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

[jt7747]

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.

 

[Andy Resnick]

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).

 

[turbo]

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.

 

[jt7747]

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...

 

[Andy Resnick]

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.

 

[sophiecentaur]

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).