[Paul the Sunman]

I have mentioned several times in these forums that my Sigma 35mm f1.4 Art habitually overexposes by between 0.7ev and 1.7ev on my K3, especially outdoors in natural light. It's been driving me crazy. None of my other lenses do this (6 Pentax and 1 Tamron).

Well, I think I've solved it (sort of). There is a basically undocumented setting in the C1 menu page: "Auto EV Compensation". All the K3 manual says is "Sets whether to automatically compensate when the proper exposure cannot be determined".... whatever that means. For some reason, I had this turned on. I don't know why, in fact I didn't even know it was there. Testing in both dull and bright natural light since turning it off sees the Sigma 35mm exposing pretty much the same as my other lenses, i.e., correctly.

But that still begs a couple of questions:

1. What is this "Auto EV Compensation" setting supposed to do?
2. Why did it screw up the Sigma but leave all other lenses untouched?

Any theories?

[Dave's clichés]

C6 is the setting specially designed to confuse us to death!

No seriously, I've read the manual and looked in the menu and I can't make head nor tail of it either! I get the impression that neither did the author, he just translated it! Best leave it off! Perhaps someone who has the read the manual in Japanese might have a more appropriate translation!

[Fogel70]

"Auto EV Compensation" is used to let the camera compensate exposure if the available exposure will give an over/under exposure.

FI if shooting with a fast lens wide open in very bright light using AV-mode, then the camera might not have fast enough shutter speed to avoid over exposure. With "Auto EV Compensation" on, then the camera will stop down the lens enough to be able to capture the shot with correct exposure.

[Dave's clichés]

"Auto EV Compensation" is used to let the camera compensate exposure if the available exposure will give an over/under exposure.

FI if shooting with a fast lens wide open in very bright light using AV-mode, then the camera might not have fast enough shutter speed to avoid over exposure. With "Auto EV Compensation" on, then the camera will stop down the lens enough to be able to capture the shot with correct exposure.

Well done Fogal70!

[Digitalis]

I don't know why Pentax chose to hide an automatic exposure setting in the custom functions menu. I have always turned this "feature" off.

Why did it screw up the Sigma but leave all other lenses untouched?

Because the transmission at each aperture setting is inaccurate. With pentax lenses, even old ones without lens/camera communication the aperture behaves in a certain way.

[Paul the Sunman]

"Auto EV Compensation" is used to let the camera compensate exposure if the available exposure will give an over/under exposure.

FI if shooting with a fast lens wide open in very bright light using AV-mode, then the camera might not have fast enough shutter speed to avoid over exposure. With "Auto EV Compensation" on, then the camera will stop down the lens enough to be able to capture the shot with correct exposure.

I don't believe that's what was happening in my case. The worst behaviour was in uniformly overcast conditions where the shutter speed was actually quite low. Nevertheless, clearly there is a mis-communication of some sort between the Sigma lens and the K3, and the camera for some reason decides to take a longer exposure than required if "Auto EV Compensation" is On.