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12-05-2008, 01:54 PM   #166
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QuoteOriginally posted by Tokina Quote
They are not affected by focal length. They are affected by perspective. If you compare 50mm on 1.5x crop and 75mm on full frame, you obviously take photos from the same position and they will be indentical.
sorry. I'm one to re-edit my posts often. I added to my original point (many times actually). But that point I made...? I don't know anymore. :ugh: I'm think I'm confusing how "cropping" an infinitely small point with an infinite focal length.

And you are 100% correct. My earlier point is only made correctly if I use & compare 50mm on both film and digital in relation to (in my thoughts) maintaining the near-field subject size.

Not only from this but mainly my inability to clearly convey what I mean, I'll shut-up now.


Last edited by m8o; 12-05-2008 at 02:06 PM.
12-05-2008, 02:07 PM   #167
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QuoteOriginally posted by Tokina Quote
What if I say that perspective distortion of photos is only caused by the viewing distance and the size/shape of the surface they are printed on? Would that be correct too? Wide angle photos should be viewed from closer distance to avoid perspective distortion. And telephotos from further away. I think you assumed standard viewing distance.
Well, yes, these points have been covered in earlier posts in the thread. I never said that focal length alone is the cause of perspective distortion. I merely pointed out that focal length changes will affect perspective distortion if all other factors remain the same. However, any alteration in the perceived angle of view will affect it. Whereas the only effect that distance to subject has on perspective distortion is to make the effect more or less obvious. None of this changes the underlying cause of perspective distortion that I just pointed out.

It has also been mentioned earlier that the extent to which viewing distance from the print affects perspective distortion is debatable (not whether it affects it, just the extent to which it does). It has been observed that up to a fairly extreme degree, changing the viewing distance to a print does not seem to affect our perception of its perspective, distorted or not (some say just that the effects are very subtle). This observation has been called Zeeman's Paradox. The idea is that our brains tend to recognize the intended point of view of any two dimensional image, and compensate for any changes from this point of view. This idea is part of the reason for the concept of standard or normal viewing distance. Where exactly the "normal" viewing distance is, and how much it is the result of some natural inclination or the result of previous experience with other images, are points open to debate.

Last edited by CFWhitman; 12-05-2008 at 02:22 PM.
12-05-2008, 02:09 PM   #168
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Well that about clears it up for me! Thanks guys!
12-05-2008, 02:18 PM   #169
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QuoteOriginally posted by drewdlephone Quote
For those of you who have had your Fifty since you shot film, how have you adjusted to this change? Has the use of your lens changed?
I adjusted by buying an FA 35/2 (and a DA 40 for travel). I rarely use the 50 - the focal length just doesn't seem to work very often for what I do, and my copy (which I bought probably 9 years ago) is awfully soft until about f/2.8 (new ones may be better ??).

Jer

12-05-2008, 03:05 PM   #170
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QuoteOriginally posted by m8o Quote
But my point in "is a 50mm a 75mm on digital" is about -where- you would stand if you shot 35mm film, vs -where- you would have to stand when you use the lens on a 1.5x cropped digital camera, to fill the frame the same amount with your subject. Imagine that point WITHOUT cropping after the fact to get the final images the same size in the frame. [!] ....And then consider how that changes the background.
Are you talking about using the same 75mm lens on both a film and digital camera? If so then yes, I agree, you'd have to change positions to get the same framing of your subject, and therefore the perspective between the subject and any foreground or background objects would be different.

But if you're talking about using a 75mm lens on a full frame camera compared to a 50mm lens on a digital camera, then no - you don't have to change position - as evidenced by my sample shots above.

Edit: OK - I think from your latest post you're saying that you agree with all this. So then this means (I think) that you agree with the OP's original statement, "a fast fifty is really a fast 75mm", at least as far as it applies to the perspective and composition of a shot when using 50mm on digital vs. 75mm on full frame.

Last edited by Sean Nelson; 12-05-2008 at 03:16 PM.
12-05-2008, 05:57 PM   #171
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I think I got it now:

Perspective is camera's point in space during the exposure.
Perspective distortion is observed when viewing a picture from different distances. Imagine yourself looking through a window and then walking backwards. Your angle of view gets narrower but there is no distortion. Then try the same with a photograph. Angle of view stays the same, only the picture gets smaller and more (or less) distorted. Picture itself has no distortion; it's you viewing it from wrong distance. Due to limitations of human vision, extreme wideangle or telephoto shots can't be viewed from the distance that corresponds the angle of view of the photograph. That makes perspective distortion unavoidable.

