two hexagonal pyramids joined at their bases).
But these descriptions are paradoxical—the gamuts of these color spaces, “plotted” in their inherent cylindrical coordinate systems, are in fact full circular cylinders. So what might be meant by the allusion to these other, tapered solid figures?
In this article, we describe these two color models and reveal the rationales by which these unusual geometric figures are invoked as representing the associated color spaces.
The article begins with a review of the principles of color, color models, color spaces, and gamuts.
An appendix describes a “different” HSL color model used by Canon in some of its image manipulation software.
Issue 3, 2008.05.12. 30 pages, 8300 words, 10 figures. PDF format, 788 kB (770 KiB)
The ISO Definition of the Dynamic Range of a Digital Still Camera Newly revised
The dynamic range of a digital camera can be simplistically defined as the ratio of the maximum and minimum luminance that a camera can “capture” in a single exposure. But when we try to quantify this property, we find that the establishment of an explicit definition is much more complicated than it seems on the surface. International Standard ISO 15739-2003 gives an explicit definition of dynamic range for a digital still camera and a procedure for determining it. This article explains the basic concept of dynamic range and discusses some of the complications in defining it. Then, the definition given by ISO 15739-2003 is discussed in detail.
Issue 2, 2008.02.06 15 pages, 4580 words. PDF format, 129 kB (126 KiB)
Improved Beamwidth Control in Canon Speedlite Flash Units
With the Speedlite 580EX flash unit, Canon introduced an “image size compensation” feature intended to optimize the automatic beamwidth control (by way of “head zoom”) with EOS digital SLR cameras having different image sizes. However, an anomaly in the scheme put into effect an inadequate beamwidth for smaller focal length lenses.
In this article, we review the overall matter of beamwidth control, the Canon image size compensation scheme, the anomaly in the initial 580EX and its impact, and how it has been overcome.
Issue 1, 2008.01.25 15 pages, 4800 words. PDF format, 137 kB (134 KiB)
JPEG Compression of Still Images
A technique known as JPEG is widely used for the compression of digital data representing photographic still images. In this article, we explain how this technique works. Appendixes give tutorial insight into several technical concepts that are involved, including the Discrete Cosine Transform (DCT), Huffman coding, and run-length encoding, and discuss the difference between reversible ("lossless") and non-reversible ("lossy") compression.
Issue 1, 2003.08.16. 19 pages, 5740 words. PDF format, 232 kB (227 KiB)
The La Crosse BC-900 Battery Charger/Tester/Conditioner
The La Crosse BC-900 is a charger, tester, and conditioner for AA and AAA size rechargeable cells of the Ni-Cd and Ni-MH types. It offers extremely flexible management of its numerous functions. In this article, we review the features of this unit and give instructions for its operation.
Issue 1.1, 2006.06.25. 6 pages, 2200 words. PDF format, 78 kB (76 KiB)
In large‑format film cameras (and smaller-format cameras whose design is derived from large‑format cameras) there are three principal types of back used: the spring (Graphic) back, the Graflex back, and the Graflok back. This article describes these three types (with illustrations) and gives insights into their implications.
Issue 1, 2007.03.31. 9 pages, 2460 words. PDF format, 225 kB (219 KiB)
Lens Principal and Nodal Points
In discussions of photographic lenses, we often hear of the importance of the principal points and nodal points of a lens. This article describes what these are and why they are significant.
Issue 3, 2004.01.21. 5 pages, 1315 words. PDF format, 53 kB (52 KiB)
M, X, and V on Camera Shutters
Often in connection with older cameras we see or hear of the markings "M", "X", and "V" on a lever on the shutter, or may see or hear of the flash sync connector on a camera labeled as "X". In this article, we explain the history and significance of these markings. In the process, we will visit other parts of the alphabet, including "F", "S", and "PC".
Issue 2, 2004.01.17. 4 pages, 1300 words. PDF format, 53 kB (52 KiB)
A Metric for Chromaticity Difference New
In many areas of photographic practice, we are concerned with the difference between two chromaticity values, especially in connection with “white balance color correction” matters. An example would be the departure of the recorded image of a “white” object from the reference white chromaticity of the color space in use, or the departure form “neutrality” of the reflective chromaticity of a “neutral target” (gray card).
