"I read an article on infinity focus. I understand the concept but I don't quite know how to find it. My lens does not have the symbol for infinity, as it is a kit lens. Is it true that if I can focus on the moon, I would be set to infinity? I have an 18mm - 55mm lens and a 50mm - 200mm lens. I want to practice with my cable release using bulb mode but I'd like to better understand this concept. Thank you in advance."
What is “infinity focus?”
“Focus to infinity” is a term you have probably heard, but its application is not really very intuitive. You will typically hear the term used when discussing a photograph where the focus point is at a great distance from the camera. Technically speaking, it’s not really focusing to “infinity” since you can’t see things that are an infinite distance away, but rather the point at which the lens will form a sharp image of an object at the focus point, as well as any other object beyond that point.
How it works
Up until the point of infinite focus, light rays diverge as they reach your lens. But as you focus further into the distance, those rays straighten up until they are parallel rather than diverging. That point where they become parallel is the point of infinite focus—it’s called that because any object beyond that point will also be in focus, and so on until (theoretically) infinity. In other words, when you focus to infinity you won’t need to refocus even if there are objects more distant than the one at the focus point.
When do you need infinity focus?
You will usually hear the term invoked in discussions of subjects that are difficult to focus on, such as lightning, stars or fireworks. When you’re shooting anything that’s at a great distance (especially at night), you’ll get the best results if you manually focus your lens to infinity rather than depending on autofocus or even your own eyes to get it right.
How it’s done
Before autofocus, all high quality lenses could be focused to infinity with a simple hard stop. In other words, you turned the focus ring, and when you reached the point of infinite focus you wouldn’t be able to turn it any more. This made it pretty simple to find the point of infinite focus because you could physically tell that you were there—you didn’t even have to look at the lens barrel to answer the question, although lenses universally had that little ∞ symbol printed on them to mark the point of infinite focus.
With the advent of autofocus, though, things stopped being so simple. Newer lenses would often focus past infinity, and to make matters worse, some of them didn’t have that little ∞ symbol, either. Now, there is actually some method to the madness—the reason why those newer lenses can focus past infinity is because sometimes it’s needed—the point of focus changes as the temperature of the lens rises, so it can be useful to focus past that infinity mark when high temperatures make this necessary. Another reason has to do with lens mechanics—Canon lenses, for example, will go past the infinity mark for the simple reason that it’s harder on the autofocus motor if it encounters a hard stop when focusing to infinity.
With some lenses, however, it’s just a simple matter of the manufacturer cutting some corners. In those old days, all lenses were individually calibrated, so that the distance scale printed on the barrel was accurate. In modern lenses this isn’t considered necessary, since the autofocus does all the legwork and the distance scale is considered obsolete. So what this means is that you may not get the point of infinity right even if you are lining up the infinity symbol correctly.
So the first thing you need to do is check to see if your lens has an ∞ symbol printed on the barrel. Some lenses actually have an additional marking that tells you how far you need to go backwards after that hard stop in order to focus to infinity. This is typically shaped like a sideways “L.” If you line up the distance indicator (a vertical line) with the infinity sign, you won’t get the correct focus. If, however, you line it up with short vertical part of the “L,” you’ll be focused to infinity. Theoretically. Remember that if you’re off even a little it could mean the difference between tack-sharp stars and pancake shaped stars, so you’ll need to double check using the method below.
What if there’s no infinity mark?
If your lens lacks the ∞ symbol, you may think that you just can’t focus to infinity or that it will have to be done using some sort of complicated calculation, but you’ll be pleased to hear that that’s not true. Focusing to infinity even without the ∞ symbol to guide you is actually reasonably simple.
Full Moon by Flickr user I am mr.k
During the day, point your camera at something very distant, such as the top of a mountain range, the moon or a cloud formation. Autofocus on that object and then switch to manual mode. Mark that point on your lens barrel. That’s your point of infinite focus.
If you’re shooting in the dark and you’d rather not rely on a flashlight to find that point on your barrel, focusing on the moon is a safe bet. If there is no moon, you can use a bright celestial object or even a distant manmade light—most modern autofocus systems are capable of focusing on even very small points of bright light. Just remember that once you have your lens focused to infinity, shut off your autofocus. What you don’t want is your lens to refocus once you’re ready to make the —and it will try to do exactly that if left to its own devices. Turning off autofocus is the most reliable way to make sure this doesn’t happen.
Now keep in mind that all zoom lenses aren’t created equally—there are parfocal zooms and varifocal zooms. Parfocal zoom lenses don’t shift focus as you change the focal length, so once you’re at that point of infinite focus you won’t have to change anything even if you zoom in and out. Varifocal lenses, on the other hand, change focus as you zoom in and out, so you’ll have to refocus as you change focal lengths.
Once you get your mind around the idea, it’s pretty easy to use the focus to infinity technique to get great results whenever you’re shooting distant objects, especially distant objects at night. It doesn’t even require much practice, just a good understanding of your own equipment. Once you’ve found that point of infinite focus for each of your lenses, you’re ready to shoot those distant objects with perfect precision.
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