identifying lens aberrations

[aizan]

whenever mike johnston says something about being able to identify all sorts of lens aberrations and really describe a lens's performance, i always get a little jealous. i just don't really know what's going on, and that would partly explain why i'm drawn to the most fully "corrected" lenses like the 50mm summilux asph and 35mm summicron asph instead of risk being disappointed by lenses that might "misbehave."

it would be really useful to know what the various lens aberrations are and how they affect the image, at least the more esoteric ones. if someone says a lens has this or that, you'd know what to expect and be able to decide whether you're ok with that or not. or if a lens you're using is doing something you don't like, you can look for one that doesn't do it.

does anyone know a good non-technical reference with illustrations from real world examples?

 

[ferider]

These are the primary ones:

1. Spherical aberration: Different focal length at lens periphery than at the centre.
2. Curvature of field: Focal surface is not a plane but a bowl shape.
3. Astigmatism: The image of a point is not a point but a short line radial from, or tangential to, the optic axis of the lens.
4. Coma: off-axis image of a point is distorted radially, typically in butterfly shape.
5. Distortion: The image magnification is greater towards the edges of the field (pincushion distortion) or less (barrel distortion) than at its centre.
6. Chromatic aberration: Different focal lengths and image magnifications for different colours.

Does this help ?

If you google, you'll find lots of references and more detailed explanations.

Roland.

 

[aizan]

well, i know the definitions from wikipedia, but i don't really know what they look like in pictures.

 

[ferider]

1. and/or 2.: you need to pixel peep. Take a test shot, with a 50 Sonnar or classic Summilux, and compare resolution in the center with the corners. I've lots of those if you want.

3. Any swirly bokeh picture, for example the Heliar shot in http://www.rangefinderforum.com/forums/showthread.php?p=1177138

4. Check out http://photo.net/leica-rangefinders-forum/00MSoT, example at very end of thread. Somebody on RFF once showed an impressive Jupiter shot like this, cannt find it now.

5. Check some of my 35/1.4 Nokton shots or many other wide angle photos for barrel distortion.

6. Check some M8 examples showing Cyan shift. Not the same thing, but this is how this aberration looks like. With modern lenses this is usually microscopic.

There are also nice overviews and examples in books by Osterloh, Puts and others.

Roland.

 

[Melvin]

This website has a decent overview.http://toothwalker.org/index.html

If you want to see aberrations at work, try searching for images taken with a Noctilux or Summilux 35mm wide open. The aberrations show up clearly in the corners in some shots, usually where there's foreground out of focus. You can see the image circle getting softer toward the corners.