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10-24-2009, 04:15 PM   #1
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Location: Prairie County, Montana
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f stop calculation with 67 lenses on dslr

I have a k10d and k20d with a multitude of pentax manual lenses. The other day I purchased a convertion mount for 67 lenses to k mount. I am under the understanding that the lens focal length will be increased by about 3x on a dslr and about 2x on my ME Super and MX. What about the f-stop? Does it remain constant?

10-24-2009, 05:28 PM   #2
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the focal length is the same and the aperture is the same as before
10-24-2009, 08:59 PM   #3
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I bought one and have done the comparisons...the VIEW of a C crop digi is about 1/6 of the VIEW you get through that same lens on 6x7 format. However, a 6x7 (or 67) lens that says 100mm on the barrel, shoots the EXACT VIEW of any other 100mm lens on your digi. Same for 300mm or any other length. So the digital image is much cropped vs the 67 image shot through the same lens, but all 100mm lenses give the same IMAGE field of view regardless of which format 100mm lens you use. I've tried it on 45mm clear up to 300 and I even have a 400mm lens for my 67 format. They all MATCH the view I get with lenses from the 35mm format. So a 100mm is a 100mm is a 100mm regardless of the format of the lens.

Regarding aperture, I accept the theory of no difference, however if the smaller format only uses the middle sweet spot of the lens, could an argument be made that the aperture opening is relatively a larger percentage of that smaller window than it is of the entire front element, thus changing the effective f stop? I could probably disprove this thought playing with lenses in the front yard tomorrow, but since that's opening day of hunting season in Montana (and you are forceably removed from the state if you don't hunt), I'll be afield without a camera tomorrow.

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