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01-13-2013, 12:50 AM   #31
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QuoteOriginally posted by Lowell Goudge Quote
Cannikon I think what Venturi means is that when you move closer to the subject, to get the same subject size when you change focal length, it increases the perspective distortion.
Correct, and I was agreeing with Baro-nite's answer. Perhaps I should have explicitly done so. I simply added the reference to Wiki with photo examples to flesh his answer out.

However, I think it is worth clarifying Cannikin's rebuttal because he's throwing around perspective and focal length without addressing the OP's question which is in regard to camera format.

Focal length does not affect perspective, but it has everything to do with distortion in the context of this discussion.
Perspective, in the scientific sense, has little to do with portraiture more often than not.
Controlling distortion has everything to do with portraiture, and generally speaking the flatter the image the better when it comes to portrait work.

The closer you are to your subject, the shorter the focal length you will need to frame your subject. The shorter the focal length you use, the greater the distortion.

This is the key to the whole mess: Larger formats afford you longer focal lengths at shorter distances, giving you less (perspective) distortion in the frame.

A 50mm lens at 5ft to subject produces a field of view at the subject distance of:
2'10" on APS-C (head shot)
4'4" on 35mm (torso)
7' on 645 (full-body)

To get the same 7ft field of view as the 50mm on the 645 @ 5ft, you would need to use:
31mm on 35mm
20mm on APS-C *

The only way to get the same subject field of view and the same level distortion as on 645 here is with a 50mm lens and changing perspective by moving back to:
8ft on 35mm.
12'6" on APS-C.

* Unless you are using a very good rectilinear lens, a 20mm lens on APS-C will have noticeable distortion of the background. If you move in closer to get a head shot, distortion of facial features will become significant.

01-13-2013, 02:04 AM   #32
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A portrait lens is any lens that you use to make portraits.

I realize that this particular discussion is about using short telephotos for making what I call "mug shots".

I also realize that use of the term "portrait lens" is shorthand for that approach.

However, I wish that just once in a while people would get out of that mindset. There are other ways to make effective portraits.

The mug shot thing wears a bit thin after a while. I wish there was a "smilie" that showed eyes rolled upward in boredom.

( I rarely read portrait lens threads, as they all say the same things. Was just waiting for my scanner to warm up and made the mistake of looking at this one!)

How about a thread on portraiture with fisheyes, just for a breath of fresh air?
01-13-2013, 02:41 AM   #33
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There seem to be a few misconceptions here: perspective is a function of subject distance only. An object at twice the distance is always going to look half the size, whatever your focal length or sensor size. For portraits, you just have to choose the best subject distance for a nice portayal (not too flat, whilst no features are too exaggerated); you then choose an appropriate focal length for your framing.

Working backwards, the popular 135 camera portrait lens was from 70mm to 100mm. For a head/shoulders shot this means a subject distance of, say, 2-3 feet. If you're happy with this, you then choose the focal length which results in your required framing. For a head/torso shot, you may end up with something like a 28mm lens on an APS-C camera, or for tighter framing a 50mm lens.

Or you may prefer to operate at a greater distance (realising this will result in flatter features). Again, just choose the focal length for the required framing.
01-13-2013, 09:04 AM   #34
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QuoteOriginally posted by Venturi Quote

The closer you are to your subject, the shorter the focal length you will need to frame your subject. The shorter the focal length you use, the greater the distortion.
The distortion is a function of camera to subject distance, not lens focal length.

01-13-2013, 09:47 AM   #35
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Distortion at 150mm ( 9mm FL) and 500mm (27mm FL) Using the Kodak Ekton on M43
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01-13-2013, 05:29 PM   #36
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QuoteOriginally posted by Venturi Quote
Focal length does not affect perspective, but it has everything to do with distortion in the context of this discussion.
Perspective, in the scientific sense, has little to do with portraiture more often than not.
Controlling distortion has everything to do with portraiture, and generally speaking the flatter the image the better when it comes to portrait work.

The closer you are to your subject, the shorter the focal length you will need to frame your subject. The shorter the focal length you use, the greater the distortion.

This is the key to the whole mess: Larger formats afford you longer focal lengths at shorter distances, giving you less (perspective) distortion in the frame.

...


* Unless you are using a very good rectilinear lens, a 20mm lens on APS-C will have noticeable distortion of the background.
I don't think you quite understand what "perspective" is, or how "distortion" works.

Perspective is the apparent size and position of objects in relation to each other when viewed from a given spot. Closer objects appear bigger, further objects appear smaller, relative size scales directly with the ratio of their distances. Here's the wiki article if you want to read a more in-depth description: Perspective (visual) (notice the phrase "focal length" does not appear anywhere in that article, or in Distortion (optics) for that matter).

Now there are three phenomena that people often refer to as "distortion", but have nothing to do with each other:

1. Perspective distortion: Ratio of object sizes does not appear the way we expect to see it from a normal viewing distance (e.g. the nose looks too big and the eyes/ears look too small). "Distortion" is a misnomer because nothing is distorted, but rather simple geometry. The scene viewed from that spot is what it is, and the ratio of sizes is determined solely by distance.

The "flattening" and "exaggerating" effects of perspective "distortion" work like this:

- A and B are the same size, B is 1 ft behind A.
- Camera is 1 ft in front of A and thus 2 ft from B.
- In this spot B looks 50% the size of A (1/2).
- The camera moves back 8 ft, A and B have not moved.
- The camera is now 9 ft from A and 10 ft from B.
- B now looks 90% the size of A (9/10).

As you can see, the only variable is the camera's position, nothing else. No focal length, no APS-C/FF/whatever format, just distance. Stand in the spots and look with your own eyes to see the truth of this.

2. Rectilinear distortion: Straight lines are not straight (barrel or pincushion distortion). This is merely flaws in the given lens' optical design. Extreme wide-angle or extreme telephoto may be harder to correct for distortion in optical design, but the relationship is not directly related to focal length (e.g. the 21 ltd has more barrel distortion than the 15 ltd or DA 14).

3. Wide-angle distortion: Objects near the edge of the frame appear stretched. This is a fundamental geometric property of gnomonic ("rectilinear") projection. The wider the angle of view, the more visual area objects on the edge take up, until you approach 180 degrees (impossible to reach) where they become infinitely large. This is directly related to angle of view, and only indirectly to focal length. The effect will be the same on any format/focal length as long as the angle of view is the same.

Everything points to the same conclusion: a shot with a 50mm lens on APS-C will look more or less identical to one shot with a 75mm lens on FF from the same spot, besides DOF and rendering characteristics unique to the lenses' designs.

A quote from the same Perspective distortion (photography) article you cited:
QuoteQuote:
Below, a series of three photos shows the same scene shot from the same position with three different lenses: a normal lens, a wide-angle lens, and a telephoto lens. Notice that because the proportions in the image don't change with the angle of view, if the photos made with the wide-angle lens and the normal lens were cropped to contain the same scene as the photo made with the telephoto lens, then the image content would be exactly the same.

Last edited by Cannikin; 01-13-2013 at 07:10 PM.
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