Lenses and distortion

[blansky]

I have a question for the technical types in this site. God knows there are enough of you, engineer, scientific types here. Some of us right brained or no brain types need your help.

Here goes;

Please explain the phenomina we know as distortion in, for instance;
When we take a full length portrait of a person with a 50mm on a 35mm camera, it is fine. If we move close and do a tight headshot, we distort the features, ie, the nose comes forward and the cheeks push back.

Does this same thing happen in larger format cameras. In my experience it seems to happen in 6x6 so we use a 150mm lens for headshots.

However in 4x5 very few photographer use a 300mm lens, most just use something like a 210-240mm. If we move in real close with these lenses do they also cause the same kind of distortion.

Thanks in advance

Michael McBlane

 

[Thilo Schmid]

Michael,
perspective distortion (the one you are talking about) is a matter of the point of view. The viewing perspective is *only* dependent on the point of view, not on focal lengths or format sizes. But certain points of view can only be handled with certain focals. A cropped WA-shot shows exactly the same view as a tele-shot as long as both are taken from the same point of view. A certain focal length can be seen as a more or less magnified crop of the same overall view.

The closer your point of view is to the subject, the larger are the size differences between nearer and farer objects. This is why WA-lenses do emphasize the foreground.

If you take a close look at people, their faces actually look "distorted" in the same way. This is nothing strange that happens inside the lens. But since this is not the normal way we look at people, portraitures taken from close distances feel "unnatural".

BTW: the normal "talking distance" between two people is a little shorter than the classical portrait distance - not only in photography.

 

[glbeas]

Yes it will. This is a phenomenon of perspective, not so much the lens. If you are attentive you can see it with your naked eye.
Consider this. At ten feet using a telephoto lens on your 35mm you can measure to the ears of your subject - 10 feet, and to the tip of the nose, 9.5 feet more or less. 5 percent. As a percentage thats not much difference so the image sizes are relatively close to what you call normal. Now put a wide angle on and move in until the head is the same size as in the telephoto shot. Lets say this gets you to 3 feet. Measure to the ears again, three feet, to the nose, 2.5 feet. Now this turns into about 17 to 18 percent, more than three times the difference. And the relative sizes of those body parts will be changed by about that much.
The lens has little to do with these relationships, a pinhole camera will do the same thing.

 

[Tom Duffy]

And to answer your other question about larger formats, a 210 mm lens on 4x5 is equivalent to a 65 mm lens on 35mm format. you could argue that 4x5 photographers should be using 300mm lenses, but they often don't. reasons being inadequate bellows extensions at close up portrait distances and very limited depth of field. same depth of field as with a 300mm lens used with 35mm format, only you have to be closer still with the larger format. with an 8x10 you'd have to use a 600mm lens to be equivalent to an 85mm for 35mm. One of reasons you see so little 8x10 portrature is that's it's so hard to do.

 

[Foto Ludens]

This is the same thing that causes the so-called greater DOF in wideangles vs telephotos. If you consider distances, not proportions in the image, the DOF is always identical. That is, what really matters is how far away the subject is form the negative, not which lens is between them.

 

[victor]

andre... what u r talking about is a magnification. there is many other aspects in optics. one of the most important (for what michael askes) is angle of view.
lets take... 24mm lense (on small format) and 50 mm lense. and lets divide our negative (horizontally) into three parts (for ease, but we can make it it in tenths and more). so we are talking about 24x36 fromat (very popular, rite?) and we talk of the 36mm base divided in 12+12+12.
now im taking a photograph of say.. some person.
1. 24mm lense, distance=70cm, magnification=0.034
2. here im going to keep the magnification, which means - i will have the same frame of person.
so: 50mmlense, distance=145cm, magnif=0.034.

so i have just the same frame - person on white clear wall or black clear cloth (so taht the background will not confuse us)

what is the defferance...
the differance is thaht the 24mm lense has angle of view of about 84degrees, while the 50mm lense has about 45degrees. (sorry that the numbers are not totally acurate, since when u move very close the focal lenght will not stay 24mm but nominally slightly longer, but it is not that important now)

now, we have made dividation of negative surface in three parts (for ease). what happens is taht the center part has angle of view about the same on both 24mm and 50mm, while the sides has totolly diferent perspective (but symetrical).
if we consider the 50mm lense, the perspective of the centere and the sides is about the same (on longer lenses like 90mm is even more equal). on the 24mm lense it is very far from being the same - (metphorically) it is as if the left side looks at left and rite side looks at rite.
of course, add to it the front and back differnce.

 

[Juraj Kovacik]

I must say, reading what you are writing about, I'm happy I'm sticked with my basic 50mm Nikor JK

 

[Foto Ludens]

victor,

I understand what you are saying, and it's pretty close to what I wanted to say, but the sleepiness got the best of me. We just went about the same thing in 2 dif ways.

What I meant was this:
If the camera does not move, and the subject does not move, only lenses are traded, the depth of field will remain the same. Sure, a wide angle will cover more area than a tele, therefore having less magnification, therefore making for an apparent greater DOF.

But say the subject is a person, and 3 meters behind the subject is a brick wall.

Say you frame and photograph with a 50mm, say, at f5.6; and the brick wall is out of focus.

If you put on a 100mm, and do not move to recompose, you will get a tighter shot, sure, but the bricks on the wall will be just as much out of focus. (provided you stick w/ the same aperture)

Now, if you go back and put a 24mm, and once again do not move, you will cover a greater area, but if you enlarge the print enough, you will see that the bricks are still just as much out of focus.

Things to keep in mind:
bokeh might play a part in this, so comparing lenses of similar designs might be a more fair comparisson (I dunno)
the smaller magnification of the bricks in the 24mm shot might make them look more focused, so enlarge them to match the size of the 50mm shot.
The opposite goes for the 100mm shot.

Once you start thinking in terms of proportions, as glbeas said, it'll all be clear. It's all math... (not that I know it)

So, this long post is just to say: yeah, I know....

 

[blansky]

Thank you all for your replies.


Michael

 

[victor]

andre... i totally agree with u as far as i understand what u say. there is only one focus plane for everything - all the rest is illusion of physical and physiological limits.
the dof marks on the lenses are very practical, but those are just a "logical" approximations.
that what i have said - that u talk about the magnification.

what i was talking about is not the magnification element of optics and its appearance on the negative surface, but about the angle of view.
angle of view is the other important element to control the perspective. wide angle lenses are have "dynamic" apearance not only because the front/back relation, but mainly because of the angle of view which gives u the sense as if u turn your head around (especially pronounced in some kinds of object arangement in the space). at the centre (the radius of the centre is upon your critical approach to this metter) the perspective is about the same for all focal lenghts - means, if u point your camera directly forward, with all focals it will look the same - "direct forward perspective" (with defferance of magnification/distance etc).
but as u go aside of the negative surface u will find differances:
the long lense for will remain almost the same (direct forward) on the all negative surface,
while the short lense will record the sides in defferent perspective - as if looking from the side, or if the camera is vertical it will be from up and dawn. the normal lense is somewhere in between. the wider u go the more pronounced is the effect of the angle of view and the longer u go the more directly u look at the whole scene. of course everything has its practical and creative use. in practice, this knowledge means nothing much, without applaying it to the arrangement of the space and what u want from this space and things in it.

 

[Foto Ludens]

exactly!