Can a Canon DSLR reliably measure small brightness or color changes over time?
Asked 8/6/2018
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2 answers
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I want to use a Canon EOS Rebel T5i to record slow, subtle brightness or color changes in a fixed subject for scientific comparison. The camera, subject, and lighting are stationary, the subject is about 20 cm away, and changes happen over roughly an hour. I care most about detecting small changes in diffracted light intensity/color between images, and secondarily about seeing where on the surface those changes occur.
My current plan is to shoot RAW, disable automatic settings, lock focus manually after initial autofocus, use ISO 100, and expose as long as possible without clipping the RAW data. I assumed this would maximize usable precision, but the resulting shutter speed is only about 1/8 s, while the experiment could tolerate exposures up to about 10 s.
Is this a reasonable approach for this kind of measurement, or are there limitations of a consumer DSLR that make it unreliable for quantitative brightness/color comparisons?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
4
Is this approach reasonable or am I missing something?
You're making the mistaken assumption that consumer cameras, even those we call "pro-grade", intended for taking creative or documentary photographs are precise enough for what you are asking it to do. The EOS Rebel T5i is not a lab-grade instrument. Neither is the Canon EOS 1D X Mark II, the Nikon D5, the Sony α9, nor any other camera made for taking pictures rather than scientific measurement.
The shot to shot variation of your camera's precision with regard to exposure may well be larger than the frame to frame difference of your subject. Tolerances that are totally acceptable for creative and documentary photography can be woefully inadequate for laboratory measurements.
For more, please see the comments following this answer to Is there a sane reason why ¹⁄₁₂₅ is not, instead, exactly half of ¹⁄₆₀?
Also related:
manual mode in DSLR with constant iso, shutter speed and aperture, intensity of the image still changes, particularly this answer
Why can't ISO, shutter and aperture speeds be controlled more precisely rather than sticking to a fixed scale?
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
8y ago
0
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Your approach is sensible as far as camera settings go: RAW, fixed manual exposure, fixed focus, and low ISO are all the right ideas for repeatability. But the main limitation is the camera itself: a Rebel T5i is a photographic tool, not a calibrated measurement instrument.
For very small changes, shot-to-shot variation in exposure and sensor response may be comparable to, or larger than, the change you want to measure. So the key issue is not just using RAW or avoiding clipping, but knowing the uncertainty of the measurement.
In particular, test how repeatable the exposure really is. If you set 1/8 s repeatedly, how much does actual brightness vary frame to frame? If that variation is significant relative to the effect you’re trying to detect, the camera may not be adequate by itself.
A practical improvement is to include a stable reference of known brightness in each frame so you can compare your subject against it and detect drift or exposure variation.
So: yes, your setup is reasonable, but you should validate repeatability experimentally before trusting the data as quantitative measurements.
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