I got myself a Darlot Petzval 10.5" to use for the 8x10. I understand that the image circle for Petzval design is approximately its focal length, thus 10.5" image circle is not enough for 8x10. Since I am using this lens for portraits mainly, how close should my subject distance be in order to cover the entire full frame 8x10 (without any movements). I calculated that I need the image circle of at least 12.8" to cover the full frame. Is there any formula for this?
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