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09-06-2009, 12:51 PM   #1
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a matter of distance...

Is there a "best" focal distance for any given lens?

First the terms I'm using:
distance = how far the camera is from the object (roughly where the photographer's VF is , not the lens front / back / sensor specifics) in this case 24" and 48"
proportion = how big the object is in relation to the image area
print (output) size = 8.5 x13" @360ppi image as a constant
viewer distance = 1.5x the print diagonal as a constant (in this case app. 24")

Now the question using those terms: how do you judge the ideal lens at which ideal distance to get the ideal proportion? Or to reiterate, is there a "best" distance for any lens?

I ask because, to my eye, the "close" image below the object (the point of the rocks) looks out of proportion to the entire image and the "far" image seems better and the "farther" image most accurate / correct / proportioned. Subjective, I know, but it got me thinking about this...

The examples (K20 with A 50 f/1.7), @ 100%:
from 24"

from 48"

and from 72"

Brian

09-06-2009, 04:40 PM   #2
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Gee, what a well posed question!!

There's more than one answer depending on desires, but here's some major considerations:
1) Correct perspective means the camera should be the same distance from the subject as your eyes (easy), AND
2) The camera's field of view should agree with your field of view (hard to pin down)
This is hard because we've each got a few "inherent" fields of view:
a. Our entire angle of view, about 190 degrees, which looks really strange on a display to me.
b. The angle of view that looks right to you.
c. The angle of view used for our precise vision (about the size of your thumbnail at arm's length.)

Unfortunately the angle of view that "looks right to you" depends on your dominant interest in the scene you are observing. So it is reasonable to follow Common Wisdom (actually highly researched by the movie/camera/video people.... 50mm for 2:3 FF)

In your photos, they ALL look about right to me (but I tend to like a smaller AOV than average I think.

#1 brings my attention to the leaves and rock texture (I can imagine being on my hands & knees looking at them (ie. 18-24" corresponding to APS-C & FF.))

#2 looks about right if I were leaning partially from my waist.

#3 looks about right if I were a little taller than I am & standing up straight; it is interesting that only in #3 I noticed the rocks form a path of sorts.

There is not enough foreground/background in these scene to evaluate perspective I think.

Dave in Iowa

Last edited by newarts; 09-06-2009 at 04:49 PM.
09-06-2009, 05:44 PM   #3
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Dave, thanks for the input. I'm betting there is a body of wisdom out there that has a lot of this figured out empirically , i.e., that 50mm works "best" for images perhaps _____ feet out. I don't know that, but that's what I'm hoping folks will contribute... or explain why that won't work,
Brian
09-06-2009, 06:33 PM   #4
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QuoteOriginally posted by FHPhotographer Quote
Dave, thanks for the input. I'm betting there is a body of wisdom out there that has a lot of this figured out empirically , i.e., that 50mm works "best" for images perhaps _____ feet out. I don't know that, but that's what I'm hoping folks will contribute... or explain why that won't work,
Brian
One of my main points is that "best" focal length per distance depends on the photo's purpose.

Say there's the cutest 4 year girl ever with ice cream cone old sitting sideways on a bearded goat, 12' from you; she's looking cross-eyed at a bumble-bee on the end of her nose. You decide which story you want to tell & that will determine the focal length.

If you want to do the best you can, you'd probably have to allow for some distance adjustment to keep the perspective right.

The empiricists seem to think 50mm for FF at a normal viewing distance for the subject of a photo is about right ON AVERAGE - this varies with individuals & scenes I'm sure. In the above example, 50mm for a FF sensor would be ok for either bee or goat if you'll adjust your distance appropriately.

If, in the above example, you must stay 12' from the little girl and want to emphasize the bee on her nose, you'll need a long lens, maybe 300mm - in which case her nose will look squished.

Dave

09-06-2009, 08:33 PM   #5
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What you are describing isn't really lens-specific, but rather, subject-specific, and also viewer-specific. That is, it's well known that most viewers find images of the human face most pleasing when shot from a certain distance range, and that's pretty much independent of focal length. That's why portrait photographers tend to choose focal length based on how much of the person the want to include in the image, but shoot from more or less the same distance all the time (yes, that's an oversimplification, but not one without an element of truth).

