Quote:
Originally Posted by Martin Pugh
Should a camera lens focus at infinity or close to?
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Yes the lens should focus at the infinity mark for an infinity target, if the Lens flange to sensor focal plane distance is correct, provided that no-one has adjusted the sometimes present hard infinity stop on the lens (typically on manual lenses).
Quote:
Originally Posted by Martin Pugh
Interesting observation. I am testing out a new lens which has a backfocus of 55mm. First I attached a QHY600 and all the necessary adapters to achieve 55mm, and the lens came to focus at infinity or very close to.
Changed the camera to a ZWO ASI6200, and all of the adapters necessary to achieve 55mm, and the lens came to focus 'significantly' away from the infinity mark on the lens.
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It seems very odd that there should a significant difference in where the lens focuses (the lens focus mark position) with one camera compared to the other. It suggests that the Flange-focal plane distance is somehow different between both cameras, either via differences in adapters in use (if they indeed differ between one camera or the the other in any way) or perhaps there is a small difference the sensor position (via possibly differently adjusted tilt adjustment screws) between the 2 cameras.
Of course I don't know your brand of lens, but one thing that seems unusual (to me) is the back focus distance of 55mm. Are you sure it is correct for your make of lens? The likes of Canon, Nikon, Sony etc.. are typically in the mid 40mm range for DSLRs, with mirrorless of course much lower. Of course lenses designed for larger formats are another story and have larger flange focal distances.
Perhaps just to confirm for your lens, have a look at the following Flange Focal Distance Table, just to make sure and good luck in your quest ....
https://en.wikipedia.org/wiki/Flange_focal_distance