I test-shot frames in Manual VS Av mode with identical aperture/shutter speed/ISO, settings and lighting, and I noticed that some frames in Av are sometimes over or underexposed compared to those in Manual.
It can easily be reproduce with spot meter, taking a reading in Av of a slightly darker or lighter tone without change in shutter speed.
I wasn't testing it with tight tolerances, but I took more than enough shots (like two hundred, at every aperture in both Av and M modes for comparison, with redundancy to make sure it was not lighting inconsistencies), and I had pretty consistent results.
Point the spot meter at any subject and take a shot in Av. Then take and lock a reading at a slightly brighter part of the subject, but not so much brighter that the shutter speed would change, reframe exactly like the first and shot. Aperture, shutter speed and ISO are the same in both shots, yet the second will be a bit underexposed. It's even more evident and easy to see that result if the camera was set to 1/2-stop increments.
ALO (and HTP) were both turned off.
I also read on a famous reviewer website, referring to the EOS 6D, that ""In any auto exposure or auto ISO mode, exposure may only be read to halves or thirds, but the camera sets it steplessly anyway."
So, is that correct that the camera fine-tune the exposure at software level in Av mode?
If so, what about image quality? For example, if I shoot a raw in AV, the camera adds +0.17 EV and I further add +0.17 EV in post. Is it the same as shooting in Manual with same aperture/shutter speed/iso, and applying +0.33 EV in post, or worse?