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04-15-2010, 09:57 PM   #61
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QuoteOriginally posted by audiobomber Quote
I totally disagree. If you take a photo of a subject in the foreground with a wide angle lens, the background will look further back than what you saw with your eye. If you use a telephoto, the background will appear larger than what you saw with your naked eye. A normal lens will not distort the perception of depth.

This photo below was taken with a telephoto lens. The water tower was nowhere near as prominent when I viewed the scene.
This is simply false. No rectilinear lens ever invented can change the relative size of that water tower relative to other objects in the scene. If it appears twice as tall as the trees in the picture, I *guarantee* it appeared twice as tall as the trees to the naked eye. Only the presence of a passing black hole (which can bend light) could possibly alter that basic physical relationship.


Last edited by Marc Sabatella; 04-15-2010 at 10:09 PM.
04-15-2010, 10:04 PM   #62
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QuoteOriginally posted by audiobomber Quote
People keep trying to relate "normal" to their field of view, but that's not what it's about.
Yes it is. It is the *only* thing it can possibly be about. But not in the sense of matching the FOV of the eye - I agree tha would be foolish, as the eye has *much* wider FOV than what anyone would call "normal". But the definition of "normal" is about matching the FOV of the *print* as viewed from a typical distance. If the FOV of the lens matches that of a typical print viewed from a typical distance, you get a "normal" view. If the FOV of the lens is either wider or narrower than the FOV of the print you create when viewed from a given distance, then you get the perspective distortion you refer to.

QuoteQuote:
A lens is normal if it shows similar perspective to the unaided eye.
Again, every lens ever invented has the same perspective as the unaided eye, since perspective is dependent on position and position *only*.

It's unfortunate that so much confusion continues to exist over this issue, but I'd encourage you to to read over any of the dozens of other threads on this topic over the years. The myth that focal length somehow affects perspective is reluctant to to die, but die it must.

Last edited by Marc Sabatella; 04-15-2010 at 10:10 PM.
04-15-2010, 10:08 PM   #63
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QuoteOriginally posted by Parallax Quote
I assume you meant diagonal, Marc
Well, no - using the diagonal is one of the common (if equally arbitrary and strange) ways of defining "normal", but the OP for some unexplained reason was proposing that 45 degree horizontally was a better definition, and that's what I was questioning.
04-16-2010, 03:51 AM   #64
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QuoteOriginally posted by Marc Sabatella Quote
Well, no - using the diagonal is one of the common (if equally arbitrary and strange) ways of defining "normal", but the OP for some unexplained reason was proposing that 45 degree horizontally was a better definition, and that's what I was questioning.
I suspect the OP was being entertaining, having a little joke both with their name and the manner of presentation and the content of the post. Bait was thrown out. And we all bit. And now this absurd discussion has run on for several pages. Maybe one of OP's purposes was to flush out those with flawed understanding of photographic optics. Well, that seems to have worked. Are we all clear now on perspective and normality?

As it happens, I'm currently reading some old books on science in pre-modern Europe. They point out that the development of the laws of perspective, dating back to Brunelleschi in 1425, were absolutely vital to the scientific revolution of the following centuries, and indeed to European dominance of the world.

Many Renaissance artists were not just students of geometry, but were working mathematicians who doubled as engineers, architects, and advisors on ballistics. Perspective drove the revolutions in astronomy and cartography, the latter a tool of conquest and trade. Make a better map projection, make better maps, and you know where to send the fleets. Know how to compute ballistics, and you know where to aim the guns. And more recently, better optics have been used for gun- and bomb-sights, satellite and ground-based surveillance cams, other weapons and tools of control.

Perspective isn't trivial. Optics aren't trivial. But let's have fun anyway.

04-16-2010, 05:33 AM   #65
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QuoteOriginally posted by RioRico Quote
Perspective isn't trivial. Optics aren't trivial. But let's have fun anyway.
OK I did have some fun.

Considering all that's been said about "normal" I did my own little test.

I'm nothing if not a practical empiricist when it comes to photography so this is what I did.

I took a series of shots indoors and out at different FL using the kit lens.

I then looked at the shots on the screen without knowing what the focal length was and judged subjectively what I felt was giving the closest feel to what my naked eye saw at the time of the shot. I did pay particular attention to FOV but also to just the overall "feel" of the shot.

The long and the short of it - all the most "normal" shots were in the 28-24mm range by what exif said.

So, at least when using the kit lens, from now on that range is now the "normal" range is far as I'm concerned. Don't know if it would be the same with another lens but I assume it would be pretty close.

This will satisfy no one but it works for me.
04-16-2010, 06:04 AM   #66
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I did some more reading and I see now where I was going wrong. I particularly like this explanation: Telephoto compression is the apparent compression of perspective.
This little article even addresses viewing distance. So from now on, if I ever comment on perspective again, I will be sure to include "apparent". Figure 1-36.–Reflecting telephoto lens

I have done the cropping thing, cropped a normal view to the same framing as a telephoto, and I know that changing focal length doesn't actually change the perspective, but it looks like it does. I have witnessed telephoto compression, which is the apparent compression of perspective, in an image taken with a telephoto lens. I have seen a distant feature recede into apparent insignificance in the background when I use a wide angle lens. The concepts of telephoto compression and expansion based on focal length are not controversial at all. From my research they appear to be pretty much universally accepted. There's a good visual demonstration here: Towner Jones Photography: Back to Basics - Intro / Lens Compression. So given that there is apparent telephoto compression and apparent wide angle expansion, it's easier to define normal, because it's neither of those.

