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11-26-2008, 11:53 AM   #76
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QuoteOriginally posted by Sean Nelson Quote
It seams you are using a different meaning for the word "perspective" than others.

This is the effect of moving the camera:




This is the effect of zooming (ie, changing focal lengths) without moving the camera:




When the TV screen gets larger faster than the background it's reflecting, that's a change in perspective.

But when you just zoom in, the perspective does not change.
This perfectly illustrates the statement you quoted. When you move the camera, assuming you started with normal perspective, the perspective remains normal (it changes in a normal way). When you zoom in, the perspective is compressed, that is, the objects in the reflection appear to be closer to the television than they actually are (perspective changes in an abnormal way, i.e., gets distorted).

As I alluded to, you may have started out with extension distortion, in which case normal perspective was somewhere in the middle of the zoom.

You seem to be under the impression that changing the apparent relative sizes of objects an equal percentage has no effect on perspective. Since perspective is all about the apparent relative sizes of objects as compared to distance, and in real life the apparent sizes do not change equally when distance changes, this seems to be an odd definition of perspective.

In the context of whether focal length affects perspective distortion is the only place I seem to ever see this definition of perspective, and it always seems to go hand in hand with the notion that perspective distortion is influenced by camera to subject distance (and distance alone, usually), which is absolutely false.

If you use a wide-angle lens to make something that is one meter away appear to be two meters away, then something two meters away will appear to be four meters away. If you move directly away from the objects until the closer is 30 meters away and you make it appear 60 meters away, then the farther object will appear to be 62 meters away. Either way you have doubled the apparent distance between them (although it will be a lot more obvious at the closer distances).

Basically, when you make one thing in an image seem twice as far away as it actually is, then you make everything in the image seem twice as far away, doubling the apparent distances between them. If you make one thing seem half as far away, then you make everything seem half as far away, halving the apparent distances between them. That is what perspective distortion is.

11-26-2008, 04:12 PM   #77
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QuoteOriginally posted by CFWhitman Quote
Basically, when you make one thing in an image seem twice as far away as it actually is, then you make everything in the image seem twice as far away, doubling the apparent distances between them. If you make one thing seem half as far away, then you make everything seem half as far away, halving the apparent distances between them. That is what perspective distortion is.
Ah, you are talking about "perspective distortion" as described in this Wikepedia article:

Perspective distortion (photography - Wikipedia, the free encyclopedia)

...whereas we have been talking about perspective (period) as described here:

Working with Perspective, Subject Distance and Focal Length - Photo Tips @ Earthbound Light


Perspective distortion is demonstrated by this photo which changes the focal length AND the subject distance, maintaining the same apparent subject size (in this case, the middle of the TV screen):

11-26-2008, 08:58 PM   #78
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Summarising, a focal length is "normal" if it doesn't cause perspective distortion when viewing the image at a normal viewing distance (equal to the diagonal of the image).
11-27-2008, 07:33 PM   #79
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I brought up perspective distortion because it is relevant to the conversation. The odd thing is that the second article you cited also discusses perspective distortion, even though it doesn't use the term. The advice it gives is useful, but it teaches that perspective is only about the relative apparent sizes of objects to each other, and is not about their apparent relative size to the frame or their size in the field of vision. This is tantamount to saying that apparent point of view isn't part of perspective, which to me seems to contradict the classic definition of perspective. (see both Perspective (visual) and Perspective (graphical) at - Wikipedia, the free encyclopedia)

As I said, I only see this definition of perspective used in this context (about focal length, perspective distortion, and related subjects). It is used to try to make people understand that changing focal length won't change the relative sizes of objects in a photo, which is good. It's just usually taken a step too far and used to claim that changing focal length won't affect perspective distortion, which does not follow and further confuses people.

However, as long as you understand that lens choice will affect the apparent distances between objects and parts of objects (because it affects angle of view) no matter how far away they are, the rest just becomes a matter of semantics. As long as you understand, there's no point in arguing over semantics.

Class A pretty much summed up what what makes a lens "normal."

