willie_901
Veteran
This is meaningless without telling us what you think distortion happens to be.
Impact of aperture on distortion. I don't get it!
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This is meaningless without telling us what you think distortion happens to be.
ndnik
Established
Aperture can have an effect on barrel or pincushion distortion in lenses. Only points on the optical axis see the full aperture (the pupil). Points away from the axis see only parts of the pupil, mostly in an asymmetric way. This is what leads to vignetting. The beam coming entirely from one side of the pupil at the image corners also moves inwards or outwards with aperture. This is what causes the distortion to change.
The effect is most pronounced in wide aperture lenses that have strong vignetting and focus shift.
If you look at the beam of a wide aperture lens at full aperture vs. the beam at small aperture (can't seem to find a proper link right now), you'll see what I mean more clearly.
Disclaimer: I work with astronomical instruments for a living, same principles apply even though we don't put apertures on out telescopes ...
EDIT: After coffee, I realize this works perfectly well without focus shift at all, it is simply the fact that different parts of the pupil form their image at diffrent distances from the image center (distortion). At large apertures, vignetting takes care of eliminating most of the pupil, leaving a half-decent image, at small apertures it is the aperture ring that cuts out most of the pupil leaving only the center, forming again a single decent image with different distortion.
- N.
The effect is most pronounced in wide aperture lenses that have strong vignetting and focus shift.
If you look at the beam of a wide aperture lens at full aperture vs. the beam at small aperture (can't seem to find a proper link right now), you'll see what I mean more clearly.
Disclaimer: I work with astronomical instruments for a living, same principles apply even though we don't put apertures on out telescopes ...
EDIT: After coffee, I realize this works perfectly well without focus shift at all, it is simply the fact that different parts of the pupil form their image at diffrent distances from the image center (distortion). At large apertures, vignetting takes care of eliminating most of the pupil, leaving a half-decent image, at small apertures it is the aperture ring that cuts out most of the pupil leaving only the center, forming again a single decent image with different distortion.
- N.
sevo
Fokutorendaburando
Sure? If I don't misunderstand you, that would be coma and CA, but not geometrical distortion.
ndnik
Established
No, it would be gemetrical distortion, not coma, CA, and higher-order aberrations. The relevant issue is that different parts of the pupil form their image at different locations in the focal plane. Aperture and distance from the optical axis control which part of the pupil you see.
A nice page with a sketch that may make things more clear is http://toothwalker.org/optics/distortion.html.
-N.
ottluuk
the indecisive eternity
Yet, copied from your link above:
"The size of the stop has no effect on the distortion, as the chief ray does not alter its route when the aperture is made smaller or larger. To be sure, Fig. 2 cannot be understood in the context of a paraxial theory. In the absence of the stop, the lens in Fig. 2 suffers from spherical aberration, coma, and astigmatism, which result in a blurred image patch for each point in object space."
I suppose strong curvature of field could create an effect that could be mistaken for distortion and is apparent at close range and large apertures but hard to notice otherwise.
ndnik
Established
I think he's right under the assumption that the chief ray does not move as the stop changes size.
- N.
- N.
ndnik
Established
It is still true that vignetting at large apertures makes your lens have barrel distrortion and as you close the aperture you move towards the orthoscopic case. I think.
-N.
-N.
ColSebastianMoran
( IRL Richard Karash )
Thanks for this link. Excellent article. And, there are other technical optics articles on his site.
ka7197
Established
It's nonsense. Geometrical distortion (barrel-shaped, pincushion-shaped, wave-form) isn't affected by aperture.
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