I have a Pentax K10D. I just got myself a Vivitar Series 1 19-35mm f/3.5-4.5 lens. I noticed, that in Av mode, when I set the aperture to f/8.0 on the body, it only stops it down to about f/5.6. This is regardless of zoom position. This is a KAF mount lens. The lens stops down properly in M mode with green button metering, the viewfinder darkens accordingly as well. However in Av mode, it doesn't work properly. I just had a full day of shooting with this lens attached to my Pentax MX and all was well. I was using the aperture ring then. I came to the conclusion, that using this lens by aperture ring is the way to go, it works well. However it's quite disturbing. It either means there's a mechanical problem, or there is one in the firmware of the camera. This isn't he first lens, with which this problem occurs: I had a sigma P&P lens too, which also gave me overexposed pictures. So does my Tokina 28-70 MF lens. I highly doubt, that all of these lenses are faulty, since they (with the Sigma's exception) work perfectly on my MX. I'm thinking that it my be my good 'ol K10D, which is acting up.
Do you have a genuine Pentax lens to test on the camera? If it works fine, there's probably nothing wrong with the body- the electronics for auto aperture are very simple.
There are some user reports of the A-type electronics failing in that Tokina lens, though (exactly the same issue with maxing out at F5.6), and some third-party lenses aren't fully compatible, either. Using the MX to compare with isn't doing you any good as it uses the aperture lever the old-fashioned way.
Yes, this is a bizarre issue, though the report on the review is just the opposite with the lens not opening up beyond f/5.6. The OP's being the AF version, the maximum aperture and aperture range information should be transmitted by the data pin with the standard KA contacts serving as backup on non-AF bodies. Why the aperture information should shift at exposure time only is a little strange. The fault is most likely with the lens and I would not expect the OP to explore further except to confirm that other lenses (18-55 kit?) work properly. That being said, I would be very interested in seeing what aperture is reported in the EXIF. That would help differentiate between an electronic failure and a mechanical issue with the aperture mechanism. I suspect something mechanical and would expect the EXIF to report f/8 to indicate proper lens-body communication.
To explain further...In normal operation with the aperture ring, the body will operated the actuator to full travel with the iris opening being determined by a mechanical "stop" inside the lens. Barring damage or impediment to the iris itself, this mechanism will usually work reliably for decades of regular use. Body-controlled aperture (so-called automatic aperture control), on the other hand, requires that the aperture actuator be precisely positioned with the lens complying by changing the iris opening in response. The lens response is passive with the lens lever following body coupler using spring tension from the lens. The lens part of the interaction may fail due to damage or wear to/in the actuator mechanism as well as degraded or misplaced lubricants.
Confirmation of smooth and snappy motion through the full range (lens off camera, aperture ring on "A" setting, actuator flicked with a finger to simulate actuation during exposure)
The aperture blades look like new, although when it's 5.6, the iris isn't centered, looks somewhat downwards looking. It goes away by f/8.
I did check this one and even unmounted while checking the DoF preview. Without lens, it was fine, but it stopped halfway with a lens mounted. (there wasn't any resistance when unmounting, so I didn't damage it)
I'm guessing that this might be an electric problem. Although the Tokina has a similar problem, that too is somewhat solved by using an EV comp. of about -0.7 or -1.0. But than again, that only solves the shutter speed.
Update: It CAN be compensated by setting the aperture setting about a stop higher than the desired one. For example setting around f/11 - f/13 gives the desired aperture opening of f/8, which is just fine for just about anything. So it's no catastrophe, one just has to pay attention.
I am looking for a knowledgeable sports writer who would be able to produce web copy about sports personalities (British legends), sports memorabilia (all types) and sporting articles (current news). A successful candidate would be responsible for updating up to 3 websites and would need to be competent enough to add images and do some small editing in a CMS system. If you think you fit this description then we would like to talk to you.