11-27-2008, 08:31 PM   #80
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I would just like to chime in to say that Marc Sabatella is 100% correct in everything he's written (that I've read) in this thread, and anyone with a completely divergent opinion is equally wrong. I'm glad he's said what I was going to before I got the chance, it saved me a lot of effort
11-27-2008, 09:09 PM   #81
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QuoteOriginally posted by wallyb Quote
I would just like to chime in to say that Marc Sabatella is 100% correct in everything he's written (that I've read) in this thread, and anyone with a completely divergent opinion is equally wrong. I'm glad he's said what I was going to before I got the chance, it saved me a lot of effort

Well put, Marc.

Er, I mean "Wally".



11-27-2008, 09:14 PM   #82
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QuoteOriginally posted by jsherman999 Quote
Well put, Marc.

Er, I mean "Wally".



Think whatever you like! I really am just glad someone has their head on straight -- to condemn ignorance but not reward knowledge is ignorant itself
11-28-2008, 06:31 AM   #83
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Just as an additional interesting note, since Marc brought it up here:

QuoteOriginally posted by Marc Sabatella Quote
I hear what you're saying, but again, now we're talking about something that is a function of print size and viewing distance - not simply of focal length, or even of the focal length / sensor size ratio. It's true the print won't look "natural" at this distance, but it would be obviously be wrong to say the perspective of the image changed just because you moved. What's the perspective of the image if there are two people at different places in the room? If there is no one in the room?
There are actually two schools of thought about this issue in classical perspective study. (And here you would have thought perspective was not even slightly controversial at this point. )

According to one school of thought the answer to this is yes, you do give a drawing (or in this case photo) a different perspective by viewing it from a different spot. As far as I know this concept is the older school of thought.

According to the other school of thought the answer to this is no, each drawing (or again in this case photo) has an inherent point of view, and viewing it from another spot does not change its perspective (Zeeman's Paradox). According to the concept of ordinary (normal) viewing distance, this inherent point of view would be centered to the image at a distance equal to the diagonal of the image. From Marc's way of looking at it, it would seem that the inherent point of view of an image would have to be the point where the perspective becomes correct (for example, farther away for telephoto images and nearer for wide angle images), but that may not be exactly the way he thinks of it.


Last edited by CFWhitman; 11-28-2008 at 11:01 AM.
11-28-2008, 09:59 AM   #84
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From a photographic perspective, given that you want to take a picture of subject "X", then you choose a focal length for the subject depending upon how you want to render it. If you want to compress the depth of the shot you use a longer lens, and if you want to emphasize the depth you use a wider lens.

But for the given subject, you have to move closer with a wide lens or further with a long lens in order to maintain the same subject size in the frame. This is why I consider "perspective" to be related to distance and not focal length.

If I merely stand in the same position and change focal lengths, then I'm not really changing the feeling of depth in terms of the subject and it's environment, instead I'm including or excluding more of the surrounding environment, either isolating the subject (longer focal length) or showing it's context in it's surroundings (wider focal length). I don't consider that to be a change in perspective.

In the end, it's kind of like a "glass half full / glass half empty" kind of distinction, and I guess it doesn't really matter as long as you understand what you want to acheive and how to acheive it. But if you're trying to communicate the concepts it's important to be clear on the distinction between the effects of focal length vs. position.
11-28-2008, 12:21 PM   #85
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QuoteOriginally posted by Class A Quote
Yes, but one cannot talk meaningfully about a "normal" focal length without going beyond the parameters you mention above.
True enough. I'm just "insisting" that we not take a word with centuries of well-defined definition and suddenly imbue it with this new meaning. Or that if we do so, we are always careful to differentiat what kind of perspective we mean. Simple statements like "perspective changes with focal length, or focal length / sensor size ratio, just fly in the face of centuries of usage of the term perspective.

