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I know that lens can come in many formats, e.g. crops vs. full frame. However, someone recently told me that the Zeiss Otus has an image circle that is larger than the intended format. This is one reason why the Otus is so good and also similar things can be said about the Sigma Art lens or so I'm told.

Besides buying the gear, and a medium format back and the adapter to fit it on, is there any easy way for me to know how big of an image circle a lens has?

It seems that using a image circle for a format that's smaller than it improves the corners a lot since you're not taking the edges of the image circle?

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    While it is true that using a lens designed for a larger medium on a smaller format does keep you in the center of that lens, and thus in the sharpest part, it's also true that lenses made for larger formats may not provide the resolution necessary for those smaller formats. Smaller formats are intended to be blown up quite a bit, so the lenses are designed for this. – Hueco Mar 16 '18 at 4:53
  • Also, some lenses are used exactly for their field curvature, like a Petzval. Using one of these lenses designed for a larger format, on a smaller format, will cause you to miss out on what makes this lens great! What I'm saying is, your premise that always using a larger format lens on a smaller format for better results is incorrect. – Hueco Mar 16 '18 at 4:55
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    I'm feeling this is an X-Y problem. Do you actually care what size the image circle is, or do you actually care about the image quality produced by the lens? – Philip Kendall Mar 16 '18 at 8:29
  • @PhilipKendall There are applications where the larger image circle will help like tilt-shifting. The goal is IQ as always but knowing if your lens can do other things doesn't hurt. – unsignedzero Mar 17 '18 at 20:12
  • Surely it depends on the application? Mount the lens further from the sensor and you have a larger image circle. Take it too far and you lose the ability to focus at infinity. So the maximum image circle you can get from a lens depends on whether you're doing landscape or macro. – Peter Taylor May 7 '18 at 10:42
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You may be able to find published data about the size of the image circle produced by a given lens. In the case of the Otus lens line, the image circle (for all three models, 28mm, 55mm, and 85mm) is about 43mm across, so a bit bigger than it has to be to cover a 35mm sensor, but not nearly big enough to cover a much larger sensor with edge-to-edge sharpness/resolution.

(The size I'm quoting for image circles produced by the Otus is from the datasheets linked here: https://www.zeiss.com/camera-lenses/us/cinematography/products/otus-lenses.html#data)

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