RobCD wrote:
Steve Spencer wrote
The astigmatism is clearly evident as well. If you look at the test chart for 24mm and f/8 in the corner you will see that the horizontal lines show pretty good resolution, whereas the vertical lines have pretty poor resolution. This discrepancy between resolution in different orientations is exactly what astigmatism is and can be measured on MTF charts from a K8 machine as differences between tang/sag lines. At 24mm there is some pretty strong CA as well. This may be caused by the astigmatism, but I am not sure. Can you explain this in terms of how it would show up in an image? I'm especially curious about landscape images stopped down.
Thx,
Rob
Strictly speaking, lateral/transverse chromatic aberration is where different colors/wavelengths have different magnifications. This is most apparent with tangential structures, where a black line crossing the corner of the frame has one color on the inside of it and another color on the outside of it. This is trivial to fix with modern software, though you lose a few pixels of chromaticity at the edges of the frame. Astigmatism is where the radial and tangential structures do not focus on the same plane. Astigmatism is a side effect of making the focal plane relatively flat, so it is usually a "better of two evils" sort of thing. In practice, the effect of astigmatism is that you can never get edges/corners that are as sharp as the center, but where edges/corners have far more depth of field at their lower resolution. If you use a heavily astigmatic lens for landscape work, it is important to check the edge rendering when finding optimal focus, rather than only the center focus. The easiest way to see the effects of astigmatism is with live view at high magnification: find a high contrast detail near the corner, then try to focus on the object, and, if the issue is astigmatism, you will see two fairly defined ghosts of the same subject come in an out of focus (as opposed to the blurring-into-discs that you'll see in the center of the frame) but the two ghosts will never align with each other to create a truly in-focus image. The two ghosts you're looking at there are the radial and tangential focus plane images. Unfortunately, this cannot be corrected in software, but you'll likely get an overall sharper image of a landscape than if the focal plane wasn't flattened. Also, astigmatism often combines with spherical aberration to create some color outlines that look similar to lateral chromatic aberration, which, as it is longitudinal chromatic aberration, can only be "fixed" by applying a desaturating notch filter.
Both of them result in separation between the radial and tangential curves on an MTF chart. Whereas lateral chromatic aberration is easy to correct for, astigmatism is more complex and, in some cases, compared to the alternatives, even desirable.
Hope this helps.
Cheers,
Jon