olyacme
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theSuede wrote:
Do you really need to be that far away from the pinhole? A really high resolution camera has got ~5µm pixel c-c distance, on a 50mm lens that gives that a 0.5mm hole (normal pinhead needle) only needs to be 0.5*50/0.005mm = 5m away before the pinhole is smaller than a single pixel in the camera...
No, but the pinhole was bright enough, and I wanted to test at infinity, for best comparison with an actual star.
theSuede wrote:
The comatization in the above example actually makes the Lateral CA much harder to judge, as Coma is often corrected very differently for the three different primary wavelengths. For lateral CA, the USAF target, or even any other high-contrast pure black/pure white edge will give more pictorially relevant information - without making things to hard on the more non-scientific target users who only want an overview of classic lenses' CA behaviour.
Maybe, but I think most photographers can produce an image like that, and see how far the various aberrations extend, and how intense they are, and then make a good assessment on how they will affect their images. For example, I knew that this lens delivered good detail but low contrast wide open, and figured this was probably due to Coma. The star test confirms this speculation, and shows its extent.
theSuede wrote:
LoCA smudges the information available in a lateral CA pinhole target so much that you almost have to deconvolve the channels separately to get a good overview of what's going on.
The images shown were chosen to compare a pinhole star with a real star, rather than look at any particular aberration, so you're right to call them a smudge. They're greatly overexposed, for one thing, to bring up the Coma. I did take additional images at f/16 in order to look at Lateral CA more or less by itself, and out of focus shots to look at SA. On-axis shots could be used to inspect Longitudinal CA in better isolation.
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