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01-22-2010, 07:55 AM   #31
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QuoteOriginally posted by Lowell Goudge Quote
as responded to by dlacouture it appears to be due to the type of viewing screen, and the impact that aperture has is that it will change the angles at which light strikes the viewing screen, although I cannot give youthe physics behind the issue, it appears to be an issue of scatter, one part caused by the micro lenses of the screen, the other caused by the F Stop of the lens and the angle light hits the viewing screen.

When I first heard about the issue, I didn't think it impacted my K10D because the images were generally close visually. It was not until I shot not a scene but a uniform lit surface which had a very narrow exposure band and measured the grey value of the peak that I could actually quantify the error,m and also test a solution.
Not totally, if it knows maximum aperture, and takes 2 metering readings, one at maximum and one stopped down, it can , with the correction curve calculate the used aperture. and set the shutter accordingly.

As a result, the green button function would change from stop down - measure - open to measure - stop down - measure - open
The discussion of the screen grinding and aperture begins to make sense. However, here is a riddle that does not.

Since I went to the K10/20d for my manual lenses (actually, mostly the K10d these days because of the K20 focus adjustment for AF) I tend to use the Cosina 1.2 almost exclusively over the Pentax M50/1.4 for low light 50ish shots. Why? On my K10d, the Pentax M50/1.4 gives an exposure response that is a shaky S-curve, while the Cosina does not. I can stand in front of a solid colored wall or card with the Cosina, and the exposure is consistent at all apertures except 5.6 and 8, which will expose about 1/2 stop higher (those two apertures are actually more accurate). The green button exposure from 1.2 through 4 renders a histogram that is so consistent as I scroll through the shots on playback that I have to look at the exposure number in the corner to know it is a different shot. Is there something about Cosina's aperture that is physically different?

01-22-2010, 08:41 AM   #32
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My brain hurts.
01-22-2010, 09:51 AM   #33
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I use both K and M series manual lenses on my K100D with excellent results.
For me, it was simply becoming familiar with the AE-L method of metering on this particular model, used in conjunction with aperture ring and histogram to obtain perfect exposure. Every Pentax model seems to vary slightly in technique to obtain the same result. I am grateful that I can still use all my old lenses, including screwfit, I don't think any other manufacturer offers such a complete retro facility.
01-22-2010, 09:58 AM   #34
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QuoteOriginally posted by Lowell Goudge Quote
thread-jack accepted.

the K7D definitely meters better than the K10D, but it is not perfect;

It is close to the *istD for fast lenses, but still worse for slow lenses, or significantly stopped down, but if you consider the average value of the K7 Metering in my curve as about 130, then the deviation either side is in worst case within +/-1 stop.

with the K10D it is more in the range of -1.5 to +2
Thanks Lowell, for the info, the K-7 curve does look better, but I still think I'll wait for the next body, hopefully metering will be even better with the K-? or whatever they call it.

NaCl(besides I'm pretty happy with my K20D)H2O


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