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06-12-2013, 02:48 PM   #1
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lens focal lengths

Hello there,
so as everyone knows the APS-C cameras have a 1.5x crop-factor so that 50mm becomes 75mm.
I understand that correctly, right?
So anyway, are the APS-C optimized lenses actually crop-optimized, so that basically a 50mm lens on APS-C becomes a ~35mm lens on FF?
I ask that because I recently made a few comparison shots between a Pentax SMC-A 50mm/1.7 and a Sigma 17-50mm/2.8-4.5 and the sigma seemed to have a wider fov @50mm than the Pentax 50mm.

Thanks in advance!

PS.: if there is already a thread about that, that I haven't found, sorry, please redirect me to that thread.

06-12-2013, 02:59 PM   #2
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Not exactly, as others most certainly will explain after this.

The lens focal length is always what it says. The "equivalent" or "apparent" focal length will change depending on the size of the sensor in the body. So yes, you can multiply or divide by 1.5 for Pentax. I think Canon is 1.6.

If everyone - including manufacturers - went by "angle of view" of a lens it would perhaps make more sense, but even manufacturers refer to the "old" film format conversion.


http://en.wikipedia.org/wiki/Crop_factor

Last edited by SpecialK; 06-13-2013 at 03:29 PM.
06-12-2013, 02:59 PM   #3
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QuoteOriginally posted by pfannkuchen_gesicht Quote
so as everyone knows the APS-C cameras have a 1.5x crop-factor so that 50mm becomes 75mm.
No, everyone does not know that. In fact 50mm is always 50mm. The focal length is a property of the lens and has nothing to do with the sensor.
QuoteOriginally posted by pfannkuchen_gesicht Quote
are the APS-C optimized lenses actually crop-optimized, so that basically a 50mm lens on APS-C becomes a ~35mm lens on FF
No, see above. However, a lens designed for an APS-C sensor MAY not cover the full dimensions of a larger sensor. It might, and in some cases does, but not always. Depends on the design.
QuoteOriginally posted by pfannkuchen_gesicht Quote
the sigma seemed to have a wider fov @50mm than the Pentax 50mm.
FOV will change with different sensors. Any lens at 50mm should have the same FOV on the same camera but might have a different FOV on different cameras.
QuoteOriginally posted by pfannkuchen_gesicht Quote
if there is already a thread about that, that I haven't found, sorry, please redirect me to that thread.
There are dozens if not more. Though I will admit the search engine here is not the best and that this is a topic hard to search for.
06-12-2013, 03:04 PM   #4
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Here are a couple of articles that might help:
https://www.pentaxforums.com/forums/pentax-lens-articles/90477-crop-factor-fo...ield-view.html
https://www.pentaxforums.com/forums/pentax-lens-articles/93714-field-view-tab...d-645-6x7.html

06-12-2013, 03:53 PM   #5
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QuoteOriginally posted by jatrax Quote
No, everyone does not know that. In fact 50mm is always 50mm. The focal length is a property of the lens and has nothing to do with the sensor.
yes, it is always the same, I just meant the difference between actual focal lenght and the perceived focal length you get by a different sensor size.

QuoteOriginally posted by jatrax Quote
FOV will change with different sensors. Any lens at 50mm should have the same FOV on the same camera but might have a different FOV on different cameras.
that's what my point is. The Pentax SMC-A 50mm/1.7 and the Sigma 18-50mm@50mm seem to have different FOVs on the same camera.
Initially I just wanted to see the sharpness difference and made some test shots. for that I used a tripod, which always stood at the exact same position(the ball-head was also tightly screwed and did not move), and mounted my Pentax K5 on that. Then I made a couple test shots with the Sigma and then with the Pentax 50mm. The Pentax definitelly had a narrower fov there.
I think I will retry this but I think the result will be the same.
06-12-2013, 04:04 PM   #6
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QuoteOriginally posted by pfannkuchen_gesicht Quote
The Pentax definitelly had a narrower fov there.
Some zooms have a little slop in them so that 50mm may be off just a bit. Not saying that is the case here but it could be. At any rate, there should be no difference. And there isn't in other lenses I have tried. I have a fair number of film lenses and anything I've tested against a similar focal length DA lens has come up the same.

We had a similar post last week where someone posted pictures to show exactly what you are talking about. But it was clear from the images that the camera had shifted slightly between shots. A proper test needs a good repeatable and measurable target, fence or house with closely spaced windows and needs to be shot straight on. Tripod and locked down of course as you know.
06-12-2013, 04:13 PM   #7
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I'll just test again, maybe the tripod moved withouth me noticing it(which would be kinda strange though). I had put a few things on my desk I photographed, so should be the same in every shot.
Thanks for the quick replies!
06-13-2013, 12:18 PM   #8
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Zooms can also use complex designs so that focal length changes with the distance they are focused to. With that design, infinity focus is at the marked distance, but when you focus closer, the field of view increases.

I remember seeing reviews of Pentax 50mm lenses where the actual measured focal length is a few percent over 50mm, so I'd expect the A50/1.7 could be say 52mm and the Sigma maybe 48mm with slightly exaggerated range for advertising. If the field of view difference changes with distance, though, it's the lens design.

06-14-2013, 01:03 PM   #9
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QuoteOriginally posted by pfannkuchen_gesicht Quote
yes, it is always the same, I just meant the difference between actual focal lenght and the perceived focal length you get by a different sensor size.

that's what my point is. The Pentax SMC-A 50mm/1.7 and the Sigma 18-50mm@50mm seem to have different FOVs on the same camera.
Initially I just wanted to see the sharpness difference and made some test shots. for that I used a tripod, which always stood at the exact same position(the ball-head was also tightly screwed and did not move), and mounted my Pentax K5 on that. Then I made a couple test shots with the Sigma and then with the Pentax 50mm. The Pentax definitelly had a narrower fov there.
I think I will retry this but I think the result will be the same.
The lens fl, which you see printed or engraved on the lens or can read in a brochure is usually "normalized" to meet the value most people expect from it. So a 50mm lens may only have 47mm fl or may have 55mm fl. You never know, unless you really measure it. If you compare two lenses, one might have a fl near the lower tolerance limit and the other might have an actual fl near the upper tolerance limit. (Just as Dave wrote above.)

Many years ago I compared a handful of wide angle lenses and the difference between longest and shortest fl (all being 20mm lenses) was in the vicinity of 15% - which is lot for such wide angles.

Another factor which influences the field of view is indeed shooting distance. If you compare two lenses, make sure, you use them at near infinity or at least 50x their focal length. Depending on the lens construction, the fl changes significantly at short distances.

Ben
06-14-2013, 02:04 PM   #10
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alright I think that explains it, thanks a lot guy!
I didn't even know that lens manufactures have so high tolerances.
06-15-2013, 02:38 PM   #11
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But please don't understand this as tolerances of the manufacturing process. These are tolerances of marketing specifications against scientificly correct specifications.

Bitte dies nicht als Fertigungstoleranzen mißverstehen! Es handelt sich um für Marketingzwecke gerundete Werte, die eine gewisse Abweichung gegenüber der streng wissenschaftlichen Definition zeigen dürfen. Diese Abweichungen sind aber weit entfernt von den Phantasiewerten, die man gelegentlich bei Blitzgeräteherstellern bezüglich der Leitzahl ("GN") findet.
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