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11-17-2008, 02:52 PM   #151
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I'm afraid that I beat around the bush a little bit in my last post. You are not correct about the direct influence of focal length on perspective distortion. If that were true, imagine the amount of perspective distortion you could get with the little point and shoot cameras with an 8.5mm lens; but you can't do that because their sensors are not big enough to widen the field of view that far beyond "normal" with that focal length lens.

If you shoot a scene with a 10mm lens on an APS-C camera, you need to shoot with a 15mm lens on a 35mm film or "full frame" sensor camera to get the same amount of perspective distortion from the same spot at the same angle. When was the last time you shot 35mm film with a 15mm lens? If you did, you would realize that you can get all the same effects as with your 10mm on APS-C. (If you want some really extreme distortion effects with a rectilinear lens try the Sigma 12-24mm lens on a full frame camera).

If you research this on the Internet, you will find more than one source that will tell you that perspective distortion is only changed by relative distances of objects in a photo to the camera (that is, the number of times further away one object is than the other from the camera), and not focal length. They will say that focal length only changes perspective distortion to the extent that it makes you move to make the same object fill the frame at a different distance and changes the relative distance it is from the camera compared to other objects in the frame. So, if you compensate for a shorter (wider) focal length by cropping the sensor rather than by moving closer to the subject, perspective distortion remains unchanged.

While this explanation is correct as far as it goes, it leaves out the fact that what makes a perspective appear "distorted" is how skewed it is from our normal perspective. That is why shots taken with a "normal" focal length (whatever that happens to be for your camera) should rarely, if ever, appear distorted (when viewed from the expected distance, anyway).

11-17-2008, 10:10 PM   #152
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QuoteOriginally posted by CFWhitman Quote
If you research ..., you will find more than one source that will tell you that perspective distortion is only changed by relative distances of objects in a photo to the camera (that is, the number of times further away one object is than the other from the camera), and not focal length. They will say that focal length only changes perspective distortion to the extent that it makes you move to make the same object fill the frame at a different distance and changes the relative distance it is from the camera compared to other objects in the frame. So, if you compensate for a shorter (wider) focal length by cropping the photo rather than by moving closer to the subject, perspective distortion remains unchanged.
Thank you for that (with a minor edit). This is the first I've read this thread and was about to jump through my monitor until I read this paragraph.

Additionally, except for a fisheye, lens manufacturers go to great lengths in their optical formulas to minimize such distortions. You only really need to come in close when you're looking for a ridiculously narrow DOF.

So knowing this, the fast 50 offers more artistic (DOF) and optical (f/1.4) versatility as a portrait lens than a longer focal length, and does it with no perceptible drawbacks.
11-17-2008, 11:24 PM   #153
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I may not be referring to what you call "perpective distortion" but here are photos taken at different focal lengths but at the same FOV (c/o thepirate):

90mm


58mm


31mm


Is this the right test for the "perspective distortion" we're talking about here?

Edit: Apparently it's about the working distance and magnification, since the wider focal lengths need you to get closer to the subject.

Last edited by soccerjoe5; 11-18-2008 at 12:16 AM.
11-17-2008, 11:32 PM   #154
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So as you can see, while the FOV is the same to cover the chessmen, the 90mm has more of that flattening look which is nice for portraits. The wider focal lengths have more of that depth where things in front look nearer/bigger and things further back look further/smaller. As to portraits, things like the nose, cheeks, forehead, and chin will look a bit bigger.

That's why I mentioned that I think the 10mm on the APS-C will always have a bit more of that deep feeling that the equivalent on a FF/35mm camera which is 15mm.

I hope we're talking about the same thing here, that's what I was referring to as "perspective distortion.


Last edited by soccerjoe5; 11-17-2008 at 11:57 PM.
11-18-2008, 12:11 AM   #155
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QuoteOriginally posted by CFWhitman Quote
They will say that focal length only changes perspective distortion to the extent that it makes you move to make the same object fill the frame at a different distance and changes the relative distance it is from the camera compared to other objects in the frame. So, if you compensate for a shorter (wider) focal length by cropping the sensor rather than by moving closer to the subject, perspective distortion remains unchanged.
I think you're right here

So basically the 50 will act like the 85 if they're taken at the same distance and the 50 is cropped to the FOV of the 85. Am I right?
11-18-2008, 10:15 AM   #156
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QuoteOriginally posted by soccerjoe5 Quote
I think you're right here

So basically the 50 will act like the 85 if they're taken at the same distance and the 50 is cropped to the FOV of the 85. Am I right?
Yes, you're exactly right, and that is what you're doing when you shoot at 50mm with an APS-C sensor compared to 75mm with a 35mm sized sensor. You are cropping the 50mm image to the same size as the 85mm image by "cropping" the sensor from 35mm size to APS-C size.

The example you found with the chess board is a perfect illustration of the type of perspective distortion that I am talking about, although you can find less subtle examples where someone did the same experiment with an even wider angle lens at the one end and an even longer lens at the other.