I stick by the assertion that human vision FOV has nought to do with the definition of a normal lens. Normal refers to normal perspective, a lack of apparent telephoto compression or apparent wide angle expansion. A normal lens which gives this normal looking perspective has a focal length in between a telephoto and a wide angle. Defining normal in terms of the diagonal size of the film or sensor simply sidesteps the reason for choosing the term "normal". There is no reason why a diagonal measurement should be called normal, or that focal lengths near that focal length are also normal. At least "normal apparent perspective" means something.
04-16-2010, 06:23 AM   #67
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QuoteOriginally posted by audiobomber Quote
I did some more reading and I see now where I was going wrong. I particularly like this explanation: Telephoto compression is the apparent compression of perspective.
This little article even addresses viewing distance. So from now on, if I ever comment on perspective again, I will be sure to include "apparent". Figure 1-36.–Reflecting telephoto lens
Exactly. Good to know, that we all arrived at common ground now.

QuoteOriginally posted by audiobomber Quote
I have done the cropping thing, cropped a normal view to the same framing as a telephoto, and I know that changing focal length doesn't actually change the perspective, but it looks like it does. I have witnessed telephoto compression, which is the apparent compression of perspective, in an image taken with a telephoto lens. I have seen a distant feature recede into apparent insignificance in the background when I use a wide angle lens. The concepts of telephoto compression and expansion based on focal length are not controversial at all. From my research they appear to be pretty much universally accepted. There's a good visual demonstration here: Towner Jones Photography: Back to Basics - Intro / Lens Compression. So given that there is apparent telephoto compression and apparent wide angle expansion, it's easier to define normal, because it's neither of those.
This perceived compression/expansion is a well-known effect and I think, nobody denied it here. It would probably be good to have example images posted, as you sometimes find in photographic teaching books.

QuoteOriginally posted by audiobomber Quote
I stick by the assertion that human vision FOV has nought to do with the definition of a normal lens. Normal refers to normal perspective, a lack of apparent telephoto compression or apparent wide angle expansion. A normal lens which gives this normal looking perspective has a focal length in between a telephoto and a wide angle. Defining normal in terms of the diagonal size of the film or sensor simply sidesteps the reason for choosing the term "normal". There is no reason why a diagonal measurement should be called normal, or that focal lengths near that focal length are also normal. At least "normal apparent perspective" means something.
Yes, "normal" means different things to different people, as this thread very clearly shows. Perhaps your definition from above is not bad: a normal lens is one, that shows neither apparent compression nor expansion. But I guess, this apparent things are subjective then.

Ben
04-16-2010, 07:37 AM   #68
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QuoteOriginally posted by audiobomber Quote
Good, we agree that telephoto lenses compress perspective. Not everyone seems to be on board.
Ummm, no. That's not what I said. What I said was that the subject distance that we tend to use telephotos for compresses the perspective.
The lens is just capturing what is there.

04-16-2010, 08:49 AM   #69
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QuoteOriginally posted by audiobomber Quote
I did some more reading and I see now where I was going wrong. I particularly like this explanation: Telephoto compression is the apparent compression of perspective.
This little article even addresses viewing distance. So from now on, if I ever comment on perspective again, I will be sure to include "apparent".
Good, we're getting much closer. Still:

QuoteQuote:
changing focal length doesn't actually change the perspective, but it looks like it does.
Correct, *if* we assume you are viewing the resulting image from a "typical" distance for that image size. This is the key. Changing the distance from which you view the image completely changes the "apparent compression" effect. Again, the technical term is "perspective distortion" - see Perspective distortion (photography) - Wikipedia, the free encyclopedia. The most relevant quote from that article: "Perspective distortion is influenced by the relationship between two factors: the angle of view at which the image is captured by the camera and the angle of view at which the photograph of the subject is presented or viewed". That's actually a very nice succinct summary of the topic. If the FOV of the lens matches the FOV of the print itself from your viewing position, it is "normal". If the FOV of the lens differ significantly from the FOV of the print itself, you get the perspective distortion - "apparent compression" in the case of telephoto lenses.


QuoteQuote:
The concepts of telephoto compression and expansion based on focal length are not controversial at all. From my research they appear to be pretty much universally accepted.
The basic idea, yes. But the *specifics* of how, when, and why are not well understood by most who observe the effect. Most of the time, the distinctions being made here in this thread aren't important to observe, and we can get lazy and refer to telephoto compression without getting specific about what exactly this means: how it depends on the relationship of the FOV of the lens to to the FOV of the print.

But when one wants to start talking about exactly what defines a normal focal length in this sense, one needs to understand the true nature of the effect. The explanation here actually makes this quite simple: to calculate the "normal" FOV, you need to first define a print size and a viewing distance, then measure the angle of view subtended by that print from that distance. A lens that produces that same AOV will be "normal". It has nothing to do with the "perspective of the unaided eye" or anything like that - it is 100% determined by what you gauge to be the print size and viewing distance you wish to choose as your standard.

QuoteQuote:
I stick by the assertion that human vision FOV has nought to do with the definition of a normal lens.
Correct. As I said previously, the FOV of the eye is much wider than normal. All that is relevant is the FOV of a given print when viewed from the distance from which you intend to view it.

QuoteQuote:
Defining normal in terms of the diagonal size of the film or sensor simply sidesteps the reason for choosing the term "normal". There is no reason why a diagonal measurement should be called normal, or that focal lengths near that focal length are also normal. At least "normal apparent perspective" means something.
The problem is that what it means depends on how big of a print you plan to make and how far back you plan to view it from. So it ends up being subjective - there's no way to define a precise focal length that is "normal" by this definition. At least using the sensor diagonal provides a way of doing that. It wouldn't be worth much if it didn't happen to be within range of what in fact *is* appropriate in terms of measuring the AOV of typical prints viewed from typical distance, but it turns out it kind of is. So it does make a reasonable placeholder for actually going out and measuring the AOV of various prints from various distances.
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