Originally posted by JeffJS I'm more inclined to believe that the aperture blades open all the way to a fixed point. When you press the DOF preview or make the exposure, the aperture is simply closing down to the set aperture. That is, an f2 lens, at rest (not stopped down at all) is open just a little bit wider than f2 by design.
I see what you mean, but if the blades stop down a little bit wide open, then a perfectly round highligh would not be possible. Also, judging by older lense designs that used preset, and from testing a new lense, the blades do not move (stop down) when the camera takes the exposure at wide open aperture.
Change of thought regarding wideopen aperture:
When the camera is instructed to take a photo, I persume that the camera knows the chipped lenses wide open aperture, thus the camera would not stop the lense down at all. But when the camera is instructed to make a depth of field preview, the camera stops it down to the wideopen aperture physically via firmware commands, thus bumping lever and thus the aperture blades.
I will take test photos observing the difference in aperture behaviour between taking normal photos and against preview, as well as changing the custom mode for using the aperture ring as this may change the way in which the camera meters in review mode.
This is all a little hard for me to explain, appologies if it is not clear.
I have found that the (all things being equal) regarding taking the same exposure using different shutter speeds and apertures sizes, that the exposures produced are not equal, thus the following things may be happening:
The aperture assembly is worn, misaligned.
The camera aperture lever is inaccurate.
The cameras shutter curtain is inaccurate.
Or the camera may post process a little bit in camera?!
Meh, will go out and take some real photos
I have also observed that when taking a photo wide open the photo is taken but before the mirror returns for the next shot, the aperture is stopped right down to the smallest aperture. Does anyone know why this is? It seems really unnecessary.