In this article, the author suggests the use of the metric “du’v’” as a single-valued measure of the degree of a chromaticity difference. The metric is defined and a rationale given for its use. An appendix defines the way this metric can be calculated from the sRGB coordinates R, G, and B of the two colors whose chromaticity we wish to compare. An available spreadsheet is also described that can be used to perform this determination.
Issue 1, 2008.02.08. 9 pages, 1850 words, one figure. PDF format, 219 kB, (214 KiB)
Mosaic Sensor Arrays in Digital Photography
Many contemporary digital still cameras use a mosaic sensor array (often called a color filter array, or CFA, or a Bayer array) to develop a digital color image of a scene. In this article we describe this device, the principles of its operation, and its implications on the nature of digital camera images..
Issue 1, 2003.08.04. 7 pages, 2200 words, PDF format, 63 kB (62 KiB)
New Measures of the Sensitivity of a Digital Camera
CIPA, the technical association of the Japanese camera industry, introduced in 2006 two new measures of the “sensitivity” of a digital camera, recommended for use instead of the ISO speed rating to date used for the purpose. They are the standard output sensitivity (SOS) and recommended exposure index (REI). These new measures are also defined, as alternatives to the ISO speed, by the 2006 version of the relevant ISO standard itself. In this article, we discuss these two new measures and their significance. The article begins with background information on related topics encountered in the discussions. A summary is included.
Issue 2, 2007.08.30. 13 pages, 3900 words. PDF format, 107 kB (105 KiB)
On the Term “Field of View Crop Factor”
In photography, the term “field of view crop factor” is sometimes used to describe the ratio of the format size of a full-frame 35-mm camera to the format size of a particular camera of interest. This article describes the underlying technical concept and why we are interested in the factor itself. The author also suggests that the term is not appropriate, and gives his reasons for that opinion, including a critical examination of the rationale often offered by the term’s proponents.
Issue 5, 2006.05.06. 4 pages, 1500 words. PDF format, 73 kB (71 KiB)
This Excel spreadsheet allows the user to determine the amount of blurring on the image (in terms of the actual diameter of the circle of confusion) for an object at a certain distance for focus at a different distance. Separate sheets are provided allowing distances to be stated in meters, millimeters, feet, or inches.
The format allows for the particulars of several "setups" to be entered on separate lines so that the results can be easily compared.
Issue 5, 2006.05.13. XLS format, 38.9 kB (38.0 KiB)
The Packard Ideal shutter is a behind‑the‑lens shutter widely used in large‑format view cameras. It was introduced in the late 1800s and is still made and used today. This article describes the shutter, its features, and how it operates. An appendix describes, with illustrations, the detailed working of the shutter mechanism.
Issue 2, 2007.01.06. 15 pages, 4100 words. PDF format, (406 kB, 396 KiB)
Parallax Suppression in a
Target Rifle Aperture Sight
by Robert J. Burdge and Douglas A. Kerr, P.E.
With an aperture sight, often used on target rifles, the shooter looks through a small hole in a metal plate that is mounted on the rear of the rifle near the shooter’s eye, observes a front sight which is typically a small vertical post located near the front of the barrel, and adjusts the aim of the rifle until the top of that post is located on the desired location on the target. Additionally, users of these sights are always urged to position their eye so that the tip of the post is positioned precisely in the center of the field of view observed through the rear sight aperture. This is done in the interest of eliminating parallax shift between the front sight and target which would lead to uncertainty in aiming.
But we find that, as we look through such a sight and move our eye from side to side (with the aiming point of the rifle fixed), we see essentially no change in the relative position of the tip of the front sight and the target. The expected effect of parallax shift does not appear. This suggests that sight alignment, in the traditional sense, does not affect the point of aim.
In this article we look into this phenomenon. In the process, we will encounter various related matters in the fields of photographic optics and human vision. The results of both “live fire” and optical model tests are given and discussed.
An appendix presents a ray tracing exercise that demonstrates the phenomenon and its source.
Issue 4, 2007.05.30. 23 pages, 6140 words, numerous illustrations. PDF format (311 kB, 304 KiB)
The following article is a companion to this one.
Parallax Suppression with a Target Rifle Aperture Sight—An Optical Demonstration
In the use of the common aperture sight on rifles in precision target shooting, common wisdom emphasizes the necessity for the shooter to carefully maintain his eye position so the tip of the front sight post appears centered within the circular field of view of the rear sight aperture. Otherwise, goes the wisdom, parallax shift will occur, which will disrupt the accuracy of the shooter’s aim.
