How to Focus at Infinity
In the old days, all we had to do to focus at infinity for astrophotography or distant landscapes was to set our lens' focus scale at infinity.
All lenses had focus scales, and they had hard mechanical stops exactly at infinity, so it was trivial to set them there, even in total darkness.
With autofocus, one of the many ways camera companies discovered to save money was to take away the infinity stops. These always had to be calibrated on an optical bench at the factory for each and every lens, but with AF, camera makers take away the stop, and save themselves the cost of calibrating each lens. Since AF works at infinity, few people notice.
Likewise, focus scales aren't really needed on lenses if you aren't trying to use a depth-of-field scale. Camera makers are also getting cheaper and taking away focus scales, too.
If it's daytime, no big deal: point it at infinity, let the camera autofocus, and lock focus.
But how do we focus a camera at infinity if it's dark?
Easy: find something at infinity which is bright enough for the camera to see. There's almost always a distant light on a building someplace, and modern AF systems easily can focus on these.
The best news is that most people haven't tried, but modern cameras are so sensitive that they usually can focus quite easily on bright objects in the sky, like a planet, and certainly on the moon.
Get your camera to autofocus on something at infinity, and then lock the focus. On most DSLRs, lock focus by sliding your AF switch to MANUAL, and be careful not to knock your lens!
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