Infinity doesn't work. How do I focus on stars at night?

Infinity focus doesn't work

So here you are trying to take photos of the night sky or attempting star trails, but when you get back home, you find out that they're all blurry:

When it should be all sharp like this:

If star trails look fat or star dots look like donuts (darker in the middle like the bad example above), something went wrong. Why did that happen, even though you are sure that you were doing everything right? (Manual everything, vibration compensation turned off, focus all the way to infinite, as stars are thought to be).
There are two reasons why the ∞ sign doesn't necessarily mean the lens is focusing properly and on infinity.

Cost

In the old days, expensive lenses were individually calibrated, or at least the distance scale of a lens series was calibrated. With modern auto-focus lenses, manufacturers no longer spend money on this for all but the most expensive lenses, as the AF takes care of proper focusing. And having the AF zoom past infinite allows for wider manufacturing tolerances. Also, most people use cameras during the day and modern cameras now autofocus down to -2 EV 1, i.e. really low light situations, so why bother? Because it's not good enough for night photographers!

Autofocus

Autofocus of a lens is an iterative process: the lens barrel quickly changes in one direction, overshoots the target a little, then goes back and settles into the right value (hence, the "hunting" when it can't find the right value in low light, the main reason we turn it off at night). This is very fast on some lenses, but for autofocus to work, the lens barrel cannot bump against the end for infinity focus, as it would not allow to overshoot and iterate the correct focus. To solve this, manufacturers built a small buffer into it, by allowing the lens barrel to travel "past infinity". also allowing subtle changes due to temperature or wear. "Past infinite" makes no sense physically, but rest assured that it is not focusing on infinity, hence, gives you those blurry shots if you put the lens barrel all the way against its stop. On some lenses, that extra buffer is very significant and can be more than the length of the entire ∞ sign on your scale.

How do I focus on stars at night?

OK, we now understand that autofocus doesn't work at night and putting the lens barrel all the way against its furthest setting does not focus on infinity.

Here are three ways for how to figure out the correct "infinite" focus setting for your lens. That will, of course, be different for each and every lens! The first is for a more permanent solution, the second for something you can apply in the field (and use to check the results of the above).

Quickest solution: Many lenses have a marker that actually tells you how much to back up from the furthest barrel stop to get infinite focus: it shows an L-shaped marker going from the middle of the ∞ sign to a shorter distance, which needs to be lined up with the focus marker to get correct focus. The left image shows the barrel all the way against the stop. Note how the marker associated with the ∞ sign does not line up with the focus marker. The right image shows the infinite focus properly lined up, this is several degrees of rotation away from the barrel stop.

Try this first and see if it produces the correct results (verify it with solution #3).

Permanent solution: Take your lens and focus on an "infinite" object during the day, for example, a very far mountain range. You can also do this at night by focusing on the moon, if available, or a very far light, if it is bright enough. Look at your lens barrel and mark the setting that it found, either with a piece of paper/sticky film or - gasp - by scraping a small line into the black coating where the center of the ∞ sign is when properly focused on infinity. Probably too involved, signs fall off, scratches don't look good and all may change over time.

Ad-hoc solution: also to be used to check the above solutions from time to time, as it may change over the age of a lens. Take photos at night with a bright celestial object in the center of your composition. Jupiter works very well for this in the northern hemisphere. Put the lens barrel all the way against the stop corresponding to the greatest distance ("past" infinite). Repeat that by backing off from that setting small amounts, for example: past the ∞ sign (where the barrel hits the stop), then just where the focus marker hits the ∞ sign on the left, then dead-center of the ∞ sign, then on its right edge etc.

View the images and zoom as far into the star you focused on and compare which setting is the sharpest. It will be difficult to nail it without doing this at 5-10 settings, but usually you can get really close to optimum infinite focus with the above 4 lens settings.

Note that I do not yet know if the infinity focusing is different at different focal lengths of the same lens. If you figured out the proper infinite focus of your 24-70 mm lens at 24 mm, I do not know if that also applies at 70 mm. Tests will be done to figure that out!

The same applies to the focus setting, which may change with aperture (f-stop) as well. However, for star dots, the fairly narrow f/5.6 to f/7.1 range produces best results, particularly if you want to include some foreground. Hence, it is sufficient to calibrate your lens for that range.

Many people across the net recommend f/2.8 or "wide open" to capture the most light and faintest stars, but that does not allow you to incorporate foreground, which makes an image much more powerful and interesting! Remember that you can expose longer than a few seconds to capture more light, up to almost a minute on some very wide angle lenses. That, however, is an entirely different lecture to be found here. And then there is also the higher ISO setting to capture more light.

As often, shooting star dots or star trails is always a trade-off between noise (high ISO), creative depth of field (f-stop) and exposure time (limited for dots, not for trails).

For More Information

If you are interested in more information about infinity focus, here are some links:

Footnotes:

1 EV numbers are a way to express the brightness of a scene in a scale that combines the shutter speed and aperture settings into one number -- letting the photographer choose what combination of shutter speed and aperture settings to use. Each EV number equals one stop of brightness, so a scene with an EV of 6 is one stop brighter than a scene with an EV of 5. EV 0 corresponds to an exposure time of 1 s and a relative aperture of f/1.0. See Wikipedia

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