A recent article by Robert J. Burdge and Douglas A. Kerr, P.E., points out that this parallax shift doesn’t seem to really occur in practice, and advances an explanation in terms of basic optical theory.
Subsequently, Kerr conducted tests in which the human eye is replaced by a digital camera in the interest of actually demonstrating the behavior involved. This article reports on these tests and discusses the results.
Issue 2, 2007.05.30. 12 pages, 3090 words, numerous illustrations. PDF format, 226 kB (221 KiB)
This article is a companion to one listed just above.
Pixel Resolution in Digital Photography
This article gives insight into a number of aspects of the concept of pixel resolution in digital photographic practice. Topics include: What do we mean by resolution, and what is pixel resolution? What is the resolution indicator in a digital image file, and what does it mean? What are resizing, resampling, and interpolation? What do publishers mean by their resolution requirements for submitted digital photographs? What is the difference between a pixel and a dot? How do we accommodate the resolution appetite of a printer?
Issue 1, 2004.11.09. 13 pages, 4300 words. PDF format, 110 kB (107 KiB)
Photographic Exposure Metering and the Infamous “18% Calibration”
We often hear it said that photographic exposure meters (including those forming part of camera automatic exposure systems) are “calibrated to 18% reflectance” (or maybe 12.8% or thereabouts). What does this mean? In this article, we discuss what this actually means in digital photography. We also discuss the closely-related matter of “18% gray card metering”.
Issue 1, 2004.07.19. 7 pages, 1800 words. PDF format, 98 kB (96 KiB)
Photographic Photometry 101 Recently revised
Photography deals with light, and we are concerned in many technical ways with light and its behavior. Especially in matters of exposure, exposure metering, and the like, our discussions often involve photometry, the discipline of describing the “strength” of light. Our discussions are often hampered by inadequate or incorrect understandings of the different concepts of the strength of light and the terms, quantities and units that are involved. This article provides a concise review of photometry as it applies to photographic matters. It also gives an introduction into how the f/number of a lens affects photographic exposure.
Issue 2, 2007.12.25. 12 pages, 3600 words. PDF format, 111 kB (108 KiB)
Polarization of Light and Polarizers
Light is a form of electromagnetic radiation, and has the property of direction of polarization. This article discusses the concept of polarization and some of its implications. Both plane (linear) and circular polarization are covered. The operation of polarizers - optical components that manipulate the polarization of light - is described, and some applications discussed. Finally, a brief introduction is given to the use of polarizers in photography.
Issue 4, 2004.09.23. 9 pages, 2600 words, PDF format, 106 kB (104 KiB)
Principle of the Split Image Focusing Aid and the Phase Comparison Autofocus Detector in Single Lens Reflex Cameras
Many single lens reflex (SLR) cameras are equipped with an arrangement in the viewfinder known as a split image focusing aid, intended to facilitate accurate visual determination of the point of proper focus when focusing manually. In this article, we explain the principle by which this arrangement operates. We also describe another related viewfinder manual focusing aid, the microprism field, and discuss the application of the split image principle to one type of automatic focus detection system, the phase comparison system.
Issue 5, 2005.08.28. 17 pages, 4700 words, 16 figures. PDF format, 151 kB (147 KiB)
Programmed Automatic Exposure in Canon EOS Digital Cameras
Many modern cameras, both film and digital, offer (usually as their basic mode of operation) a “programmed automatic exposure” mode. In this mode the camera, after measuring the luminance of the scene, sets both aperture and shutter speed with no further intervention on the part of the photographer. This article discusses the details of this operation as found in the Canon EOS 10D, 20D, 300D (Digital Rebel), and 350D (Digital Rebel XT) digital single-lens reflex (SLR) cameras. It also discusses the related matters of exposure compensation (exposure bias) and program shift, tools that allow the photographer to “tweak” the programmed automatic exposure control mode to deal with the special needs of a particular shot.
Issue 1, 2005.07.04. 13 pages, 4000 words, 4 figures. PDF format, 133 kB (130 KiB)
The Proper Pivot Point for Panoramic Photography Recently revised and expanded
When doing panoramic photography with a conventional camera, multiple, slightly‑overlapping shots of the overall scene are taken by pivoting the camera in steps, and the images are joined to make a single large-scope image. In order to be able to properly join the images, we must avoid parallax shift between them. To do so, the camera must be pivoted about the camera’s center of perspective, which turns out to be the center of the entrance pupil of the lens.