Normal lens means a lens that has focal length the same as the diagonal of the film/sensor it's designed for. This is the most simple of lens designs and has approximately the same field of view as human eye. Can someone explain why? I'm guessing there is some kind of symmetry and magnification is somehow normal. I can't quite figure it out...
12-05-2008, 07:24 PM   #172
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QuoteOriginally posted by Tokina Quote
Imagine yourself looking through a window and then walking backwards. Your angle of view gets narrower but there is no distortion. Then try the same with a photograph. Angle of view stays the same, only the picture gets smaller and more (or less) distorted. Picture itself has no distortion; it's you viewing it from wrong distance.
This doesn't seem like a good example to me, since (at least in my mind's eye) the frame of the picture still seems square no matter what distance I view it at. Perhaps this is "Zeeman's paradox".

The reason behind "Perspective Distortion" is simple - our eyes take in a wide view projected onto a spherical "sensor" (the retina). "Perspective Distortion" comes into play when you try to flatten that spherical image onto a flat sheet of paper. The only way to do it is to stretch or compress the image somewhere, hence the "distortion".

When the field of view is small, the difference between that portion of the "sphere" and a flat piece of paper is small, so there's little or no distortion, But when you start getting to fields of view in excess of, say oh, 40-50 degrees or so, you start getting into more significant warping of the spherical image to get it onto a flat piece of paper.

It's the same problem cartographers face when they try to make maps. Maps of small areas - no problem. Maps of continents or the whole world printed on flat paper - well something's got to give.

This is why focal length alone has nothing to do with "Perspective Distortion" - the real factor is field of view. Focal length is only relevant when you hold everything else constant (ie, sensor size), so that a change in focal length equates to a change in field of view.

So everyone repeat after me: Perspective = where you take the picture from. "Perspective Distortion" = field of view. That's it. Focal length only enters the equation when it causes you to change your shooting position or results in a different in field of view.
12-06-2008, 01:58 AM   #173
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QuoteOriginally posted by Sean Nelson Quote
This doesn't seem like a good example to me, since (at least in my mind's eye) the frame of the picture still seems square no matter what distance I view it at. Perhaps this is "Zeeman's paradox".

The reason behind "Perspective Distortion" is simple - our eyes take in a wide view projected onto a spherical "sensor" (the retina). "Perspective Distortion" comes into play when you try to flatten that spherical image onto a flat sheet of paper. The only way to do it is to stretch or compress the image somewhere, hence the "distortion".

When the field of view is small, the difference between that portion of the "sphere" and a flat piece of paper is small, so there's little or no distortion, But when you start getting to fields of view in excess of, say oh, 40-50 degrees or so, you start getting into more significant warping of the spherical image to get it onto a flat piece of paper.

It's the same problem cartographers face when they try to make maps. Maps of small areas - no problem. Maps of continents or the whole world printed on flat paper - well something's got to give.

This is why focal length alone has nothing to do with "Perspective Distortion" - the real factor is field of view. Focal length is only relevant when you hold everything else constant (ie, sensor size), so that a change in focal length equates to a change in field of view.

So everyone repeat after me: Perspective = where you take the picture from. "Perspective Distortion" = field of view. That's it. Focal length only enters the equation when it causes you to change your shooting position or results in a different in field of view.
I guess all that is true if the picture is captured and viewed with flat surface.
In my example every single spot of the picture must have the same viewing distance. That means spherical surface. If not, picture is distorted. And even when every spot has the same distance from viewers eye, the distance/picture size must match the angle of view of the picture or there will be perspective distortion. I don't think that perspective distortion is the same as angle of view. It's a subjective observation. Compare pictures from a corrected wideangle and a fisheye. The distortions with the fisheye lens is caused by the flat sensor. Normal wideangle lens has corrective lens elements that (intend to) eliminate those distortions. If we view flat pictures from "normal" distance, the wideangle shots will have too much flatness in them and the tele shots too much magnification. Pictures with normal lens are not distortion free either, but it's the best compromise for our eyes


