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09-03-2007, 08:22 AM   #1
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zooms underexposing at longer focal lengths

I put a split prism focusing screen in my K100D, and while I haven't had any problems with metering using prime lenses, zooms are a different story. On both my zoom lenses (DA 18-55 and FA 28-90) metering is accurate at the short end of the zoom, but underexposed by a couple of stops at the long end. I'm guessing that for some reason the combination of focusing screen and variable aperture are fooling the meter into thinking it's getting more light than it is. It happens both wide open and stopped down. I don't really understand why this is happening or what I can do about it...any ideas?

09-03-2007, 10:18 AM   #2
and
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it doesnt really have anything to do with the fact that its a zoom, it has to do with the aperature. if u get a constant f2.8 zoom then u will not have this problem. constant f4 also seems ok.

I believe its related to the fact that half of the center prism and half of the microprisms will go dark if the lens is too slow, especially indoors. now the metering is behind the focusing screen, so if half of the center if black and the other half normal then the meter seems to go crazy and underexpose..

The split prism screen i have seems ok for up to f4, so my da 12-24 works ok, but at f5.6 everything goes dark. the worst is a cheap sigma 18-50 3.5-5.6 lens. even at +3 ev its still slightly underexposed,although its fine at 18mm.

solution? get faster lenses...if u dont want any problems then u should not have anything slower than f4. I have replaced the f5.6 lenses i used with faster zooms or primes so I dont have much of a problem now.
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