PERSPECTIVE "distortion"

[johnielvis]

here's 1000 words in a pic---the one ruler is at the tip of the nose--the other at the eye socket...shot from like a foot away...cut out of the nose ruler bottom and copied it to the other ruler to compare size LOOK!!!!! the nose ruler is BIGGER than the other ruler!!!!!

that means that there's more magnification on the nose tip than the things behind the nose tip!!!!!

SEE....this was shot from like 1 foot away....with the apple built-in cam

BUT...you see...the short distance causes the magnification to change greatly over the depth of a face....

if you use a longer lens and get farther away, the change in magnification is smaller, therefore you see it in more "proper perspective".....

so there is no "distortion" here--it is pure projection--a pinhole will do this...i think the lens on the apple IS a pinhole actually--this is pure perspective and solely due to distance from the subject--absoluetly NO distortion--this is what happens when you project a 3D object onto a 2d plane---then you look at the 2d plane and your brain thinks there's something screwy going on cause that's now how you "saw it".

so anybody thinking they got 'barrel distortion" or any kind of "distortion" is misguided....it's pure projection/perspective...things closer to the lens are bigger because they are magnified more. PERIOD.

[Paul Fitzgerald]

Correct, that's the 'Z axis' distortion, you can use it to advantage or it can make you crazy.

"if you use a longer lens and get farther away, the change in magnification is smaller, therefore you see it in more "proper perspective"....."

Both reasons why longer lenses have preferred for portraits, the extra distance gives a nicer perspective AND a deeper apparent DOF for the same f/stop=magnification. You do start to run into telephoto compression from 12'(4m) or further.

[ic-racer]

And the effect is independent of the lens. It depends only on subject distance.

[johnielvis]

right--it seems that people see write-ups on perspective...and there this very good one that was on wikipedia with bottles as an example--BUT, this, for some reason, apparently leads people to believe that the effect is only seen for two separate objects.

they seem to miss the fact that the SAME object can have varying magnification over it's surface if it is sufficiently 3-dimensional. One object is NOT all at the same distance from the lens.

This is what I think is going on in the minds of some---they understand it all with the "separate" objects, and the effects, but then the connection is missed when there is one object that looks "distorted"--the implicit assumption is that all parts of the same object are the same distance away, since its all connected on the same object--therefore it should record perfectly like it was all at the same distance---the way you "see it" when you look at it.

Then you get the picture and your brain interprets the projection as 2-d and it looks strange--and you are confused---it don't look like how it was first seen with the eye.

[polyglot]

I assume you're referring to my comments in the barrel distortion thread, my point was that walls are flat, unlike faces. If shot straight-on, they should look square not barreled, whereas the OP there was alleging barrel distortion (I think he imagined it).

My second post even described this effect you've illustrated nicely with the big nose.

The reason I disagreed with you in that thread was because you asserted that a flat wall would appear bulged in the middle because the middle is closer, and that is not the case.

[unixrevolution]

Very interesting blurb. I'm glad I read it...though, the apple iSight's lens is no pinhole. It's got an f/2.8 aperture, with a 3-element glass lens with 2 aspherical elements. The focal length is just unbelieveably short thanks to its 1/4" sensor.

[Paul Fitzgerald]

"The reason I disagreed with you in that thread was because you asserted that a flat wall would appear bulged in the middle because the middle is closer, and that is not the case."

Actually it is the case. Lens companies can adjust the lens to be flat-field but only at it's intended focus distance / magnification, going outside that range the distortion does show.

Draw it out or graph it:
center of lens to center of target = 100%
center of lens to far corner of target = 110%
10% magnification difference
adjusted and/or compounded by:
center of lens to center of film = 100%
center of lens to far corner of film = 110%

the difference from center to corner expands exponentially the closer you get.
'And the effect is independent of the lens. It depends only on subject distance.'

[johnielvis]

wouldn't the extra distance EXTRA magnification on the film side exactly compensate for the extra disttance which reduces magnification towards the subject?...there's some sort of compensation going on there as well...the extra length inside causes the numerator to get bigger (increase magnification), but the extra distance on the outside to the subject causes the denominator to get bigger (decrease magnification)....do these EXACTLY cancel each other out? I'll have to check the calculations--I know I calculated the exponential distance increase in my younger days and even wrote a computer program that printed out what distroted graph paper would look like as you chance the proximity.....I'll see about calculating the %'s but it does seem that they should exactly cancel each other out--so a pinhole, pure projection, wont' distort---but we'll see....busy now to think about it or draw diagrams....

[E. von Hoegh]

Perspective is one thing.

Artifacts due the lens are another issue entirely, better not to confuse the two. Brunelleschi knew all there was to know about perspective 600 or so years ago.

[Paul Fitzgerald]

"wouldn't the extra distance EXTRA magnification on the film side exactly compensate for the extra disttance which reduces magnification towards the subject?...there's some sort of compensation going on there as well...the extra length inside causes the numerator to get bigger (increase magnification), but the extra distance on the outside to the subject causes the denominator to get bigger (decrease magnification)....do these EXACTLY cancel each other out?"

Depends on how the lens was optimized at the factory. They can optimize for color correction, flat field and rectilinear at 1 focus distance / magnification. Outside of that all bets are off.

Tele and aero lens = inf = 1 fl
General purpose = inf - hyperfocal = 1 fl +
Wide angle = hyperfocal = 1 fl ++
Process = 1-1 = 2 fl
Macro = 2-1 = 2 fl+