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08-23-2013, 06:23 AM   #1
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What causes "normal perspective?"

The Abnormal Lens | Japan Camera HunterJapan Camera Hunter

The above article implies that the normal perspective of the 50mm lens (through the viewfinder) on SLR's is due to the pentaprism? Is this true? If so, then why don't APS-C cameras have a pentaprism calibrated to make 30mm or 35mm lenses have "normal" perspective through the viewfinder?


Last edited by konraDarnok; 08-23-2013 at 10:19 AM.
08-23-2013, 07:49 AM   #2
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What causes "normal" perspective is having a lens on your camera that matches the diagonal of the image area, either film or digital sensor. With 35mm film that diagonal was 43mm (or close enough), which is why Pentax made the FA43 limited, to give a true "normal" perspective. With an APS-C sensor the diagonal is more like 29mm, so a 28mm~31mm prime is about as close as you'll get, or a zoom set to 29mm.

Field of view is different though, many times you'll hear that a "normal" lens "represents what you would see with your eyes". That's only true in the sense of perspective, but since we have two eyes, and their "font elements", if you will, are shaped differently than a camera lens, our field of view is much greater. It varies a bit from person to person, but it's close to 180 degrees if you use both eyes.

[edit: I believe that when it comes to viewfinder magnification a 50mm lens is used, presumably because it was close-enough-to-normal on film, to calculate how large the image appears in the VF. Surprisingly, I don't think they changed that when smaller formats like APS-C, micro 4/3, and so forth were introduced.]

Last edited by maxfield_photo; 08-23-2013 at 07:57 AM.
08-23-2013, 08:14 AM   #3
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The 50mm lens became the normal lens on 135 because Oskar Barnack needed a lens that could produce a sharp 24x36 negative and that didn't create distortion of perspective. There was an existing 50mm design that produced satisfactory results.

QuoteQuote:
For still photography, a lens with a focal length about equal to the diagonal size of the film or sensor format is considered to be a normal lens; its angle of view is similar to the angle subtended by a large-enough print viewed at a typical viewing distance equal to the print diagonal;[2] this angle of view is about 53 diagonally.
08-23-2013, 08:23 AM   #4
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Misleading article. The writer is conflating the image the camera captures, with the image you see through the viewfinder.

Most cameras do not have a big viewfinder because of cost. Same reasons that cheap entry-level Canons and Nikons don't even use pentaprisms, instead they use cheap pentamirrors. If you take out the cost factor, there isn't really any reason to stop at 100% magnification, you could make a much bigger viewfinder and that would almost certainly be better, ergonomically.

08-23-2013, 08:38 AM   #5
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QuoteOriginally posted by konraDarnok Quote
The Abnormal Lens | Japan Camera HunterJapan Camera Hunter

The above article implies that the normal perspective of the 50mm lens on SLR's is due to the pentaprism? Is this true? If so, then why don't APS-C cameras have a pentaprism calibrated to make 30mm or 35mm lenses have "normal" perspective through the viewfinder?
perspective is a function of camera to subject distance only. don't believe me, then take a shot of a subject with a 24mm, 50mm, and 100mm lens from exactly the same position, then crop in on the shorter FL lenses to get the same framing as the 100mm lens. look at the background, and the way lines converge etc. The perspective of all three images will be the same.

what gives perspective therefore is not related at all to the viewfinder, and in fact perspective is really only the visual appearance of the magnification ratio of near and far subjects.

If you change focal length and keep the magnification of a near subject the same by moving your position, this changes the magnification of the far subjects and changes the perspective.
08-23-2013, 08:51 AM   #6
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QuoteOriginally posted by maxfield_photo Quote
What causes "normal" perspective is having a lens on your camera that matches the diagonal of the image area, either film or digital sensor. With 35mm film that diagonal was 43mm (or close enough), which is why Pentax made the FA43 limited, to give a true "normal" perspective. With an APS-C sensor the diagonal is more like 29mm, so a 28mm~31mm prime is about as close as you'll get, or a zoom set to 29mm.

Field of view is different though, many times you'll hear that a "normal" lens "represents what you would see with your eyes". That's only true in the sense of perspective, but since we have two eyes, and their "font elements", if you will, are shaped differently than a camera lens, our field of view is much greater. It varies a bit from person to person, but it's close to 180 degrees if you use both eyes.

[edit: I believe that when it comes to viewfinder magnification a 50mm lens is used, presumably because it was close-enough-to-normal on film, to calculate how large the image appears in the VF. Surprisingly, I don't think they changed that when smaller formats like APS-C, micro 4/3, and so forth were introduced.]


This isn't true. I have a 43mm limited, and it definitely expands the perspective from normal vision like a wide angle lens does. It's slightly wide angle. Things in the viewfinder appear slightly further away with the 43mm, whereas, if you mount a 50mm and look though with your left eye, the perspective will be the exact same as your right eye on either APS-C or 135 SLRs.

The guy in the article says this is due to how the pentaprism is designed.

