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07-29-2009, 03:57 PM   #1
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Focal length on M-series lens

I have an old M50-f2 coupled with k20d and when it asks for focal length should i put in 50mm or 75mm for the crop factor or doesn't it matter?

07-29-2009, 04:11 PM   #2
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Put in 50mm. The camera automatically calculates the crop factor.

RD
07-29-2009, 04:11 PM   #3
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That crop factor you're thinking of is really just a shorthand way of thinking about field of view -- i.e., your 50mm lens on an APS-C DSLR will have the same approximate field of view as a 75mm lens on a full-frame (D)SLR, when the photo is taken from the same location.

So to answer your question, crop factor doesn't enter into it. Focal length is focal length, regardless of format. So input 50mm for your M 50.
07-29-2009, 04:12 PM   #4
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QuoteOriginally posted by jackbullet Quote
I have an old M50-f2 coupled with k20d and when it asks for focal length should i put in 50mm or 75mm for the crop factor or doesn't it matter?
Put in 50mm. The crop factor does not affect the focal length of the lens, only the field of view.

07-29-2009, 04:14 PM   #5
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some where can we get a great big sticky thread.

only one line

FOCAL LENGTH IS ALWAYS FOCAL LENGTH IT IS A PHYSICAL PROPERTY OF THE LENS AND IS NOT AT ALL RELATED TO WHAT CAMERA IT IS ATTACHED TO
07-29-2009, 04:16 PM   #6
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That is my mantra. But I'm not quite to Nirvana yet for some reason.
07-29-2009, 04:20 PM   #7
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This should be a sticky.
Look at the lens bezel. It tells you the focal length to enter.
There is no such thing as a "crop factor".
07-29-2009, 04:32 PM   #8
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QuoteOriginally posted by Lowell Goudge Quote
some where can we get a great big sticky thread.

only one line

FOCAL LENGTH IS ALWAYS FOCAL LENGTH IT IS A PHYSICAL PROPERTY OF THE LENS AND IS NOT AT ALL RELATED TO WHAT CAMERA IT IS ATTACHED TO
My apologies oh great one!!!

07-29-2009, 04:38 PM   #9
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This question comes up regularly, moving to Beginners Corner where it might help others.
07-29-2009, 04:55 PM   #10
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QuoteOriginally posted by jackbullet Quote
My apologies oh great one!!!
You have to excuse us old guys!

This and similar questions come up frequently and it is tempting to get cranky at times.

When the camera asks for focal length with a non-AF lens, it stores that information for use with the shake reduction system (SR). How SR is applied is dependent, in part, on the focal length of the lens...specifically the lens' magnification.

Mounting a 50mm lens on a camera with a APS-C sensor does not change the optical qualities of the lens. A 50mm lens regardless of whether it was made for 35mm film or APS-C sensor will still have focal length of 50mm and the same magnification. Since it is asking for the focal length, you should enter the focal length.

So, what about the "crop factor". In simplest terms, the "crop factor" is just a convenience calculation that allows a person familiar with the 35mm format to know what field of view (FOV) they can expect on the smaller APS-C sensor. It is called the "crop factor" because using a smaller sensor is equivalent to the process of cropping an image to a smaller size in software or in the darkroom. Make a print from a 35mm negative, trim a third from length and width and you have the same subject as viewed on APS-C.

This probably is as clear as mud, but it is a try.

Steve

Last edited by stevebrot; 07-29-2009 at 05:02 PM.
07-29-2009, 06:09 PM   #11
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QuoteOriginally posted by stevebrot Quote
You have to excuse us old guys!

This and similar questions come up frequently and it is tempting to get cranky at times.

When the camera asks for focal length with a non-AF lens, it stores that information for use with the shake reduction system (SR). How SR is applied is dependent, in part, on the focal length of the lens...specifically the lens' magnification.

Mounting a 50mm lens on a camera with a APS-C sensor does not change the optical qualities of the lens. A 50mm lens regardless of whether it was made for 35mm film or APS-C sensor will still have focal length of 50mm and the same magnification. Since it is asking for the focal length, you should enter the focal length.

So, what about the "crop factor". In simplest terms, the "crop factor" is just a convenience calculation that allows a person familiar with the 35mm format to know what field of view (FOV) they can expect on the smaller APS-C sensor. It is called the "crop factor" because using a smaller sensor is equivalent to the process of cropping an image to a smaller size in software or in the darkroom. Make a print from a 35mm negative, trim a third from length and width and you have the same subject as viewed on APS-C.

This probably is as clear as mud, but it is a try.

Steve
Nice explanation! It couldn't be more understandable! Now if we can just put it in a spot where everyone who will ask the question will see it.
07-29-2009, 10:26 PM   #12
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QuoteOriginally posted by Wheatfield Quote
There is no such thing as a "crop factor".
Of course there is - it just doesn't means what some people think it means.
07-30-2009, 06:41 AM   #13
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QuoteOriginally posted by Marc Sabatella Quote
Of course there is - it just doesn't means what some people think it means.
It's lazy thinking.
07-30-2009, 09:04 AM   #14
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If by "lazy" that you mean "gets the job done with no unnecessary effort", then indeed, it's lazy thinking - of the best kind. Unless you know of even less complicated way to compare FOV between formats. You could of course actually calculate, or look up, the FOV for every lens/camera combination to compare FOV directly. I don't consider that easier, so if lazy gets the job done - and it most definitely does - I'm all for it.
07-30-2009, 09:12 PM   #15
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QuoteOriginally posted by Marc Sabatella Quote
If by "lazy" that you mean "gets the job done with no unnecessary effort", then indeed, it's lazy thinking - of the best kind. Unless you know of even less complicated way to compare FOV between formats. You could of course actually calculate, or look up, the FOV for every lens/camera combination to compare FOV directly. I don't consider that easier, so if lazy gets the job done - and it most definitely does - I'm all for it.
But it only applies when the person looking at the combination knows the field of view on a film or "full frame" (I hate that expression - 11X14 Deardoff is full frame!). I would wish that we could just come up with the angle subtended and leave it at that.
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