Optics question - eyes and cameras, and the mythical 50mm lens

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  • #26
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If you try to create a portrait of a person with a 28mm lens, the person's face going to be very distorted.
I'm not sure "distorted" is the right word, just differently perspectived. Because apparent size is a function of the, hmm tangent?, of the angle at a certain distance what we see is different from different distances. An intimate portrait of a _very_ close friend (in both meanings) might be more the way one wants to remember the friend, no?

I think a practical point got glossed over too. It is easier to make long lenses with little spherical aberration, so in fact wide-angle distortion might sometimes be the appropriate appellation after all...

Now on to a new can'o'worms:
The 24mm shot does have greater depth of field...
This surprises me and I knew we could get to circles-of-confusion sooner or later. Just like the perspective brou-haha, depth of field should be a function of relative angle as well. As you blow up a wide-angle frame the same relationship between distance and fuzzyness (based on the resolution of the eye, etc) should hold.
 
  • #27
turbo
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You can't shoot decent portraits with a wide-angle lens because there are stringent limits on how much you can enlarge the subject, and how much film grain or sensor noise you can tolerate.

When I was doing portraiture, I couldn't afford to spend tons of time cropping, enlarging, etc to produce the prints. Every image had to be composed to fit the frame. The notion that I could produce a portfolio for an aspiring model using a wide-angle lens was out of the question. The minimum focal length for such work was 100mm, which gave me enough separation from the subject to flatten distortions cause by perspective. Practicing photographers know this.
 
  • #28
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turbo-1... I don't think we are arguing with you about the practical aesthetics:
You can't shoot decent portraits with a wide-angle lens
Since this is a physics forum we are waxing theoretically on the subject and
ignoring the limits of lens and film. Or at least I thought I was...
 
  • #29
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You can't shoot decent portraits with a wide-angle lens because there are stringent limits on how much you can enlarge the subject, and how much grain or sensor noise you can tolerate.

When I was doing portraiture, I couldn't afford to spend tons of time cropping, enlarging, etc to produce the prints. Every image had to be composed to fit the frame. The notion that I could produce a portfolio for an aspiring model using a wide-angle lens was out of the question. The minimum focal length for such work was 100mm, which gave me enough separation from the subject to flatten distortions cause by perspective. Practicing photographers know this.
Correct. Only a fool would embark upon a portrait shoot with a wide angle lens. That's why most will use something like an 85 or 135 mm equivalent. Of that there is no doubt.

But it would be twice the fool to claim that wide angle lenses are unflattering, or the oft-cited corollary that telephotos compress distance and depth. They do no such thing.

I am not denying that long focal lengths are well suited to portraiture. But I am flatly rejecting the notion that focal length controls perspective. It's a simple distinction, but one which surprisingly few photographers appreciate. Is it a problem? Well, it's not the end of the world. Partly it's disappointing that some photogs have such poor grasp of optics. But it would be a problem if a photographer was unwilling to shoot a wide group shot using a wide angle lens for fear that somehow noses would be rendered 'unflattering' by that lens. That is the genuine belief that many photographers have because they read so often in second rate journals and mags that you shouldn't use a wide angle lens because they're not flattering.

So I guess it is a big deal when people get it wrong, and invariably they're mighty reluctant to relinquish their faulty understanding.
 
  • #30
turbo
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This "faulty understanding" comes from a former professional photographer and an ABO-certified optician. Yes, me.

I have been trying to explain that in practical terms (not merely theoretical terms) the focal length of a lens has a great deal of influence on the perspective of the image. A photographer doesn't always have control over the distance that he or she can approach their subjects.

I often shot group photos with a 24mm Auto Zuicko, so your example of a brain-dead photographer refusing to shoot a group with a wide angle lens is more than a bit off the mark.
 
  • #31
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It is fortunate that you do understand the concepts. But surely you must appreciate how many photographers latch onto the 'focal length changes perspective' bandwagon without out actually understanding why? Surely it would be better for people to actually understand that perspective changes due to position change, not due to focal length.

By all means educate people that focal length may dictate where you go, but for heavens sake don't tell people that focal length determines perspective because they'll take you literally and their understanding will be impaired. You don't need to look far to find photographers who are thus misguided, and I would argue that this confusion is widespread because people fail to emphasize the distinction between perspective, position and focal length.

It's a common failing of education and teaching, and it's stated time and again in photography books. It doesn't have to be that way. If people were more aware of how perspective works, then they'd be better able to use it creatively, to understand how to apply perspective effects, and to capitalise on the fact that, most of the time, people don't view photographs from the correct center of perspective.
 
  • #32
Andy Resnick
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I confess I have no idea what the OP is referring to anymore. There appears to be a lot of mixing different concepts (blur circle, distortion, magnification, etc) and no clear question to answer. Possibly this is due to conflating the focal length and angle of view without understanding how this relationship works.

Imaging a rectangular grid that is parallel to the sensor plane will tell how much distortion there is in the lens, but that obscures a critical element regarding depth perspective- objects are at different distances from the sensor. Tilting the grid (or the sensor) will address this, but then one must account for the numerical aperture of the lens, which can have several effects- the total irradiance decreases, but so do the aberrations. In any case, perhaps the OP should try it and comment.

