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12-20-2014, 09:13 PM   #1
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Understanding perspective vs lens size vs sensor size (lens mechanics?)

Ok here is what I think I understand. If I'm shooting portraits with the da 50mm 1.8 on a K50 (should be 77mm full frame equivalent) and another guy comes up with the new full frame sensor KFF1 (or whatever) and has a 77 limited 1.8. He decides to shoot the same portraits as me...

Assuming the same lens sharpness etc etc etc...

Would the perspective be the same? My question stems from the fact that I am looking at a K50 and understand the crop factor makes the lens behave like a short telephoto on a crop sensor. I also have read that for portraiture photography, a, 85mm lens (for example) is good because the perspective makes a person's face look better than a wide angle lens. It seems like although the captured area would be the same on both of these shots, the perspective would still be preferable on the full frame 77mm as that would be a tighter perspective... Am I right or am I not understanding something?
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12-20-2014, 09:25 PM   #2
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Yes, the perspective would be the same, though you'd get a shallower DOF with the 77mm on a FF compared to the 50mm on APS-C. You should still be able to get good results with either format.

The one area in which FF outshines APS-C is for wide-angle photography, since longer focal lengths can be used, and thus won't be subject to as much optical complexity/geometric distortion (generally speaking). On the other hand, APS-C has an edge for tele photography, since shorter and lighter lenses deliver the same FoV.

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12-20-2014, 09:53 PM   #3
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Traditionally on a full frame 35mm, a 135mm is considered a portrait lens. For a crop C sensor A 90mm would be equivalent. Any prime lens close to that with a sufficient aperture size should do find. you can shoot portraits with a 50 mm lens, but you will not get the best prospective. to calculate the lens focal length equivalent, multiply the actual lens focal length by 1.5 . To maintain an aspect ratio between full frame and crop "C" sensor, Divide the full frame lens focal length by 1.5 .

Note: if you take a 4 x 6 card and hold it approximately 12 inches from your eyes that will give you the basics you that you get from a 50 mm lens on a full frame camera. On the crop "C" sensor, move the card out to 18 inches from your eyes. If you divide the card in half, both horizontally and vertically, you'll see the view of a 100 mm lens. Every time you divide the card in half, you will see the view of double the previous focal length.
12-21-2014, 01:27 AM   #4
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QuoteOriginally posted by promacjoe Quote
Traditionally on a full frame 35mm, a 135mm is considered a portrait lens. For a crop C sensor A 90mm would be equivalent. Any prime lens close to that with a sufficient aperture size should do find. you can shoot portraits with a 50 mm lens, but you will not get the best prospective. to calculate the lens focal length equivalent, multiply the actual lens focal length by 1.5 . To maintain an aspect ratio between full frame and crop "C" sensor, Divide the full frame lens focal length by 1.5 .

Note: if you take a 4 x 6 card and hold it approximately 12 inches from your eyes that will give you the basics you that you get from a 50 mm lens on a full frame camera. On the crop "C" sensor, move the card out to 18 inches from your eyes. If you divide the card in half, both horizontally and vertically, you'll see the view of a 100 mm lens. Every time you divide the card in half, you will see the view of double the previous focal length.
Not sure where you get 135 as the standard for portraits in film days? The 85 was considered king of portraits when I was learning photography in the late 70s. Yes, any short telephoto could he used and many liked 100 and even 105mm for that role, but the high speed 85 was, and still is, viewed as The portrait lens.

12-21-2014, 03:13 AM   #5
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4 pics which help to understand perspective and DOF (DA x FA relation).











12-21-2014, 05:51 AM   #6
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QuoteOriginally posted by zzeitg Quote
4 pics which help to understand perspective and DOF (DA x FA relation).
Nice pics.
However in No.1 I'd add that the DoF will be thinner in DX when enlarged (i.e. printed or viewed on same monitor) to match the FX one.
Then again, the two cars in No.3 look exactly the same on paper (I'm talking shape, not size), but won't in real life (different distance = different P.o.V. = different perspective).
12-21-2014, 06:47 AM   #7
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Perspective is due to subject distance, not focal length.

"Note that linear perspective changes are caused by distance, not by the lens per se – two shots of the same scene from the same distance will exhibit identical perspective geometry, regardless of lens used. However, since wide-angle lenses have a wider field of view, they are generally used from closer, while telephoto lenses have a narrower field of view and are generally used from farther away."

http://en.wikipedia.org/wiki/Perspective_distortion_(photography)

The reason 85-105mm lenses are favoured for portraiture, is because the distance needed for "head & shoulders" framing provides a slightly telephoto compression effect that is considered flattering. Note that using a crop camera requires adjustment of the focal length to provide the same perspective and framing. APS-C classic portrait lenses would be in the range of 55-70mm.