Last edited by Tokina; 12-06-2008 at 02:07 AM.
12-06-2008, 08:46 AM   #174
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QuoteOriginally posted by Tokina Quote
the distance/picture size must match the angle of view of the picture or there will be perspective distortion. I don't think that perspective distortion is the same as angle of view.
Well, if I'm understanding the use of the term correctly, it's a factor of angle of view, print size, and viewing distance.
12-06-2008, 09:54 AM   #175
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QuoteOriginally posted by Tokina Quote
The distortions with the fisheye lens is caused by the flat sensor. Normal wideangle lens has corrective lens elements that (intend to) eliminate those distortions.
Ah, but you see the "correction" of a rectlinear wide angle lens keeps straight lines straight, but that's not really the way we view the world. For example, imagine taking a straight-on picture of a fence with a rectlinear wide angle lens that covers say, 60 degrees horizontally. The picture should show the top and bottom lines of the fence to be parallel. But in real life, if you turn left or right by 30 degrees you'll see the lines of the fence converging.

This is the difference between our spherical view of the world and trying to represent it on a flat piece of paper, and it's why a rectlinear projection onto a flat surface causes distortions at wide angles. The greater the field of view, the more distortion you get.
12-06-2008, 12:25 PM   #176
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QuoteOriginally posted by Tokina Quote
Normal lens means a lens that has focal length the same as the diagonal of the film/sensor it's designed for. This is the most simple of lens designs and has approximately the same field of view as human eye. Can someone explain why? I'm guessing there is some kind of symmetry and magnification is somehow normal. I can't quite figure it out...
This has been talked about a bit, but basically a normal lens is one that has the same ratio of focal length to sensor size as "normal" viewing distance is to print size. So the definition of normal lens depends on what you think normal viewing distance is. In general, it has been estimated that normal viewing distance is about equal to the diagonal of the image, so that would make a normal lens one with a focal length about equal to the diagonal of the sensor. However, if you feel that normal viewing distance is really a bit further away than that, then perhaps a normal lens would be of a focal length a bit longer than the diagonal of the image.

This stems from the fact that maintaining this ratio equality (focal length/sensor size = viewing distance/print size) is the way to avoid any kind of perspective distortion (at least theoretically) because the photo shot and viewed that way will look exactly like the actual scene viewed from the same point.

The complication to this idea is, as mentioned, Zeeman's Paradox, which can be interpreted to say that viewing a photo from another distance than "normal" will not change (at least up to a certain point) how we perceive its perspective. If you subscribe to that idea, then you can't always compensate for changes in focal length with changes in viewing distance because people unconsciously assume "normal" viewing distance and compensate accordingly (even when doing so makes the perspective appear incorrect).

Of course a lot of this is theoretical and somewhat subjective.

Last edited by CFWhitman; 12-06-2008 at 09:31 PM.
12-06-2008, 02:03 PM   #177
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I think the lot of you are reading a lot more into this than is called for. What does the viewing distance of the image have to do with whether the focal length of the lens is "normal" or otherwise? The print hasn't changed if you view it from one foot or 10 feet.
Most of this thread has devolved from the sublime to the absurd.
12-06-2008, 02:40 PM   #178
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Just get a fast fifty, use it, leave it on the camera for a week, you will wonder how you ever did without one.
12-06-2008, 02:43 PM   #179
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QuoteOriginally posted by Wheatfield Quote
I think the lot of you are reading a lot more into this than is called for. What does the viewing distance of the image have to do with whether the focal length of the lens is "normal" or otherwise? The print hasn't changed if you view it from one foot or 10 feet.
This depends on your definition. If you take it to be one of the classic definitions where a "normal" focal length is one that produces a field of view such that a "typical" print viewed from a "typical" distance appears to make the field of view of the scene depicted, then obviously viewing distance matters. Of course, this isn't the only possible definition of "normal", but it is at least that is not arbitrary. It is, however, pretty darned subjective.

Anyhow, frustrating as this discussion has been on so many levels, I have enjoyed it a lot, and learned a thing or two in the process.
12-07-2008, 01:11 PM   #180
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I have learned a lot from this thread and I'm sure it will help my photography. Thank you all!
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