How to optimize detection of brightness changes in an immovable object?

Question

My task

I want to use a digital camera to capture slow changes of an object for scientific purposes, subject to the following constraints:

• The camera is a Canon EOS Rebel T5i.
• The position of the object, camera, and lighting are fixed. Changes in lighting are negligible.
• The object is about 20 cm from the camera.
• My main interest is to capture small changes in brightness or colour of the light diffracted by the object by comparing subsequent pictures.
• My secondary interest are details of the object’s surface or where the above changes happen, respectively.
• I do not need to normalise photographs for comparison to references or similar. I only need to compare between photographs taken with the same camera.
• Changes of the object happen on a timescale of roughly an hour.

Note that this is not for purposes of scientific illustration, but to gather scientific data. I essentially abuse the camera as a photon counter.

What I did so far

• Use the raw CR2 format from the camera since it has a higher intensity resolution (about 14 bit) than any RGB-based formats (8 bit).
• Disable all automatisms to avoid them getting in the way of comparing pictures.
• Set the focus using autofocus on one exemplary object. Then switch to manual focus to freeze it.
• Use the largest aperture (F2.8) for maximum sharpness.
• Set ISO to 100 (the camera’s lowest native ISO) to reduce noise.
• Set the exposure time to the highest value for which the raw data is not capped (except maybe for a few pixels).
• Ignore other settings such as white balance, as they do not affect the raw data (but are applied in digital postprocessing).

Question

Is this approach reasonable or am I missing something?

My biggest concern is that the resulting exposure time is about ⅛ s, which does not use the full potential of what the context of the scientific experiment allows for, which would easily be 10 s of exposure time.

I know that I could technically combine data from several pictures, but this would be very tedious.

Answer 1

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?

Answer 2

I essentially abuse the camera as a photon counter.

Keep in mind that you are using a consumer grade camera, not a measurement device.

My main interest is to capture small changes in brightness or colour of the light diffracted by the object by comparing subsequent pictures.

[...]

My biggest concern is that the resulting exposure time is about ⅛ s

If you want to measure something, you certainly want to know the uncertainty of the measurement, especially when you are looking for small changes. So when you dial in ⅛ s, is it ±1%? ±10%? Or an absolute offset? Is it negligible?

I'd run some tests how the shutter speed varies and then estimate if it is significant for the expected amount of brightness that you want to measure.

If it is significant, you could include a light source of known fixed brightness in each shot as a reference to normalise the brightness measurements throughout all your images.