Lenses for photographing people...

Started Apr 8, 2013 | Discussions thread
OniMirage
Contributing MemberPosts: 990
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Re: Lenses for photographing people...
In reply to clengman, Apr 10, 2013

clengman wrote:

OniMirage wrote:

clengman wrote:

OniMirage wrote:

clengman wrote:

clengman wrote:

The full frame equivalent of ideal focal length for portraits is < 50mm. It has only to do with rendering the subject accurately without distortion. That's it.

This is bull. First, where do you come up with "less than 50mm?" Second, what's your definition of rendering accurately? If you take a head shot with a 14mm lens, the camera just recorded exactly what that subject looks like from that distance. That is an accurate rendering. The features may appear distorted relative to the image you have of that person in your mind's eye, but if you put your eye at the same distance that the camera was when you took that head shot at 14mm, your eye will see the same "distortion" that the camera did.

You're talking about aesthetics, not accuracy. The camera is always accurate. The human eye/brain combination applies all sorts of non-optical a priori information before presenting the "perception" that you think you are "seeing".

Um the opening in the symbol is greater than.

http://www.mathsisfun.com/equal-less-greater.html

Distortion...

http://en.wikipedia.org/wiki/Wide-angle_lens

The camera captures a flat plane, a lens adds focal length, a lens is not perfect and can add distortion. In the case of wide angles this can be bad up close for portraits if you intend to be try and capture a faithful head shot for instance.

Sorry buddy that's a "less than" as in 5 < 10.

Quote from the link I gave you, math is fun:

Changing Sides

We saw in that previous example that when we change sides we flipped the symbol as well.

This:   Becky Spends > $0 (Becky spends greater than $0) is the same as this:   $0 < Becky Spends ($0 is less than what Becky spends)

... I added no value less than 50 the symbol to represent nothing is 0 therefore anything between 1 and 49.

Just make sure the small end points to the small value!

This: < is a "greater than" symbol in the same way that this: p is a "lower case Q".

Let 's take the example that you provided above. The statement is comparing two values. The first is "the amount of money Becky spends." The second is "$0." Let's assume that the first statement is true ("Becky spends > $0" OR "Becky spends greater than zero dollars."). If we reverse the statement and reverse the symbol this new statement will also be true ("$0 < Becky spends" OR "$0 is less than the amount Becky spends."), but please note that reversing the direction of the symbol also changes the meaning of the symbol.

In your original statement, "The full frame equivalent of ideal focal length for portraits is < 50mm," you are comparing two values. The first value is "the ideal focal length for portraits." The second value is "50mm." The only correct way to read your statement is "the ideal focal length for portraits is less than 50mm."

You're right that you should just be sure the small end points to the smaller number. In your case the small end pointed to "the ideal focal length for portraits" implying that this number was smaller than 50mm. I hope that clears that up.

A math class would be good, not for me though. Your just stretching the reach at this point, though I suppose I was at fault assuming you would know a numerical character would represent focal length measured in MM.

Look, I explained it the best I could using language from your own example. If you still don't get it, I'm sorry. I certainly don't need any help with this. I learned it just fine in third grade. The short answer is, "<" always represents "less than," ">" always  represents "greater than."

Focal length doesn't add distortion.

Wikipedia:

The focal length of a lens determines the magnification at which it images distant objects. It is equal to the distance between the image plane and a pinhole that images distant objects the same size as the lens in question. For rectilinear lenses (that is, with no image distortion), the imaging of distant objects is well modelled as a pinhole camera model.[3] This model leads to the simple geometric model that photographers use for computing the angle of view of a camera; in this case, the angle of view depends only on the ratio of focal length to film size. In general, the angle of view depends also on the distortion.

Rectilinear lens

In photography, a rectilinear lens is a photographic lens that yields images where straight features, such as the walls of buildings, appear with straight lines, as opposed to being curved. In other words, it is a lens with little or no barrel or pincushion distortion. At particularly wide angles, however, the rectilinear perspective will cause objects to appear increasingly stretched and enlarged as they near the edge of the frame. These types of lenses are often used to create forced perspective effects.

So I suppose a stretched face isn't distorted?

