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| p.1 #9 · when sagittal and tangential Spatial frequency curves don't coincide |
S Dilworth wrote:
S Dilworth wrote:
Longitudinal chromatic aberration is also pretty low, allowing nice MTF curves at f/2 for white light. That doesn't mean the lens doesn't produce false colour in out-of-focus regions, which might be what you're thinking about.
But that is what longitudinal chromatic aberration is, is it not?
Well, no, because it's defined at the focal plane. A lens with longitudinal chromatic aberration would produce false colour just outside the focal plane, but so might a lens with very little longitudinal chromatic aberration, for other reasons (such as spherochromatism). This is why modern lenses, despite having less longitudinal chromatic aberration than older lenses, often have more saturated false colour just outside the focal plane.
Even some apochromatic lenses (with three colours brought into focus and very minimal tertiary colour) have lots of false colour just outside the focal plane.
Sorry to keep off topic but isn't this the common misconception about LoCA, that it is the purple/green colour cast in OOF areas whilst my (admitedly poor) understanding is that it's actually spherochromatism, as you've described? It appears to be a very common error in terminology.
But it is the purple cast in front and green cast behind that is LoCA is it not? They adjust for B and bring it up to match R's plane of focus and then you have G plane of focus and R/B plane of focus just a bit in front. As you stop down the issue becomes less and less problematic. Lenses such as the 300 2.8 IS that also try to bring G to R/B don't really show this affect at all even under some pretty nasty circumstances.
I just looked and even in technical documents from Canon's own optics lab this is how they define Longitudinal CA.
EDIT: yes I just looked up some info on spherochromatism and as I thought it's not like LoCA at all and the whole purple in front and green behind across the entire frame is LoCA. Spherochromatism is said to be generally less present than LoCa and it tend to be worse in certain parts of the frame, which makes sense, than others and you might have say B in front of R in some parts and R in front of B in others.