Then there's the simple matter of geometry- in order to have a face fill half my frame using my 24mm lens, I have to be about 1 foot away. For my 85 mm, I can be 10 feet away. Thus, if there are only two objects in my frame- the near and another 30 feet away, the ratios of distances are clearly very different between the two lenses. Thus, the relative subtended angles appear very different.

What I haven't seen is the OP present any photographs demonstrating his/her claim. What I can find and generate myself definitively disproves what (I think) the OP claims- short focal length lenses exaggerate the change in subtended angle between near and far, while long focal length lenses understate the change, compared to the focal length of the human eye (about 20 mm).
 
  • #33
turbo
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Due to the moving goal-posts, I have not addressed the comparison between the human eye and a camera lens. A "normal" lens can only approximate what we see with our eyes.

The lens-maker's ideal is to build a lens that will be free of distortion and is tack-sharp all across the image plane. The human eye is nothing like this. We have greatly enhanced acuity and color-sensitivity at the fovea, which drops off quite sharply in every direction. We don't "see" this distortion as distortion because our brains have evolved to interpret the images on our retinas as accurately as possible with respect to relative size, distance, color, etc. As an extreme example, the fovea of raptors like eagles give them such a view of distant detail that it's like they have built-in binoculars in the center of their field of view, to detect prey from great heights.
 
  • #34
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Then there's the simple matter of geometry- in order to have a face fill half my frame using my 24mm lens, I have to be about 1 foot away. For my 85 mm, I can be 10 feet away. Thus, if there are only two objects in my frame- the near and another 30 feet away, the ratios of distances are clearly very different between the two lenses. Thus, the relative subtended angles appear very different.

What I haven't seen is the OP present any photographs demonstrating his/her claim. What I can find and generate myself definitively disproves what (I think) the OP claims- short focal length lenses exaggerate the change in subtended angle between near and far, while long focal length lenses understate the change, compared to the focal length of the human eye (about 20 mm).
I'm afraid it's not a claim, it's basic science. Have you checked out those introductory textbooks I cited earlier? If not send me your email address because I have a few pages scanned which will help you understand a little better. You are almost there. When you 'get it' you're going to kick yourself, but it's good that you're thinking experimentally at least.

The scenario with your 24mm and your 85mm is exactly true. It stands to reason that, to fill the same amount of frame with a wide angle lens, you'll have to stand closer to the subject. So you have the answer right there. You moved your feet, and you changed the perspective. When you move you obviously alter the depth relationships and ratios between objects in the scene. Hence the ratio of nose to ears as a proportion of total scene depth changes as you move towards or away from the subject.

Lenses do not alter depth relationships. How the hec is the lens meant to 'know' which light is distant and which is near? It can't discriminate. It is ludicrous to attribute perspectival changes to the lens when it's perfectly clear that perspective is determined by position. You move your feet = new position = new perspective. Bingo.

The photographic examples that prove this 'claim' are so well covered that I don't need to post them here, but they are in all the decent textbooks.

I would like to make one correction though: I stated previously that this basic geometry has been understood since the 1400s. That was a lie. In fact it goes back to Euclid, 300 BC. How he would turn in his grave....
 
  • #35
Andy Resnick
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I'm afraid it's not a claim, it's basic science.
I think one problem I am having is not understanding which optical effect you are referring to:

http://toothwalker.org/optics/distortion.html

'Distortion' can refer to many optical effects, none of which are related, and all of which refer to different image properties.

Specifically, there seems to be blurring in this thread between 'perspective distortion' and 'geometric distortion'. And not much discussion about how to compare a telephoto image, a wide-angle image, and normal vision since all three subtend different angles. It can be argued that 'geometric distortion' (which I think is what you are discussing) is a matter of holding the picture a certain distance from your eye.
 
  • #36
sophiecentaur
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This thread is a classic example of nerdy vs arty. (CP Snow would turn in his grave) Both views are right - from their 'perspective' or wrong. Why are people getting so aerated about it?
In practice, many pictures taken with short focus lenses look more distorted (bulbous) and many of those taken with long focus lenses look 'flat'. That's just because you stand in different places to take the pictures of the same object.
If you watch a lot of TV pictures and then see just how close up the camera is and how small the studio sets are, it goes against intuition and experience with standard (pre-zoom) film cameras. If you correct for typical lens distortions - such as 'barrel', you can avoid much of the close-up distortions. Photoshop etc. can do a great job of fooling the viewer as to the focal length of the lens that was used.
"Perspective" is a red herring; it is not the quantity that counts here. I don't think it is really even a Scientific Term; it was used by artists who were trying to describe how they could draw pictures which made distant and near objects look 'right. I think the term was taken up by the technical camera brigade and is now used too loosely by non technical users - just like 'momentum', 'power' and 'energy'.
Long shots of a subject show more of the edges and sides of it than will close shots. (Just as the Horizon gets nearer and nearer, the lower you are standing. Noses 'stand out' with a close / wide angle shot because there are bits of the sides of the nose that you just don't see (and bits of the inside can be seen which can't be seen with a long shot).
You (eye and brain) use what can be termed perspective and parallax to give a clue, in real life, about the relative positions of objects. A single photograph or even a movie sequence, can mislead the brain into all sorts of conclusions and paradoxes which could easily be resolved if the viewer had more control of the camera position at the time - e.g. move from side to side a bit. Once your brain has made sense of a scene, the relative sizes are more or less eliminated from your consciousness of the scene (all cricket players look the same size to the crowd).
 

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