I'm uncomfortable with the term "effective perspective" because it makes it seems like what we are talking about here is just some sort of natural extension of the existing idea as modified for digital or APS-C, like "effective focal length" to refer to the whole "crop factor" effect on FOV. The latter is a technically incorrect use of terminology that I am actually completely comfortable with. That is because while there *is* a specific definition of focal length, it has no "real world" meaning for most people aside from its effect on FOV. That is, for most *practical* purposes, focal length is a synonym for FOV. 99% of the time when we refer to focal length, we have in mind FOV concerns and no others. So I'm OK with blurring this distinction. But the correct definition of perspective has some very important real world applications that I do *not* want to see blurred by confusing it with this whole business of print size and viewing distance.

QuoteQuote:
As you mentioned works of art, it would be interesting to know whether the "diagonal of the image" rule also works for paintings. Surely, there is an intended and optimal viewing distance (range) for paintings.
Probably so, but I'd never heard the "diagonal of the image" suggestion before. My gut feel is that this understates the "normal" distance for small photos (do we really look at 4x6 prints from only 7 inches away?), but probably understates the difference for large paintings - a painting that is, say, 6 *feet* by *feet* might indeed be viewed from 7 feet away but will also be viewed closer, because we want to look at brush strokes, etc.
11-28-2008, 12:48 PM   #86
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QuoteOriginally posted by CFWhitman Quote
It doesn't skew the geometry from what it is with the other lens; it skews it from what it should be if it were actually closer.
I *do* understand what you are saying, and it is a very good point. I'm just continuing to argue that we not co-opt the word "perspective" to describe this effect, because that terms has too long a history of a specific meaning that is rather independent of the effect you are describing.

QuoteQuote:
Looking at it the way that you do, you could say that the apparent point of view changes, while the perspective does not, and that is what causes the distortion (distance is still not what causes perspective distortion). However, the definition of perspective is not nearly as specifically and exactly what you are saying as you seem to suppose.
I don't know; I've read many books, chapters, and articles on the subject of perspective over the years, not to mention sat through art school lectures and demonstrations on the subject, and they all deal with the spatial relationship of the viewer to to the physical subject itself and how this gets translated into flat art. Not even a passing mention is made to the admittedly equally interesting subject of the spatial relationship between the viewer and the piece of flat art. But I don't doubt that other folks in the art are aware of this issue and talk about it. I just know they are usually treated as independent subjects, with much more attention given to the one I'm defining as "perspective".

Anyhow, one reason I make a big deal of this is that, as we all know, there is a whole lot of confusion around these issues, and whenever I hear claims that a 50mm lens on APS-C isn't really similar to a 75mm lens on FF because the "perspective" is only that of a 50mm lens on FF, I cringe. I realize *you* weren't claiming that - you were speaking of the *ratio* between focal length and sensor size, meaning you do of course get that there is no difference in perspective (in *any* sense of the word) between these two scenarios. But there *are* a lot of people who believe there is some sort of perspective difference caused by focal length itself, despite the identical spatial relationships and FOV. We need language that is capable of sorting through these issues. Being able to differentiate what I am referring to as "perspective" from the effects you are referring to - ones that are actually dependent on print size and viewing distance - is important to this. And, of course, other endeavors.

So anyhow, as long as we're clear on the what the differences we are are talking about are - and I think at this point I we all pretty much are clear - I don't care that much what we call them. As long as we don't use language likely to cause confusion among those who aren't clear on all this.

Oh, and thanks, Wally! :-)

Last edited by Marc Sabatella; 11-28-2008 at 12:53 PM.
12-01-2008, 09:44 AM   #87
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QuoteOriginally posted by Marc Sabatella Quote
I don't know; I've read many books, chapters, and articles on the subject of perspective over the years, not to mention sat through art school lectures and demonstrations on the subject, and they all deal with the spatial relationship of the viewer to to the physical subject itself and how this gets translated into flat art. Not even a passing mention is made to the admittedly equally interesting subject of the spatial relationship between the viewer and the piece of flat art. But I don't doubt that other folks in the art are aware of this issue and talk about it. I just know they are usually treated as independent subjects, with much more attention given to the one I'm defining as "perspective".
In my experience with discussions of perspective, the spatial relationship between the viewer and the piece of art has generally been explained away with Zeeman's Paradox (as I referred to in my last post).

