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12-09-2013, 04:48 PM   #1
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film lenses vs digital lenses focal length

Hi friends,

I know this has been discussed a lot but this is very interesting.
I made a comparison test between film era SMC Pentax M 50mm f1.7 prime and the two kit lenses 18-55 WR and 50-200 WR regarding focal length.
Now for the film lens I should get a 75mm focal length in APSC equivalent considering the 1.5 crop factor.

But the results are definitely not as expected: the film lens has quite the same focal length effect as the crop lenses on my K5ii. The 50mm film lens is even a little bit wider than the 50-200 at the same 50mm focal length. I was expecting something like 75mm; I made a picture at around that FL (72mm as the camera doesn't seem to recognise the 75 mm).
Please watch the attached pictures. All the pictures are made with the K5ii on a tripod, not moved, jpgs from the camera. Sorry for the tricky light in my room. The picture is a watercolor I made some years ago.

Has anybody done such a comparison test or somebody has any idea what is going on?
Also, from what I understood the Limiteds are designed for FF film? How a Limited lens will behave on crop sensor regarding focal distance? I plan to buy one.

Thank you,

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12-09-2013, 04:54 PM   #2
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50mm is 50mm no matter what size sensor is behind it. The pictures should look the same as they were all recorded on the same size sensor. The "crop factor" refers to the differences when the lens(s) are compared using different sensors e.g. 35mm film vs APS-c.
12-09-2013, 04:55 PM   #3
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Focal length is focal length, it is a function of physics and sensor size has nothing to do with it. A 50mm designed for any format is going to look roughly the same on the same camera. You could put on a 50mm lens from a 6x7 medium format camera on the K-5 and it would still be the same as the DA 50mm designed for APS-C.

The image circle changes, but the focal length is always the same.

Unless you shoot with two different formats, forget crop factor ever existed.
12-09-2013, 04:55 PM   #4
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Focal length is focal length. It doesn't matter what image circle the lens covers, the field of view on the same camera will be the same for any 50mm lens- even one designed for medium format.

The whole crop factor thing just means that on film, a 75mm lens gives the same field of view as a 50mm on apsc.

12-09-2013, 05:01 PM   #5
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50mm is 50mm. Has nothing to do with crops, films, sensors or anything except the physical properties of the lens. So if it says 50mm on the lens, it's a 50mm.

Now what a 50mm looks like on film versus digital is different. Your film era 50mm will look the same as your da 18-55 at 50mm on digital because they are both 50mm lenses. Now if you put both those lenses on a film camera they would again look the same, but not the same as they looked on digital. (Assuming the DA 18-55 would work on film where I assume it vignettes badly because the lens was not designed to cover as big a "sensor" as a film lens.
12-09-2013, 05:08 PM   #6
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Thank you all. I finally understood. I almost believed that a 300mm film lens will be a super telephoto 450mm )
12-09-2013, 06:45 PM   #7
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A lot of people happily contribute to the longevity of that myth, usually invoking the term reach in the process.

I call it virtual reach: increasing the apparent magnification not optically, but via increased pixel density on the sensor. Ultimately it comes down to semantics, and your personal choice of what constitutes reach.

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