It is widely, but incorrectly, said that the proper pivot point is “the nodal point” of the lens.
In this article we discuss the optical principles involved, and demonstrate why the center of the entrance pupil is the proper pivot point.
Issue 3, 2008.02.23. 15 pages, 4975 words, 7 figures. PDF format, 315 kB (308 KiB)
Replacing the Focusing Screen on the Canon EOS 20D Digital SLR Camera
Various suppliers provide alternative focusing screens for the Canon EOS 20D single lens reflex (SLR) cameras, including such focusing aids as a split-image prism or a microprism field. This article gives step by step instructions for replacing the focusing screen on the 20D digital SLR , with illustrations.
Issue 2, 2005.09.13. 8 pages, 2260 words, 7 figures. PDF format, 234 kB (229 KiB.)
Sampling, Aliasing, and the Blur Filter
The process of capturing a photographic image in digital form with a fixed number of pixels is equivalent to the sampling used in digital audio practice. This process can introduce a type of corruption in the reconstructed image knows as aliasing. This article explains some fundamental principles of the sampling process, including the impact of the Nyquist-Shannon sampling theorem. The cause and nature of aliasing is described, along with its prevention by means of a pre-sampling low pass filter (anti-aliasing filter). The blur filter, an implementation of the low pass filter in a digital camera setting, is explained. Also discussed is the concept of the digital anti-aliasing filter.
Issue 3, 2003.11.14. 4 pages, 1400 words. PDF format, 56 kB (55 KiB)
We can tilt the lens of a camera in order that the plane containing objects in perfect focus will not need to be parallel to the film plane, a desirable situation for many types of work, including architectural photography. It is often said that the required relationship between lens, film, and the desired plane of object focus is prescribed by “the Scheimpflug principle”. It fact, there are actually two principles of Scheimpflug that must be applied to achieve the required relationship. In this article we describe these two principles and their application.
Issue 1, 2006.05.24. 14 pages, 3100 words. PDF format, 172 kB (168 KiB)
Setting Camera Exposure in Terms of Ev
In the 1950s, several camera and shutter manufacturers adopted systems for setting camera exposure through a single number that reflected the joint effect of both shutter speed and aperture. This quantity eventually came to be known as exposure value (symbolized Ev). In this article we see what this is all about and how it has been supported in practice.
Issue 2, 2007.05.15. 6 pages, 1754 words, one illustration. PDF format, 106 kB (108 KiB)
Shift of the Plane of Best Focus With Shift In Film Position
In considering the behavior of a camera, we may be concerned with how a shift of the lens-to-focal plane distance affects the distance to the object plane of best focus. In this article, we show how this can be calculated. The derivation of the relationship is given in the appendix—an opportunity for the reader to brush up on his freshman calculus.
Issue 1, 2004.10.20 7 pages, 1230 words. PDF format, 103 kB (101 KiB)
Some Principles of Photographic Optics
In this article, we review a number of areas of optics that are especially pertinent to the field of photography, including focal length, focus, magnification, exposure, aperture and f/number, field of view, and depth of field. Basic mathematical formulas for factors of interest are given.
Issue 2, 2004.09.05. 13 pages, 3380 words. PDF format, 144 kB (141KiB)
The sYCC color space is an alternative representation of the “sRGB” color space, but with a special wrinkle though which it can represent a larger color gamut than the sRGB space proper. In this article we review the definition, principles, and implications of the sYCC color space.
Issue 1, 2005.10.04. 5 pages, 1750 words. PDF format, 89 kB (87 KiB.)
An important class of photoflash unit automatically controls exposure by regulating the duration of the flash output pulse based on measurement, with a photosensor on the unit, of the light reflected from the main subject. Such flash units are often described by their manufacturers and others as “thyristor” flash units. In modern flash units that also offer more sophisticated modes of exposure control, the more basic mode is often spoken of as the “thyristor” mode. In this article, we discuss what a thyristor is and why its name has come to suggest a kind of flash unit and a particular exposure control mode.
Issue 1, 2007.06.28. 9 pages, 300 words. PDF format, 90 kB (87 KiB)
Vision Correction in Camera Viewfinders
Many camera viewfinders are equipped with a lever or knob that controls “adjustable vision correction”, primarily to allow users who are nearsighted or farsighted to obtain a sharp view of the viewfinder image without their eyeglasses. In this article, we examine how this works and learn about the unit “diopter” which is used to quantify the amount of correction in effect.
