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12-21-2007, 11:25 PM   #16
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RiceHigh's Pentax Blog: When the Focusing Screen Lies (for Focusing and Metering)

QuoteOriginally posted by Duck Dodgers Quote
I normally use my Vivitar Series 1 70-210mm lens on my K10D in fairly low light, so I'm normally shooting near wide-open. This is a "non-A" lens (no A setting on aperture ring). Today I was playing around with it in the afternoon sunlight, and noticed something odd. If I set the aperture to f/2.8 press the green button in "M" mode, I get a decent exposure (center-weighted, on tripod). As I stop down in aperture, meter and shoot, the shots are progressively more and more over-exposed as I approach f/22. Thinking there might be oil on the aperture blades, I checked, and sure enough there is the slightest bit on one of them.

However, now comes the weird part; I set the K10D f/16 @ 1/ISO (sunny 16 rule) and took a sequence of shots at the same EV (i.e. open one stop, increase shutter speed one stop, repeat, rinse). No metering, strictly manual. They all exposed fine. So the oil is not preventing the blades from closing during exposure...so why the discrepancies in metering? Why would the diaphragm response be slower when metering (as it appears to be) than during the actual exposure?


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