This is the phenomenon I'm talking about, not field of view.
08-23-2013, 08:57 AM   #7
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QuoteOriginally posted by Lowell Goudge Quote
perspective is a function of camera to subject distance only. don't believe me, then take a shot of a subject with a 24mm, 50mm, and 100mm lens from exactly the same position, then crop in on the shorter FL lenses to get the same framing as the 100mm lens. look at the background, and the way lines converge etc. The perspective of all three images will be the same.

what gives perspective therefore is not related at all to the viewfinder, and in fact perspective is really only the visual appearance of the magnification ratio of near and far subjects.

If you change focal length and keep the magnification of a near subject the same by moving your position, this changes the magnification of the far subjects and changes the perspective.


I know that, and it's not what I'm talking about. A 50mm lens, thought the viewfinder has the same perspective as what your eye sees. Why?
08-23-2013, 09:27 AM   #8
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50mm doesn't really give a perfectly "normal" perspective. The reason for choosing 50mm as "normal" is because it's good enough as a normal, and is easy to design with large apertures (on SLR), and cheap to produce.

On early SLR it was usually 55-58mm that was used as normal, but that is probably because they are even easier to produce.

08-23-2013, 09:42 AM   #9
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QuoteOriginally posted by konraDarnok Quote
This isn't true. I have a 43mm limited, and it definitely expands the perspective from normal vision like a wide angle lens does. It's slightly wide angle. Things in the viewfinder appear slightly further away with the 43mm, whereas, if you mount a 50mm and look though with your left eye, the perspective will be the exact same as your right eye on either APS-C or 135 SLRs.

The guy in the article says this is due to how the pentaprism is designed.

This is the phenomenon I'm talking about, not field of view.
This has probably more to do with the magnification used on the OVF, but magnification is not the same on all cameras.

What cameras are you comparing? Could it be some of the largest OVF on MF SLR with a pentaprism DSLR?
08-23-2013, 09:49 AM   #10
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QuoteOriginally posted by konraDarnok Quote
What causes "normal perspective?"
Within wide limits there is no objective "norm" only subjective idiosyncratic visual perception.
08-23-2013, 10:10 AM   #11
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QuoteOriginally posted by Fogel70 Quote
This has probably more to do with the magnification used on the OVF, but magnification is not the same on all cameras.

What cameras are you comparing? Could it be some of the largest OVF on MF SLR with a pentaprism DSLR?



Well for example, a K1000 and a K5, thru the viewfinder, have the same perspective, or nearly so. The K1000 however has a wider field of view for a given focal length.
08-23-2013, 10:14 AM   #12
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I don't believe for a second that the design of the pentaprism has anything to do with perspective or anything else. Even more so, the concept of a "calibrated pentaprism" in this context is pure fantasy.
The pentaprism is a completely neutral optical element because all it's surfaces are flat. It only directs the light from the lens via the mirror to the viewfinder. It has exactly the same function as a bunch of mirrors (pentamirror).

Last edited by Kobayashi.K; 08-23-2013 at 10:17 AM. Reason: Typo
08-23-2013, 10:22 AM   #13
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QuoteOriginally posted by konraDarnok Quote
Well for example, a K1000 and a K5, thru the viewfinder, have the same perspective, or nearly so. The K1000 however has a wider field of view for a given focal length.
K5 has 0.62x and K1000 has 0.88x, so they are very close in magnification. 0.62 x 1.5 = 0.93x
If you instead used a AF SLR with 0.7x magnification you would have to use longer focal length to show the same (~70mm).
08-23-2013, 10:36 AM   #14
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QuoteOriginally posted by Fogel70 Quote
K5 has 0.62x and K1000 has 0.88x, so they are very close in magnification. 0.62 x 1.5 = 0.93x
If you instead used a AF SLR with 0.7x magnification you would have to use longer focal length to show the same (~70mm).
can't say I'm really following you here. But I did find this article:

Understanding Viewfinders

How does the magnification affect perspective?

If the perspective can change depending on the viewfinder, then why don't we have viewfinders on APS-C cameras that create normal perspective on 30mm primes? (the rough diagonal of APS-C format) Seems to me that'd be great.

So if I understand this correctly, a viewfinder would need 1.6x magnification (relative to the 50mm standard) for a 30mm prime to look normal? Assuming of course a 90%+ field of view. so it doesn't crop the image (which is what I assume those magnifying eyepieces do)

Last edited by konraDarnok; 08-23-2013 at 10:44 AM.
08-23-2013, 11:31 AM   #15
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QuoteOriginally posted by konraDarnok Quote
can't say I'm really following you here. But I did find this article:

Understanding Viewfinders

How does the magnification affect perspective?

If the perspective can change depending on the viewfinder, then why don't we have viewfinders on APS-C cameras that create normal perspective on 30mm primes? (the rough diagonal of APS-C format) Seems to me that'd be great.

So if I understand this correctly, a viewfinder would need 1.6x magnification (relative to the 50mm standard) for a 30mm prime to look normal? Assuming of course a 90%+ field of view. so it doesn't crop the image (which is what I assume those magnifying eyepieces do)
With lower magnification things in the viewfinder will look further away, but it's not the perspective that changes with magnification, only magnification. Perspective only changes with the distance to the subject.

The problem in using higher magnification on APS-C is that the OVF get less light than a on FF, so the APS-C OVF might get too dark.
It's one reason why AF DSLR has lower magnification, as on those cameras often slow zoom lenses was used, but also that ~30% of the light from the lens is sent to the AF-sensor.
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