It does not follow, however, that because Zeeman's Paradox accounts for changes in viewing distance and print size that it will also account for changes in the angle of view of the capture (that is, things like changing focal length or cropping a photo). In fact, it seems to be just the opposite.

According to Zeeman's Paradox, up to a point at least, an image's perspective will still look "right" when viewed from a different distance or angle because our mind recognizes the appropriate viewpoint and compensates (because it is a two dimensional image). However, wide-angle and telephoto images, rather than still looking "right" when viewed from a different distance or angle, seem to still look "wrong" (compressed or extended) because of Zeeman's Paradox.

You can claim that the actual (relative) point of view where the camera was placed when the photo was taken is the one that matters, and I can claim that the apparent point of view is the one that matters (otherwise the phenomena of compression and extension distortion wouldn't exist). Either way, Zeeman's Paradox is part of perspective study, and I haven't co-opted the definition of perspective.

But of course, as you said, the understanding of the way it all works is the important part.

(Edit: Incidentally, for anyone who is interested in discussions of perspective, I have discovered a page besides Wikipedia that seems quite excellent (although, so far, I have not read through the entire thing) here: perspective in the world.)

Last edited by CFWhitman; 12-01-2008 at 02:17 PM.
12-01-2008, 07:42 PM   #88
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QuoteOriginally posted by CFWhitman Quote
You can claim that the actual (relative) point of view where the camera was placed when the photo was taken is the one that matters, and I can claim that the apparent point of view is the one that matters
I never said actual point of view is the only one that *matters*. I said it was the only one that has any effect on the conventional definition of perspective. Apparently you are familiar with a more expansive definition that also gets into issues of print size and viewing distance, and that's fine.

I've never heard of anyone named "Zeeman" before. None of the dozens of art books I have read make any mention of him, and I'm guessing I'm not alone in thinking of perspective only in the more limited way I have been. So I'm just wanting to make clear that these really are two different things, and that one should be be careful when tossing around the term "perspective" for the broader definition in contexts where people might assume you are referring to the narrower one.

QuoteQuote:
(Edit: Incidentally, for anyone who is interested in discussions of perspective, I have discovered a page besides Wikipedia that seems quite excellent (although, so far, I have not read through the entire thing) here: perspective in the world.)
I smiled when that site came up in my own Google search for this Zeeman fellow, because the discussion of *color* theory on the handprint site is by far the best treatise I have ever read on the subject. Awesome site!
12-01-2008, 08:11 PM   #89
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QuoteOriginally posted by Marc Sabatella Quote
That's not really "normal" in any classic sense. Not by the completely arbitrary notion of 50mm as normal, nor by the technically defensible but still equally arbitrary notion of 42-43mm as normal.
In the classic cameras of the day the focal length of a 'normal' lens was not 'arbitrary.' The focal length for a normal lens was determined by the diagonal of the film size. Hence a 'normal' lens for 35mm is 50mm, the approximate diagonal of a 35mm frame. In the same way a normal for a 2 1/4 (120) frame is 80mm, and for a 4x5 is 150mm.
12-01-2008, 10:45 PM   #90
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QuoteOriginally posted by wtlwdwgn Quote
In the classic cameras of the day the focal length of a 'normal' lens was not 'arbitrary.' The focal length for a normal lens was determined by the diagonal of the film size. Hence a 'normal' lens for 35mm is 50mm, the approximate diagonal of a 35mm frame
Actually, the diagonal is 43mm, which is why I mentioned that figure. But anyhow, that's what I meant about being arbitrary - what is the actual photographic *point* in having a focal length equal to the diagonal? I mean, why should any photographer care about that? Now, if that turns out to be the magic focal length that provides the field of view for which this whole business about "typical" viewing distances for "typical" prints sizes, that would be a reason to care. But given the considerable amount of looseness in any definition that talks abut typical viewing distances, putting a specific number nuber on it based on film diagonal still seems very arbitrary.
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