Issue 1, 2007.05.07. 8 pages, 2235 words. PDF format, 80 kB (78 KiB)
White Balance Diffusers in Digital Photography
When we photograph an object illuminated by light whose chromaticity does not match the “reference white” chromaticity of the color space used to record the image, then when the “published” image is examined by a viewer, familiar objects will not seem to have their expected chromaticity. To overcome this undesirable effect, we apply color correction (often called “white balance correction”) to the captured image. In digital photography, we may actually have the camera do this for us “on the fly”. In order for the camera to do so, it must know the actual chromaticity of the incident light—the light that illuminated the subject during its photography.
Although we can measure this with a specialized laboratory instrument, we can also equip the camera temporarily with a special “front end” (often called a white balance diffuser) that will equip it to make the needed measurement itself. There is considerable misunderstanding about the technical principles involved in doing so. In this article we review and explain these principles and show how they pertain to the actual workings of this technique. The article does not discuss the operation or performance of specific available white balance diffusers.
Issue 1, 2008.02.17. 12 pages, 3850 words. PDF format, 108 kB (105 KiB)
Mathematics, science, and engineering
A recreational problem in statistics describes a situation in which an intelligence operative, wishing to know (on very short notice) the fraction of the inhabitants in a certain city who lived north of a river running through the city, contacted one inhabitant at random and determined that he lived on the north side of the river. From that, it was determined that “the best estimate” of the fraction of the inhabitants living north of the river was 2/3. In this article we discuss the possible meanings of that answer and, choosing one for further study, analytically derive that same value.
Issue 1, 2005.12.19. 5 pages, 1385 words. PDF format, 96 kB (94 KiB)
Rail Signal Aspects and Indications
Wayside rail signaling practice in the U.S. is a nightmarish web of operating rules, signal types, aspects, aspect names, and indications, differing between the different roads and even their individual divisions and locations. Much of the “vocabulary” involved is extremely curious and counter-intuitive, a result of the long historical evolution of this field and of the industry. In this article, after an examination of some of the history of this field, a consistent (if tortured) thread of syntax is identified for the mainstream of current practice, and its principles are discussed at length. Extensive charts illustrate the majority of the vocabulary with explanatory notes under two widely-used “dialects”.
This issue is extensively revised. Additions include: information distinguishing "speed signaling" and "route signaling" paradigms; a discussion, with examples, of "Common Western Practice"; sketches of various signal types; and a description of the fascinating (if largely obsolete) B&O color position signal system.
Issue 3, 2007.03.20. 40 pages, 8600 words. PDF format, 267 kB (260 KiB)
Information systems and computers
The ASCII Character “Octatherp”
Starting in the early 1960’s, the term “octatherp” came into use for the ASCII character “#”, and this practice continued for many years. The term in fact was coined as a practical joke on the author of this article by two colleagues. This article tells the story, and names names.
Issue 1, 2006.05.08. 3 pages, 850 words. PDF format, 70.1 kB (68.5 KiB)
The ASCII and "ANSI" Character Sets
In connection with personal computer technology we often hear reference to the "ASCII" and "ANSI" character sets. This paper describes what these are and explains the basis of the acronyms used to identify them, including why "ANSI" is an inappropriate designation for the second of them.
Issue 2, 2004.07.18. 4 pages, 1300 words. PDF format, 79 kB (77KiB)
Doug and Carla - an Urban Legend
Many of my friends and colleagues have expressed interest in how I got together with my wife, the lovely and fabulous Carla. Here's a brief recount of that deal.
Issue 2, 2004.09.25. 4 pages, 1100 words. PDF format, 42 kB (41KiB)
You may have noticed that I no longer show "P.E." after my name, the indication of a licensed (or registered) professional engineer. No, I didn't lose my license for driving at 75 dBm in a 55 dBm zone. At the end of 2007 I put my Texas Professional Engineer License into "inactive" status to better reflect my "fully retired" status (and save a lot of paperwork). I'm actually allowed to use "P.E. (inactive)" after my name, but I don't want to make it seem that it is I who is inactive.
This Web site maintained by
Douglas A. Kerr
Questions? Suggestions?
Complaints? Bug reports? Location of buried treasure?
Click here to e-mail der Webmeister
817-594-2552