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01-25-2010, 05:09 PM   #1
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focal lenght on digital vs 35mm lenses

Hi all there,
I recently bought K-x, my first dslr, along with the da 18-55 kit lens, mostly because of possibility to use "virtualy any Pentax lens ever produced", however, how is it with use of older, 35 mm lenses? I found in manual: The captured area (view angle) will differ between the a and 35 mm SLR cameras even if the same lens is used because the format size for 35 mm film and CMOS sensor are different.
Sizes for 35 mm film and CMOS sensor
35 mm film: 3624 mm
CMOS sensor: 23.615.8 mm
Angles of view being equal, the focal length of a lens used with a 35 mm camera
must be approximately 1.5 times longer than that of a.
Does it mean that "old" 50 mm lens will delivers same as "new" 75 mm? And which focal lenght I should input while mounting manual lens?
Thank's in advance

01-25-2010, 05:49 PM   #2
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It means that your image, taken with a 50mm lens, looks like it was taken with a 75mm lens on a 35mm film camera.

However, that doesn't change the fact that the lens is a 50mm lens. It's always a 50mm lens no matter what, and when you enter the information into your camera, you select 50mm.
01-25-2010, 06:22 PM   #3
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Graystar is correct, choose 50 on the menu for a 50mm lens (regardless if it was made for 35mm film, APS-C digital sensor, 120 or 220 film).
01-25-2010, 07:04 PM   #4
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Original Poster
Thanks guys, I've got that sorted now

01-25-2010, 07:05 PM   #5
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The crop factor is real but it's something that you shouldn't think (or worry) about. The idea is to take pictures, not make comparisons.

Welcome to PF.
01-26-2010, 03:21 PM   #6
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If you don't own a film camera, you can forget you ever heard about a crop factor. If you've got a film camera, you can see for yourself the difference between how a 50mm lens looks on film and how it looks on digital. If you don't have a film camera, it is of no concern to you how your lens would perform on a camera you don't own. All 50mm lenses produce the same FOV on your camera, whether those lenses were originally produced for a film or digital camera.
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