>  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.