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12-05-2017, 04:04 PM   #1
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Full frame (manual focus) lenses on crop sensor camera

I have a couple of manual focus prime lenses (a Samyang 500mm F6.3 DX mirror lens and an old SMC Pentax-A 50mm F1.7) and a K-S2 body. I have been told you get a longer focal length with a full frame lense on a crop sensor, but not really found any good info on this so far. With my mirror lens I assume i would multiply the 500mm by the crop factor (1.5) and set the focal length to the end number, in this case around 750mm?

Sorry if I sound a bit vague, I'm kind of hazy on this.


12-05-2017, 04:11 PM   #2
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That's not correct, the focal length always stays the same. It's a common misconception. Just set the focal length to whatever it says on the lens.

With an APS-C camera, a 500mm lens delivers the same field of view as a 750mm would on full frame, but that's only because the latter sees more of the frame. The lens delivers the same degree of magnification.

That said, a 500mm lens is pretty long, and it should give you plenty of reach A 50mm on APS-C is also more of a short telephoto rather than a normal field of view.

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12-05-2017, 04:11 PM - 1 Like   #3
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The terminology is confusing, talking about equivalent focal length on apsc blah....
Focal length never changes, it's a fixed property of the lens. What changes is how much of the image circle that the the lens projects into the camera is used by the camera sensor to produce the image. The smaller sensors use a smaller part of the image circle. So what you get is a narrower field of view than a camera with a larger sensor would get with the same lens.
See the "crop factor" articles here

The Crop Factor Unmasked - Articles and Tips |
12-05-2017, 04:48 PM - 1 Like   #4

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Sigh... another victim of the "equivalence" nonsense...

Lisa, you're an APS-C user, right? Crop factors are somewhat useful only when you compare two formats; forget about them.

12-05-2017, 04:59 PM - 1 Like   #5
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The bottom line is that you set the focal length to the actual value written on the lens. Ignore crop factor from that point of view.
12-05-2017, 05:07 PM   #6
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The only time "crop factor" has any bearing is if you shoot both full frame and APS-C and need to understand both. If you always shoot APS-C then look through the lens and that is what it is. Forget you ever heard of the crop factor nonsense.

Last edited by jatrax; 12-05-2017 at 10:47 PM.
12-05-2017, 05:28 PM   #7
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QuoteOriginally posted by WondringLisa Quote
I have been told you get a longer focal length with a full frame lense on a crop sensor, but not really found any good info on this so far.
That's because it would be *bad* info, Lisa!

Enjoy your shooting.
12-05-2017, 05:29 PM - 1 Like   #8
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The only time you would set focal length is for lenses where the camera does not detect focal length. This is mainly for IBIS (in body image stabilization) and you would just input the focal length of the lens (ie 500). The ‘crop factor’ is relevant if you have many years of 35 mm experience to know what a full frame FOV is for your lenses. The crop sensors will shrink that FOV and so the 1.5 crop factor can give one some idea of the equivalent FOV. I don’t think of the crop factor as silly; it can serve a purpose. If you shoot only using crop bodies, don’t sweat the crop factor

12-06-2017, 04:35 AM   #9
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Thanks for the replies, clearly things could have been worded clearer when i was told about it lol.


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