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Forum: Pentax DSLR Discussion
05-21-2009, 01:57 PM
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I think pentax elected to "live with the issue" so to speak.
I advised them of the problem in writing, but had not fully investigated it. What is interesting is that an A lens does the same think, when it is taken out of A.
I think what they did, was realize the error existed, and in the quest for a brighter viewing screen, account for it in exposure compensation based on maximum apature. What is interesiting is that my sigma TCs are simply feed through for apature data, and as a result putting a TC on my 70-200 introduces a constant exposure error because the real apature has changed byt the body does not know it. Note the SMC-F 1.7x AF TC modifies the apature reported to the body of an A lens so metering is not impacted.
I also asked if they would re-calibrate my metering based on having an *istD screen installed, the answer was no to that also. that was called a modified camera.
for me personally, I would have preferred being given the choise, just like with focus adjustment etc.
Edit note
one add on point, if we had on the input menu maximum apature as well as focal length, pentax could program the green button to follow the error curve and fix the whole issue
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Forum: Pentax DSLR Discussion
05-21-2009, 10:12 AM
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No split image makes spot metering especially, worse. What I do is use the histogram, and select a suitible grey surface. (tree trunk or road is good, although I have again doug out my 18% grey card also) I then do a test shot on that surface, check the histogram, and manually adjust either shutter or aperture so that the peak of the histogram (from the grey surface) is in the middle. Once set, unless you change lighting conditions, you just leave it alone. I did the same approach (except without histogram obviously) when I shot film i.e. meter off a grey surface and go and shoot. Many people who shoot manual do a similar thing.
With some combos, if I ever get a K20, that allows exposure compensation, I will be able to just dial in an offset. When I use my SMC 300F4 anmd SMC-F 1.7x AFTC the effective aperture is F6.8 (Ithink) and always tends to over expose by 1 1.2 stops. right now, I simply set aperture, meter and then dial up the shutter speed, but it would be nice to have it always entered as compensation
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Forum: Pentax DSLR Discussion
05-21-2009, 09:45 AM
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I spent a lot of time understanding exposure errors with K mount lenses as I have 8 of them.
I do 2 tests for each lens I own, on each body (*istD and K10D at present)
test #1
for each aperture click on the lens I meter with the green button, and take a shot. my "test subject" is a block wall or paved road, uniformly lit by sunlight.
I then check the greyscale histogram value in a photo editor (I use PSPX2) on a selection of about 10% of the total area in the middle of the shot. PSPX2 lets you view the histogram for either the selection or full frame.
I plot greyscale vs apature (apature on a log scale. that's how I produced the graph for this post:
https://www.pentaxforums.com/forums/241716-post69.html
Test #2
is to meter at something around the middle of the metering range (F5.6.-F8) and as you change aperture, change the shutter speed correspondingly, i.e. each 1/2 stop click should be replicated with a 1/2 stop change in shutter speed. make the same type of plot. THis will tell you if your aperture is absolutely accurate.
Usually there are errors in both ends, or a gradual drift, i.e. a tendancy to over/under expose progressively as you stop down, but this error is usually less than 1/2 stop over the entire range.
Note between about 25 grey scale and 225 greyscale each F stop is between 40 or 50 greyscale value (depending on low or high contrast respectively)
The bottom line is to know your lenses.
As for changing the focusing screen, I swapped between K10D and *istD in about 10 minutes. It's pretty easy.
right now, I have a split image screen in my K10D
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