Originally posted by monochrome I’m not so certain it is strange behavior. I think you are experiencing Exposure Value algorithm decisions which affect multiple parameters beyond just Av, Sv and ISO, and are controlled by multiple Menu settings beyond just eDials. It is a highly complex subject. Full EXIF exports will help track down the variables.
Agreed. Unless the camera is confirmed to be in fully manual mode with no dynamic range automation or exposure compensation (auto or manual), flash-on, or auto-ISO, it is hard to say what is happening. The example photos of the diaphragm show a difference of about 1 stop between body-controlled and aperture ring controlled. A full stop might indicate intent.
Originally posted by Andrea K The problem can't be the sticky blades because is related to the size of the iris, the second test checks this.
I beg to differ based on a fair amount of experience with vintage lenses and troubleshooting experience on this site. Your images show different opening sizes. They do not indicate why. It is quite possible that there is an issue with the aperture actuation linkage that results in an improper opening size at exposure time. After all, the lens is probably at least 30 years old.
If it were me doing the testing, I would do the set up in M mode with fixed ISO, no EV comp, no auto-DR adjust, and no auto EV comp. I would then do a blank wall/constant light exposure series with equivalent EV through the range of available apertures with the expectation of similar capture histogram for all exposures. I would also test for physical resistance in the mechanism first since it is easy to do and becomes the stop point for fault identification if the movement is anything other than smooth and snappy within the range. There is no sense looking further.
To be clear, unless similar problems have been seen with other lenses, it is highly unlikely this is a problem with the camera body. If the lens is a recent purchase and fault may be reasonably assigned, a return to the seller might be in order. Otherwise, repair might be appropriate.
Steve
Last edited by stevebrot; 08-28-2018 at 03:37 PM.