How can I quantitatively compare two cameras' color accuracy with a Macbeth ColorChecker if their exposures differ?
Asked 4/24/2017
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I want to compare the color reproduction accuracy of two cameras quantitatively, using either RGB or CIE Lab values from images of a Macbeth ColorChecker. The setup uses the same standard light source, the same lens (same focal length and aperture), one-time auto white balance, and no gain. The issue is that each camera may need a different shutter speed to avoid clipping, so the exposure times are not identical. Since patch brightness changes with exposure, how should I compare the color differences fairly? Should both cameras be forced to the same manual exposure, or can differing exposures still be valid if the measurements are normalized somehow?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
1
While I don’t think a minor difference in shutter time will be a major influencing factor, I suggest setting both test cameras to the same f-number and shutter speed, in manual mode. Should you observe differences in the exposure levels, mount neutral density filters on the camera that yields the greater exposure. For many years I managed an operation that made test films for the photofinishing industry. These were super accurate as to color and density. They were used worldwide to calibrate high speed photofinishing color printing machine, so I have some experience in these matters.
Use ND filters to balance the camera with the greater exposure to the other. I suggest you use Kodak Wratten Neutral Density filters. These are of high quality and as neutral as you can get. Some should be available on the web or you can procure others.
ND filters are labeled in density units. These seem cryptic however the values are exponents of base 10 logarithmic notation.
0.0 = No density change used to balance a system keeping the number of filters used the same.
0.025 = 1/12 f-stop = 1.06X
0.05 = 1/6 f-stop = 1.12X
0.10 = 1/3 f-stop = 1.26X
0.15 = 1/2 f-stop = 1.41X
0.20 = 2/3 f-stop = 1.58X
0.30 = 1 f-stop = 2.0X
Let me add -- Use constant voltage transformer to power the lighting system. The slightest voltage change will greatly effect the color output of your lighting system.
Originally by user44949. Source · Licensed CC BY-SA 4.0
user44949
9y ago
0
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Different shutter times do not automatically invalidate the comparison. A standard approach is to expose each camera so the chart is well below clipping, then normalize measurements using the white patch on the ColorChecker. In color-difference workflows, values are typically scaled relative to the white patch, so absolute exposure can differ as long as highlights are not saturated.
A relevant reference is ISO 17321-1, which describes color characterization of digital still cameras; it allows fixed exposures that place peak output roughly between 50% and 90% of saturation.
If you want the simplest direct comparison, put both cameras in manual with the same aperture, shutter speed, and ISO. If one camera overexposes, you can reduce its light with a good neutral-density filter. That helps isolate differences in sensor response rather than exposure.
Best practice: shoot RAW, keep lighting and optics identical, avoid clipping, and compare normalized patch values (often converted to Lab) against the chart reference. The key is consistency and normalization, not necessarily identical exposure times.
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