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12-01-2008, 11:27 PM   #91
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QuoteOriginally posted by Marc Sabatella Quote
what is the actual photographic *point* in having a focal length equal to the diagonal
The whole concept of "normal" looses much of it's significance in a world where almost every camera has choice of focal lengths. But in the old days of fixed-lens cameras, it meant a compromise between wide and tele - a sort of "sweet spot" of focal lengths which could suffice as the one and only field of view that a photographer could make do with under most circumstances.

12-02-2008, 09:59 AM   #92
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QuoteOriginally posted by Sean Nelson Quote
But in the old days of fixed-lens cameras, it meant a compromise between wide and tele - a sort of "sweet spot" of focal lengths which could suffice as the one and only field of view that a photographer could make do with under most circumstances.
That's been my assumption as well. FWIW, with my 18-55, I tend to shoot around 28 and around 40 at least as much as I do right around 33. And I'm finding 28mm and 40mm prime lenses make a nice complementary pair that I find more versatile than I would a single 33mm lens. But even the 28mm is closer to the diagonal measurement of my sensor, I find myself wanting to shoot at 40mm more often.
12-02-2008, 03:32 PM   #93
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QuoteOriginally posted by Marc Sabatella Quote
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.
A 'normal' lens has about the same field of view as the human eye.
12-02-2008, 09:51 PM   #94
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QuoteOriginally posted by wtlwdwgn Quote
A 'normal' lens has about the same field of view as the human eye.
lol... and here I was sure it has the same perspective as the human eye... (ok, ok, this must be the 10th thread I've entered into on this same subject so I can't help but have some fun with it!)

Kidding aside, put a zoom that goes at least 30 - 70mm (or both shorter & longer) onto the camera. Put camera to your right eye. With left eye open look at a chair at an isometric angle with both eyes. Zoom in out & Stop when the two images are the same perspective & size thru the viewfinder as with the naked eye (when the two images perfectly overlay). ...the lens always stops @ 50mm for me.

12-02-2008, 10:04 PM   #95
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QuoteOriginally posted by m8o Quote
lol... and here I was sure it has the same perspective as the human eye... (ok, ok, this must be the 10th thread I've entered into on this same subject so I can't help but have some fun with it!)

Kidding aside, put a zoom that goes at least 30 - 70mm (or both shorter & longer) onto the camera. Put camera to your right eye. With left eye open look at a chair at an isometric angle with both eyes. Zoom in out & Stop when the two images are the same perspective & size thru the viewfinder as with the naked eye (when the two images perfectly overlay). ...the lens always stops @ 50mm for me.
Exactly what I was saying earlier. A 50mm lens matches the perspective of the human eye with a 1x (or close to it) viewfinder. Therefore it's a comfortable and happy perspective for photographers, hence the tradition that 50mm is the "standard" lens.
12-03-2008, 12:05 AM   #96
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QuoteOriginally posted by wtlwdwgn Quote
A 'normal' lens has about the same field of view as the human eye.
No, it doesn't, at least not if you consider peripheral vision - the eye is *much* wider than normal. If you do try to exclude peripheral visions, then it's sort of in the ballpark, but extremely subjective - anything from around 20mm to around 50mm on APS-C (30 to 75 on film) might be legitimately claimed to be about the same as the eye.
12-03-2008, 12:07 AM   #97
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QuoteOriginally posted by m8o Quote
Kidding aside, put a zoom that goes at least 30 - 70mm (or both shorter & longer) onto the camera. Put camera to your right eye. With left eye open look at a chair at an isometric angle with both eyes. Zoom in out & Stop when the two images are the same perspective & size thru the viewfinder as with the naked eye (when the two images perfectly overlay). ...the lens always stops @ 50mm for me.
I thought you said kidding aside? For the record, if perspective changes one iota when zooming, your lens is horribly defective.

As for size, that all depends on the viewfinder.
12-03-2008, 12:08 AM   #98
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QuoteOriginally posted by m8o Quote
Kidding aside, put a zoom that goes at least 30 - 70mm (or both shorter & longer) onto the camera. Put camera to your right eye. With left eye open look at a chair at an isometric angle with both eyes. Zoom in out & Stop when the two images are the same perspective & size thru the viewfinder as with the naked eye (when the two images perfectly overlay). ...the lens always stops @ 50mm for me.
I thought you said kidding aside? For the record, as I have said many time now, if perspective changes one iota when zooming, your lens is *horribly* defective.

As for image size, that all depends on the viewfinder.

