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Camera lenses are converging lenses, meaning they have positive power. A simple convex lens will do this job, however, the image is flawed. I am talking chromatic aberration whereby each color will come to a focus at a different distance downstream of the lens. Now a negative lens diverges and a positive lens converges. Additionally they display opposite ...


2

What is the actual combination of aberrations at play here, The "spinning top" bokeh near the edges appears to be tangential astigmatism. Because the bokeh balls don't change their "width" (i.e., the diameter of the spinning top doesn't substantially change) further from the image center, the sagittal focus is fairly constant. But because the bokeh balls ...


2

Because in many cases the flat glass needed to lengthen the back focus by the needed amount would need to be thicker than the entire registration distance. The effect of flat glass is so miniscule that it would take glass thicker than the space available to get the desired additional back focus distance for most applications. Flat glass would also ...


1

It would appear that a thick, planar optical glass window is also capable of lengthening back focus - and would probably introduce less new problems. Why is this not done in practice? Because it won't work. It won't lengthen the back focus enough. That is, a flat optical element shifts the focus by an amount less than the thickness of that flat element....


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