Academic question: If I run out of turn attempting to achieve an infinity focus (close, but no cigar) would I:
- Move the lens away from the sensor? OR
- Move the lens closer to the sensor?
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If the lens can focus on closer objects, but not achieve infinity focus, the lens needs to be brought closer to the sensor or film plane.
This often happens with adapted lenses that move the lens too far from the imaging plane.
This also happens when you insert focusing bellows or extension tubes between a camera and its lens — you trade infinity focus for the ability to focus closer than the lens normally would.
For so-called unit focusing lenses (many but not all prime lenses are), moving it closer to the sensor is what will make it focus to infinity, and that is what the lens mechanics are actually DOING to focus.
Lens designs that focus only by moving the front element or group closer to the rest of the lens can usually (if these are simple designs like tessars) be focused the same way, with the front element as close as possible.
So called internal-focusing lens designs that focus only or additionally by moving elements between the front and rear groups can also be focused that way, but this will cause problems. Most zooms are of that type, focusing them this way can compromise parfocality or sometimes even image quality. Prime designs using internal focusing will most of the time have image quality problems if the rear group is even minimally the wrong distance to the sensor (ultrawides can give horrible results if they are a fraction of a mm off!).