Beyond that, though, there's not likely to be much consensus on the "best" distance for other subjects. I much prefer your first image of the rocks, for instance; the last is by far the least interesting to me. That has as much to do with the specific position of the rocks, though, as the fact that they are rocks at all - and again it has nothing to do with the focal length, except to the extent that the focal length determined how far you had to be in order to get the framing you wanted.
09-06-2009, 10:42 PM   #6
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Thanks Marc, this got me thinking and googling and I stumbled across "size constancy" that seems to fit with some of your comments. There's also a lot of work on "color constancy" that seems apt as well. I think my sense of "proportion" is more perceptual, an incongruence between what I know about the relative size of the object and my perception of that object in the image. In other words, it depends on how you look at things hehehehehe

Here are some paraphrased constancy comments for those interested:
----------
Size constancy refers to the fact that our peceptions of the size of objects are relatively constant despite the fact that the size of objects on the retina (sensor) vary with distance.
-----------
Psychologists explain size constancy with Emmert's Law: known distance determines apparent size. Normally you know how far away things are, thanks to depth cues. Therefore you can compensate for the size of the image entering your eye, producing size constancy.
-----------
Also see "color constancy" etc, all based on the theory that no organic system expands more of its capacity on a given perceptual task than is necessary (Sayre 1968, pp.151-152)

Last edited by FHPhotographer; 09-06-2009 at 10:50 PM.
09-06-2009, 11:11 PM   #7
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Moved to lens forum, this is not about style or technique.
09-07-2009, 10:59 AM   #8
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QuoteOriginally posted by Damn Brit Quote
Moved to lens forum, this is not about style or technique.
I disagree, see PM
Brian

09-07-2009, 11:02 AM   #9
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One of the early conventions in describing focal length was that an FL equivalent to the diagonal measurement of the 35mm negative/image would be "normal" since it approximated the typical FOV of human eyes. This was a convenience derived from the inclination to describe things relative to some common condition.

Some folks insist on making conventions into implied standards where none was intended. As discussed above, what looks mo' betta is a personal thing.

There are some other generalizations which might also warrant consideration in lens preference: for instance faster and/or less optically complex lenses tend to have more curvature of field, telephoto lenses appear to compress DOF, etc.

H2
09-07-2009, 06:09 PM   #10
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until I read this I was going to say my SMC 300F4 works best from minimum focus to 100 feet.
09-07-2009, 07:15 PM   #11
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Although at a given distance any 2 lenses should give the same view I can think of at least 2 reasons to pick a different lens for a given distance. That is to say if I used a 50mm lens and cropped it to the same FOV of a 100mm lens (in a prefect world) they would look the some (other then the FOV).

The first is for macro. To get as close to 1:1 as a given lens can give I usually need to be as close to the minimum focal distance for that lens. So I have to pick a lens for a given distance. There is more to it then just this but you get the idea.

The next is if I have more then one subject at two different distances. If I use a wider lens it will make things farther from the near subject look even farther away. If I use a long lens then to thing farther away will look a lot closer to the near subject. This is not DOF but how the subjects look in relation to each other and it can even aptly to the nose on your face.

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09-07-2009, 08:09 PM   #12
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Another complicating factor is how do you want to view your pictures? The distance you view the photos from affects your perspective. The perspective in wide angle photos looks correct if viewed from a closer distance than a telephoto photo. If you are doing wall size enlargements and viewing them from a close distance, a wide angle perspective will look more normal than a telephoto perspective.

You can see this yourself by taking a wide angle photo and viewing it at a relatively close distance - the perspective no longer looks distorted. If you are viewing a photo from across the room, the telephoto perspective will look correct.
09-07-2009, 09:35 PM   #13
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QuoteOriginally posted by pacerr Quote
One of the early conventions in describing focal length was that an FL equivalent to the diagonal measurement of the 35mm negative/image would be "normal" since it approximated the typical FOV of human eyes.
Thing is, our eyes actually have a much wider FOV than that. I can easily see more than my 1855 takes in at its widest setting. But much of that is "peripheral". In some vague sense, the normal FOV does more or less correspond to the "non-peripheral" portion of our vision, though.

But actually, a different aspect of "normal" might be relevant here. One of the things claimed about a "normal" FOV is that if you make a "typical" sized print of an image taken with a "normal" lens, and then view that print from a "typical" distance, the FOV of the print will matches the FOV of the scene depicted. I believe another way of say this would be, a normal lens has the same FOV as a typical print viewed from a typical distance, and hence in that sense, the resulting perspective seems natural. And conversely, images taken with a wider or longer lens will appear to have "unnatural" perspective in a typical print viewed from a typical distance.
09-12-2009, 11:10 AM   #15
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Yeah, I've been wrestling with this for a while now. This question was put differently, I hoped, and came at the issue in terms of perception instead of trying to gather quantitative data (and you may notice that I eventually got frustrated with the earlier comments and simply begged off the whole thing). I still think it would be helpful to share this kind of empirical information, but the problem is asking the right question to get at it. Thanks for the response,
Brian
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