Another issue in explaining this is the term "field of view." I used it in my first post about perspective distortion to mean one thing, while you used it in your next to last post to mean a different thing. This leads to misunderstanding, so I will refer to what I was talking about as "angle of coverage" instead (i.e., how wide of an angle is covered by the camera setup in question, which is determined by the focal length to sensor size ratio), and I will use "field of view" to refer to what you meant: roughly how much of the subject and it's surroundings are visible (I say "roughly" because this can't remain perfectly constant at different angles of coverage unless you are looking down a tunnel or something like that).

I would like to clarify my thoughts on this. I don't really agree with the standard explanation you find on the Internet that I referred to in my last post. It is not that it is exactly incorrect technically (although it sort of is), but it is looking at it the wrong way. I referred to it because I thought that the way they explain it would be a verifiable way for you to see that taking a picture with two different sized sensors from the same place with equivalent focal lengths would give you exactly the same perspective distortion. However, I think that the way that they explain it and look at it opens the way for confusion, and that people could understand it better by looking at it a different way.

My problem with that explanation is that it claims that changing the relative distance of objects to the camera changes the perspective distortion. This is not really true. Changing the relative distances of objects changes the perspective, but not the distortion. Unless you compensate for changing these relative distances by also changing focal lengths or film/sensor size you do not change the distortion. To illustrate I will refer to camera effects used in movies. There are two effects that are really just the opposite of each other: the Vertigo effect (because it was first used in that movie), and the reverse effect (which I don't know of distinctly different name for). These effects both make use of perspective distortion in opposite ways.

The Vertigo effect is accomplished by moving a camera forward into the picture (in Vertigo, downward from a height while looking down) while simultaneously zooming out to keep the field of view the same. This uses perspective distortion to create the effect of making farther away objects appear to grow even farther away while you are looking at them. According to the standard explanation of perspective distortion, however, this effect should work even if you did not change focal lengths by just moving the camera forward (perhaps with a near object in the center of the frame so that it would remain visible, although in reality this changes nothing). This is obviously not the case. If you did that, it would be clear that you were merely moving the camera forward, and perspective distortion would not change. You need to change the focal length of the lens to get the desired effect (a change in perspective distortion). *Edit* I had been thinking that the Jaws shot was the reverse of the Vertigo effect, but I realized it is actually the exact same effect by a different name. To see the reverse effect you can view the scene where Frodo and his friends first encounter a Black Rider on the road in The Lord of the Rings: the Fellowship of the Ring, or apparently in a diner scene in Goodfellas (although I have never actually seen that movie).

That is why in reality people who say that changing focal lengths changes perspective distortion are closer to correct, in a way, than the standard explanation is. I say this because if you change the focal length of your lens and nothing else, you change the perspective distortion (although it may not always be incredibly apparent), while if you change the relative distances of objects and change nothing else, you will not change perspective distortion (you will only change perspective).

What actually changes perspective distortion, however, is the angle of coverage in the image when it is viewed at the expected size from the expected distance and how far it is from the angle of coverage our eyes would see in real life. The angle of coverage is dependent on the ratio between focal length and film/sensor size and not directly related to either one by itself. Changing the relative distance of visible objects (the perspective) only makes existing perspective distortion more or less apparent; it does not change it.

Last edited by CFWhitman; 11-20-2008 at 02:32 PM.
11-18-2008, 11:26 AM   #157
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QuoteOriginally posted by TourDeForce Quote
Thank you for that (with a minor edit). This is the first I've read this thread and was about to jump through my monitor until I read this paragraph.

Additionally, except for a fisheye, lens manufacturers go to great lengths in their optical formulas to minimize such distortions. You only really need to come in close when you're looking for a ridiculously narrow DOF.

So knowing this, the fast 50 offers more artistic (DOF) and optical (f/1.4) versatility as a portrait lens than a longer focal length, and does it with no perceptible drawbacks.
This is exactly why I don't like the standard explanation of perspective distortion you find on the Internet these days. It causes confusion about what perspective distortion is. Lens manufacturers go to great lengths to minimize optical distortions in their lenses, not perspective distortion. It is neither possible nor desirable to eliminate perspective distortion when you vary a lens from the "normal" focal length (well, there may be some cases where it could be desirable, but it is still not possible). Perspective distortion is part of what makes a telephoto lens a telephoto lens and a wide angle lens a wide angle lens.

Class A does have a valid point. I was just saying that I think that an "85mm (or 55mm or 58mm on APS-C) and not one mm less" attitude is taking it a bit too far. Using a 75mm (or 50mm on APS-C) lens for portraits is fine as long as you know what the disadvantages (and advantages) are, and you can get good results. If there were no advantage to the slightly longer focal length, Pentax would not bother to come out with the 55mm lens.

For what I think is a better explanation of what causes perspective distortion, see my above post. I don't know that it's perfect, but I think it will help you to understand it better.
09-19-2009, 01:39 PM   #158
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Back to pictures...

























09-19-2009, 01:54 PM   #159
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And the final one, a toast to this lens, because it almost never leaves my camera.

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