Some people define a wider range for portrait lenses, e.g. 70-135mm FF equivalent. I find my FA 50mm (75mm FF) uncomfortably close for portraits, but not because of perspective. I'm uncomfortable because I feel like I'm crowding the subject.

Last edited by audiobomber; 12-21-2014 at 07:03 AM.
12-21-2014, 06:54 AM   #8
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Quick question...... what if we used a 30mm on an aps-c and a 20mm on a FF. Would the perspective and distortion be "equivalent"

Wide angles do distort the image......


Last edited by devouges; 12-21-2014 at 08:13 AM. Reason: had the 20 and 30 interchanged
12-21-2014, 08:23 AM   #9
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QuoteOriginally posted by Docrwm Quote
Not sure where you get 135 as the standard for portraits in film days? The 85 was considered king of portraits when I was learning photography in the late 70s. Yes, any short telephoto could he used and many liked 100 and even 105mm for that role, but the high speed 85 was, and still is, viewed as The portrait lens.

This is what I remember as well. What makes a mild telephoto good as a portrait lens is the fact that you can move back from the subject so that the field curvature is less and the perspective is somewhat compressed or "foreshortened." Facial features in particular are rendered in a more pleasing way. As I recall from my Spotmatic and later film camera days, 80 - 100 mm was kind of the sweet spot. Of course for a crop sensor camera those must be adjusted downward by the crop ratio (53.3 - 66.7mm at 1.5 crop ratio.)

Last edited by dakight; 12-21-2014 at 08:42 AM.
12-21-2014, 08:31 AM   #10
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QuoteOriginally posted by devouges Quote
Quick question...... what if we used a 30mm on an aps-c and a 20mm on a FF. Would the perspective and distortion be "equivalent"

Wide angles do distort the image......
If distance to subject is the same and those are both rectilinear lenses, then yes, "distortion" would be equivalent.
12-21-2014, 08:40 AM   #11
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Crop does not make for a different focal length, a wider lens pushes things back and a longer compresses regardless
Of the crop factor.
The reason for the around 80mm for portrait is to keep the proper facial proportions not distance to the subject.
Keep the same framing and try a 28, 50 and 80mm
12-21-2014, 09:15 AM   #12
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QuoteOriginally posted by bobpur Quote
Crop does not make for a different focal length, a wider lens pushes things back and a longer compresses regardless
Of the crop factor.
The reason for the around 80mm for portrait is to keep the proper facial proportions not distance to the subject.
Keep the same framing and try a 28, 50 and 80mm
In order to keep the same framing, distance will vary, hence proportions.
A 28, a 50 and a 80mm from the same distance (face framed with the 80mm, the others cropped in Photoshop to match) will all yield the same picture, minus the MP difference...

Last edited by LensBeginner; 12-21-2014 at 09:21 AM.
12-21-2014, 09:23 AM   #13
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QuoteOriginally posted by bobpur Quote
Crop does not make for a different focal length, a wider lens pushes things back and a longer compresses regardless
Of the crop factor.
It's your distance to the subject that matters for 'perspective distortion'. Try taking a head shot with the same framing with a 20mm on a P&S camera with a tiny sensor (or a Pentax Q) and then with a 20mm on a FF or aps-c.
12-21-2014, 10:12 AM - 1 Like   #14
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Perspective is based on the position of the front element of the lens relative to the subject. It is the same regardless of crop or field of view and is the same for the human eye as for a camera. As such here are a few bullet points
  • Crop factor (I hate that term) has no affect on perspective. All it means is that the subject is framed with a different field of view.
  • There is no such thing as wide angle perspective
  • There is no such thing as telephoto perspective
  • There is no such thing as telephoto "compression". A wide angle shot taken at the same distance and cropped to the same field of view will have the same "flatness" or "compression" as a photo taken with a longer lens.
  • Wide angle "distortion" for portraits is the natural result of being close to the subject.* That which is nearer (noses, for example) are closer and appear larger than those that are further away (ears, for example). This is not magic and can be demonstrated with a longer focal length lens on a stitched image.


Steve

* There is a type of true distortion that is present with rectilinear wide angle lenses. It called volume anamorphosis and is a side-affect of the straightening of normal perspective with these lenses resulting in a objects at the center appearing natural while those towards the margins are "stretched". There is a good example on the PTLens Web site: http://epaperpress.com/ptlens/distortion.html

Last edited by stevebrot; 12-21-2014 at 10:23 AM.
12-22-2014, 05:36 PM   #15
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It is useful for me,thanks.
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