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02-20-2009, 05:33 PM   #1
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What do I input when using old manual focus lens?

When I'm using my old 50mm f/1.7 on my k200d and then thing comes up to input the focal length, should i put it 50mm or should i put in 75mm because of the 1.5x crop factor?

02-20-2009, 05:41 PM   #2
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It's a 50mm lens no matter what body it is on. So put in 50mm.
02-20-2009, 06:58 PM   #3
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QuoteOriginally posted by rparmar Quote
It's a 50mm lens no matter what body it is on. So put in 50mm.
Absolution true. The 1.5 crop factor just says that a 50 mm lens on an APC sensor camera will give you same photo as a 75 mm lens on a 35 mm senor or film.
02-20-2009, 10:24 PM   #4
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I think this is one of the most asked questions for a good reason: it is the subject of great confusion. The crop DSLR world must be a daunting one for the new folks.

02-22-2009, 04:02 PM   #5
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Thank you.

I was just wondering because of the change in field of view with the crop factor.
02-22-2009, 11:33 PM   #6
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QuoteOriginally posted by lurchlarson Quote
I was just wondering because of the change in field of view with the crop factor.
It's only a "change" if you compare between a film SLR and a digital SLR. But you digital SLR doesn't even know that film SLR's existed. As far as it is concerned, what it sees is all there to see.
02-23-2009, 06:34 AM   #7
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I was supposed to write a sticky about this.

02-23-2009, 07:56 AM   #8
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QuoteOriginally posted by lurchlarson Quote
When I'm using my old 50mm f/1.7 on my k200d and then thing comes up to input the focal length, should i put it 50mm or should i put in 75mm because of the 1.5x crop factor?
The 75mm is the FOV comparison anyway that is often confused in brochures including Pentaxes to be the focal length that has changed.
02-23-2009, 01:59 PM   #9
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QuoteOriginally posted by Blue Quote
The 75mm is the FOV comparison anyway that is often confused in brochures including Pentaxes to be the focal length that has changed.
Do we take it then that in a lens of a given focal length, say 50mm, the size of the image remains the same for an APS-C sensor frame when compared with a 35mm film frame or a 35mm digital sensor frame ; only the amount of the image visible, or the field-of-view (FOV), is less in the APS-C sensor?
02-23-2009, 02:51 PM   #10
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QuoteOriginally posted by chhayanat Quote
Do we take it then that in a lens of a given focal length, say 50mm, the size of the image remains the same for an APS-C sensor frame when compared with a 35mm film frame or a 35mm digital sensor frame ; only the amount of the image visible, or the field-of-view (FOV), is less in the APS-C sensor?
Correct. The sensor can't magically make the lens project a different image; the lens doesn't know if its projecting onto APS-C or 35mm. But when the image is projected onto the smaller "screen" of APS-C, the edges of the images are lost. If you then take this smaller/cropped image from an APS-C and the larger image from 35mm and blow them up to the same size, then effect is very similar to what would happen had you shot with a 1.5X longer lens on 35mm, which is where the idea of using a "crop factor" to compare focal lengths comes from. It works very well as it accurately captures the change in FOV. It does not, however, account for the difference in DOF - the 1.5X longer focal length lens on 35mm would give less DOF than the shorter lens on APS-C, all else being equal. But "most" people are more concerned about FOV than DOF "most" of the time.
02-23-2009, 02:57 PM   #11
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QuoteOriginally posted by chhayanat Quote
Do we take it then that in a lens of a given focal length, say 50mm, the size of the image remains the same for an APS-C sensor frame when compared with a 35mm film frame or a 35mm digital sensor frame ; only the amount of the image visible, or the field-of-view (FOV), is less in the APS-C sensor?
Yup.

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