Originally posted by micl161 As this lens have fixed aperture (f 3.5) , so what's the difference if I use Av mode (wide open as its also f3.5) and the M mode ?
The short answer is that the difference is the same as with any other manual aperture lens and that there is a difference.
That being said, yours is a good question that leads directly into the realm of unverified (wild) claims by people like me. My personal experience with Pentax dSLRs (post-2009) as well as others on this site is that stop-down metering using the optical viewfinder is more consistent in M mode (green button or EV scale) than Av. Yes, there is a difference and often the difference is notable.*
Another difference for stop-down metering is between the optical viewfinder and live view with live view holding a distinct edge. My practice in recent months has been to get my exposure settings using M mode and the green button in live view and then shoot using the optical viewfinder in M mode. The settings should remain good until either the subject or light changes.**
Steve
* Sometimes as much as 1-2 stops. A good way to test is with a light gray or white blank wall in good natural light (e.g. north facing at noon) filling the frame. Actual behavior depends heavily on the lens being used and the aperture setting. A proper exposure will yield a centered histogram for settings measured at all apertures.
** This approach is
very old-school, but works quite well in actual practice.
Last edited by stevebrot; 10-03-2018 at 08:15 AM.