Distorted relative to what? Like I said already, if you take a head shot using a 14mm lens (For the purposes of this thought experiment, place the subject's nose in the center of the frame.) then replace the camera with your head, then look straight ahead with the subject's nose centered in your field of vision. (You will have to use your peripheral vision to see the same angle of view captured by the camera.) the perspective you see with your eye will be the same as the perspective captured by the camera.

Fine but does this distort the features of the subject? For 4/3 sensors (7mm) 14mm would distort features.

No. In fact the human visual field (including peripheral vision) covers a similar angle of view as a rectilinear lens in the neighborhood of 6 or 7mm focal length in terms of a 35mm film format. Objects at the periphery of your vision will appear similarly "stretched" as objects at the periphery of a photo taken with a 6 or 7mm lens on a 35mm sensor.

The perspective effects are entirely due to subject distance.

If you took the image captured by the camera and printed it and displayed it so that the print covered the same real angle of view in your field of vision as the angle of view represented in the print, it would look almost identical to your perception of the subject when you put your face all up in your subject's face. (I say almost identical because there are aspects of your perception resulting from parallax and binocular vision that obviously wouldn't be present.)

Only if the exact focal length is used matching the field of view via two iris's to create a single image.

It doesn't matter if you use a single wide angle image, or several longer focal length images stitched. If they are both created from the same point in space and they both cover the same angle of view, they will look the same.

The reason that people think that wider angle lenses introduce "perspective distortion" is that the the image that you have in your mind of a particular person is a composite, an average. You see faces from varying distances, and your mind sums up all these views with all the accompanying changes in perspective and averages them out. When most people recall a face, they see the face as it would appear from maybe 10 or 15 feet away. (Ken Rockwell has a little article that discusses this briefly here. I don't know if I agree necessarily with a specific number of "15 feet" as he states, but the general idea is valid. I think it'd be interesting to learn if someone has studied this. It would certainly be possible to design an experiment to arrive at an average "natural viewing distance" for a group of individuals.)

Anyway, you don't commonly look at faces from a distance of less than around 3 feet, so when you get into focal lengths that require a subject distance of less than this for head and shoulders framing, the subject starts to look a little unusual, but like I said already, this has nothing to do with "accuracy of rendering" or any kind of optical distortion introduced by the lens. This is only a result of the subject distance and angle of view. A rectilinear lens (with no barrel distortion) records an accurate rectilinear projection of a scene. When you "see" (and I'm speaking here of your field of vision, including your peripheral vision, as you look at a point straight ahead) what you are seeing is essentially a rectilinear projection. If you spend some time trying to see this way (Look straight ahead as you're walking around town, but try to pay attention to the appearance of objects in your peripheral field of view as you pass them. Try not to actually direct your gaze upon them.) you'll realize that things at the edges and corners of your visual field really do start to look stretched out as they come closer to you.

Would need perfect vision for something like this. Glasses or contacts may introduce variables that would make it hard for such an experiment to be consistent.

No. It's got nothing to do with visual acuity. I realize that no one can see with the same detail in the periphery of the visual field that they can in the central portion of the visual field. That's not the point. What you can see is the general shape of large objects on the periphery. I think if you try the experiment I described that you will be able to see what I mean. For instance, walk on a sidewalk next to a high wall and notice the "perspective effect" on the wall immediately to your side.

The camera's position relative to the subject can give the appearance of distortion. People think that focal length adds distortion because you have to move closer to the subject for equal framing with a wider lens compared to a longer lens. That's it.

And this is different.

As I tried to explain above, it really isn't different.

A rectilinear lens with no barrel distortion just records a rectilinear projection of the scene in front of it.

Perceived "perspective distortion" is purely a result of 1) recording an image from a vantage point closer than what one is accustomed to seeing, or 2) including subject matter from a wide field of view that one might not normally pay attention to because it is at the extreme periphery of the individual's field of vision.

Does it not distort? We know an object doesn't look a certain way when we look at an image, our brain knows there is something abnormal about what we are seeing.

No it doesn't distort. Our brain tells us something doesn't seem right because it is not accustomed to seeing the object from so close, or because it's not usually as easy to see what is in the extreme corners of the visual field. If you could freeze your entire field of vision, print it out, hang it on the wall and look at it, you would see that it looks very much like a photo taken with a very wide angle lens. Only the central few degrees of the image would be sharp, but the "distortion" would be the same.

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