12-03-2008, 04:50 AM   #99
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QuoteOriginally posted by Marc Sabatella Quote
I thought you said kidding aside? For the record, as I have said many time now, if perspective changes one iota when zooming, your lens is *horribly* defective.
But if a zoom lens extends while zooming, then front glass goes closer to subject. Does that count as small perspective change?
12-03-2008, 06:36 AM   #100
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QuoteOriginally posted by Marc Sabatella Quote
No, it doesn't, at least not if you consider peripheral vision - the eye is *much* wider than normal. If you do try to exclude peripheral visions, then it's sort of in the ballpark, but extremely subjective - anything from around 20mm to around 50mm on APS-C (30 to 75 on film) might be legitimately claimed to be about the same as the eye.
Actually, yes it does. It's just that you insist on setting up this peripheral vision straw man as if it means something.
Get over it, your wrong.
If it was any other way, then (in 35mm), a normal lens would be considered to be around 24mm.
12-03-2008, 09:19 AM   #101
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As I understand it, "normal" perspective has nothing at all to do with the width and height of the image when looking through the viewfinder. I see a larger FOV with one eye than I can see at 16mm on my K20.

The view through my viewfinder at about 55mm (roughly 80mm full-frame equivalent) shows objects in the focal plane at roughly the same size as my naked eye. This also has nothing to do with a "normal" perspective.

Perspective: The relationship of aspects of a subject to each other and to a whole.

"Normal" perspective in photography simply means that the relative size of the objects in the frame are reproduced in the same relative size as we see with our normal vision. If I look at a scene through a 16mm lens, the objects furthest from the focal plane of the lens look proportionately smaller compared to objects in the focal plane. If I look at the same scene at 250mm, the objects further back from the focal plane look proportionatly larger compared to objects in the focal plane.

Photographers use this effect in selecting "portrait length" lenses. The most accurate portrait will be obtained with a normal lens. A wide angle lens will stretch a person's face in an unflattering manner, and a telephoto lens will flatten the face in a flattering manner. Hence the preference for mild telephoto portrait lenses. If you're photographing a beautiful model with an unusually long nose, pull out that 300mm prime.

Last edited by audiobomber; 12-03-2008 at 09:36 AM.
12-03-2008, 09:30 AM   #102
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Wow - that was hard work reading this far.

Keep it up guys, I am learning a lot.
12-03-2008, 10:04 AM   #103
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QuoteOriginally posted by Wheatfield Quote
Actually, yes it does. It's just that you insist on setting up this peripheral vision straw man as if it means something.
Well, of course it means something. I don't see how you can say it doesn't. "The field of view of the eye" can mean entirely different things depending on whether one considers peripheral vision or not. And even if you decide it doesn't, it's still an *extremely* subjective determination as to where "normal" vision stops and where "peripheral" vision starts.

QuoteQuote:
If it was any other way, then (in 35mm), a normal lens would be considered to be around 24mm.
Only if you insist on defining "normal" as "equivalent to the FOV of the eye". If you prefer, you can define it as "equivalent to the FOV of the non-peripheral portions of the eye", but that's simply not something you can pin down to a specific number. And while sure, you *could* define "normal" that way, there are several other "definitions" of normal being proposed here that are similarly imprecise. The so-called "normal" field of view is sort of a rough approximation of a lot of things - it "sort of" resembles the non-peripheral field of view, it "sort of" provides a field of view that, when printed at "typical" sizes and viwed at "typical" distances, resembles the view of the print itself, and it "sort of" provides a decent compromise between wider and longer focal lengths if you were forced to use only one focal length. But imagining that all these different possible definitions would all converge on exactly the same exact focal length: in particular, either the diagonal of the film/sensor, or perhaps 50mm/33mm if you prefer - is just ludicrous. The different "definitions" of "normal" I gave are all very inexact and broad.
12-03-2008, 10:09 AM   #104
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QuoteOriginally posted by audiobomber Quote
Photographers use this effect in selecting "portrait length" lenses. The most accurate portrait will be obtained with a normal lens. A wide angle lens will stretch a person's face in an unflattering manner, and a telephoto lens will flatten the face in a flattering manner.
As I have said many times now, this is quite simply a myth. it's not about focal length, it's about distance to subject. Changing focal lenth has *no effect whatsoever* on perspective. The change we perceive when shooting portraits with different focal lengths comes not from the different focal lengths themselves, but rather, from the fact that when shooting with a wide angle lens, you'll tned to come in closer, and when shooting with a telephoto lens, you'll tend to back up, in order to maintain the head the same size in the frame. It's this change in position that affects perspective, and *NOT* focal length. This is trivially easy to demonstrate. Take pictures of a person using your favorite wide angle (well, not a fisheye) lens and your favorite telephoto lens, *without* changing your position. Now crop the image from the wide angle lens to give the sme FOV as the telephoto. There will be *no difference whatsoever* in the perspective of the two pictures.
12-03-2008, 10:11 AM   #105
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QuoteOriginally posted by m8o Quote
lol... and here I was sure it has the same perspectivePut camera to your right eye. With left eye open look at a chair at an isometric angle with both eyes. Zoom in out & Stop when the two images are the same perspective & size thru the viewfinder as with the naked eye (when the two images perfectly overlay). ...the lens always stops @ 50mm for me.
And then adjust your 50mm by the 0.85x magnification of the viewfinder and you get 42mm ... which is still quite long ... interesting experiment though.
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