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Academic question: If I run out of turn attempting to achieve an infinity focus (close, but no cigar) would I:

  1. Move the lens away from the sensor? OR
  2. Move the lens closer to the sensor?
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  • This is a false dichotomy because there are different focusing mechanisms and lens designs that may have reasons other than incorrect distance between rear element and film/sensor for inability to focus to infinity. – xiota Jan 13 '20 at 22:14
  • "No more turn left to get focus at infinity" – Try turning the focusing ring the other way. You might be using a lens that turns "right" to focus to infinity. – xiota Jan 13 '20 at 22:16
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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.

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  • Some lenses (with large bulbous front elements) require a filter between the film/sensor and rear element (rather than decreasing the distance between the two) to focus to infinity. – xiota Jan 13 '20 at 22:10
  • @xiota assuming you're not talking about wavelength-specific photography (i.e., IR), do you have any examples? I'm not aware of any optical-wavelength lenses that, given they are mounted at the correct flange focus distance, require a rear filter in order to focus to infinity. – scottbb♦ Jan 13 '20 at 22:21
  • Thanks scottbb. That answers the question. My setup is for an Vivitar lens with Nikon F mount attached via adapter to dedicated astro camera. Needed to know if I should shorted or lengthen the optical train. The lens will always be wide open aperture wise and always set to an infinity focus. Due to the lens to T thread adapter I was just shy of achieving focus. Thanks again. Getting a smaller spacer should do it. – Woxof Cavu Jan 14 '20 at 3:08
  • @scottbb Zenitar 16/2.8 Fisheye requires the rear filter to focus correctly. The helicoid cannot be adjusted to focus to infinity without the filter. (All copies sold without filter are pretty much good only for parts.) I'm pretty sure I've seen some other fisheyes that require similar filter placement (also of Soviet origin). There are also fisheyes that have internal filters that affect focus (Sigma). It's possible there are ultrawides with similar designs, but I don't pay attention to UWAs. – xiota Jan 14 '20 at 6:30
  • @WoxofCavu That info should have been included in the question instead of claiming it as an "academic question". – xiota Jan 14 '20 at 6:33
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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!).

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  • Some lenses (with large bulbous front elements) require a filter between the film/sensor and rear element (rather than decreasing the distance between the two) to focus to infinity. – xiota Jan 13 '20 at 22:11

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