Originally posted by Marc Sabatella That's the part I don't get. I've never heard anyone claim that any PKA lenses "heavily underexpose" wide open (or anywhere else, for that matter). I mean, there is the common beginner complaint of underexposure in general from people who don't understand about protecting highlights, aiming for 13% or so reflectance,etc. But it sounds like you are making a different claim, one I've never heard before, and one not supported by anything in my own experience with several different Pentax cameras. But I don't have a K20D, so I can't directly say you're wrong about this. I can only wonder why no one else has ever reported this.
I observed this with both a k10 and a K20, and on both, changing the focus screen greatly improves the exposure.
My FA50/1.4 easily underexposes 1 stop compared to the kit 18-55 set at 50, for the same scene...
When you say : "you are making a different claim, one I've never heard before, and one not supported by anything in my own experience with several different Pentax cameras", you surprise me, as you know about the stop-down problem of the k10, and this is just another aspect of this problem...
To clarify my example:
- do you agree that a stop-down m42 50/1.4 set to f/4 will expose just like a regular 50/4 lens (apart from slight discrepancies in light transmission)?
- do you agree that the same m42 50/1.4 will expose differently when set at f/1.4 and f/4?
- And last, do you agree that a FA50/1.4 will then underexpose, compared to the 50/4 above, even when the aperture is set to f/4 on the body (as the exposure is metered wide-open)?
That's what I noted on my Pentax K10 & k20 bodies.
Mind you, the exposure problems appeared with the k10/K20 generation. It was perfectly fine with the *ist generation, and is, luckily, linked to the focus screen, as changing it for the *ist LL60 screen solves the k10/k20 exposure problem.
I don't know about the k1x0-k200-k2000 family, though...