Originally posted by petez I just got off the phone w/ Pentax. The gentleman was kind enough to test a K20D and a manual lens. He could not reproduce the problem. I explained my frustration and the frustration of others who experience the problem and he is willing to help find a solution. The only catch is it must be with a Pentax lens. So, if you are seeing manual exposure or manual metering errors with a Pentax lens, please reply w/ the lens model and the problem and I'll get him to test it.
Pete
I went through this with my K10D, and sent pentax a spread sheet with all the lenses I had tested up to that time.
As I have bought more I have kept the spread sheet updated.
In the end, I have seen nothing on the K10D to resolve the issue.
I even offered them the technical concept, simply allow us to enter minimum and maximum apature and the camera then controls the stopping down of the lens as it does with KA mounts and newer. The only catch is for manual lenses pentax has to recalibrate the apature actuator. This is because KA mounts and newer have the apature area linear with movement of the lever, where as K mounts have the apature diameter move linearly with the lever. As a result, unless they make this change in calibration, the exposure will be off by the same amount.
They could of course, simply avoid all the software issues and give us a viewing screen that works, but at the rate they shipped K10Ds (I think about 10-15000 per month) they have to retrofit a LOT of cameras.
Lots of luck with them and your K20. I have learned to live with the issue, but wish I didn't have to.
Lenses tested to date on K10D
Pentax K 50mm F1.4
Pentax K 105mm F2.8
Pentax K 135mm F2,5
Pentax K 300mm F4 with and without 2x Takmur TC
Pentax M 100mm F4 Macro
Tamron XR LD 28-75 F2.8 Di (note tested using the apature ring)
Vivitar Series 1 70-210 F3.5
Vivitar 400mm F5.6
All lenses exhibit the same behavior
the general curve is -1 to -1.5 stops under expose at F1.4 moving to correct exposure at about F4 to +1.5 to +2 stops from F8-F16 then coming back slightly to +1 to +1.5 stops by F22/32
All these same lenses are within +/- 1/2 stop over the entire range of f-stops on my *istD.
It is curious to note that my Tamron 28-75 when using the manual apature ring as opposed to the camera controling the apature exhibited the same problem, but is perfect when using the apature controlled by the camera.
Still to test.
Tamron adaptall 2 25mm f2.5
Sigma APO 70-210mm F2.8 EX
Sigma APO 10-20mm EX DC
Pentax SMC - FA 28-105 f4-F5.6
Pentax SMC - FA 28-80mm F3.5-4.5
For all those interested, the most reliable way to perform the test is to take a series of photos, one at each manual F-Stop detent, using the green button to set exposure in manual mode.
Your subject should be a block wall, paved or concrete roadway or path.
For each shotuse a photo-editor's histogram to calculate the grey scale value of the central 10% of the image area.
The camera should be set to 1/2 stop increments, because this matches the mechanical detents of the lens.
ISO should be set to one value. preferrably below 800. Contrast shouldbe set to neutral the camera should not be in bright mode.
With these settings you should have the grey scale (for no exposure error) at about 110, and each stop is a change ingrey scale of 40-45 over the greyscale range of about 25 and 230. You can actually test this out by setting your camera to the middle F stop, set the exposure, and then mechanically changing apature both ways without changing the shutter speed. If you plot grey scale vs f-stop (Fstop on a log scale) you will get an s shaped curve streight in the middle, showing the fariation of grey scale with F stops.