Alley in Pisa, Italy

by Lars Kotthoff

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All lenses create a circular image, it's just that most of them have an image circle large enough that it covers the entire sensor. Vignetting at wide apertures is a manifestation of the image circle encroaching on the corners of the sensor as the circle edge is not as sharp as it would be with a narrower aperture. With a fish-eye lens, the image circle is ...


This is not "obviously irrelevant to normal photography" at all; we just don't normally worry about the sort of precision that you'll need to deal with. There are two numbers that we ordinarily take at face value, knowing that they're slight fibs: the focal length of the lens (which is usualy rounded to a "friendly" value except on data sheets), and the ...


The "telecompressor" you mentioned is a focal reducer, a device that concentrates the image in order to project it onto a smaller sensor. This approach reduces the backfocus distance (the distance from the back of the lens to the sensor). So it only works on mirrorless cameras using lenses designed for DSLRs (which have sufficient backfocus distance to make ...


Consider this review of the Canon 8-15 f/4L USM fisheye, which can shift from circular at 8mm to diagonal at 15mm. Yes, it's circular because the lens's entire image circle is inside the area of the sensor, rather than covering the entire sensor. I'm not sure there's going to be an entire explanatory webpage other than Wikipedia on this, because it's such a ...


Your second diagram is exactly on point here. Or at least it is for traditional lenses (that is, lenses that are not internal focus). All you need to do is scale the diagram's width according to the focal length of the lens. Longer focal lengths mean that you are focusing closer (at distances less than infinity); that is, the distance between the lens and ...

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