Infinity focus is mainly used to get distant objects in focus. During night photography or
it is necessary to get the stars in focus. Let us understand what is infinity focus and how to get your lens to focus at infinity.
What is infinity focus?
Before getting into the real technical definition of infinity focus; let me try to simplify things for easy understanding.
When you focus your lens on infinity, it means you have focused on things that are very far away from you. It also means that everything in your frame is in focus up to infinity. Infinity focus is mostly used for star trail photography or Astrophotography or most commonly while photographing the Milky Way
With infinity focus, you make sure that the faraway subjects are in focus and will thereby appear sharp in your pictures. Technically, at infinity focus, the light rays coming from your subject are parallel to your lenses.
Until the point of infinity focus, light rays tend to diverge or spread while they reach your lens. When you set infinity focus, you make those light rays parallel by virtue of your lens adjustment feature, so that once they are parallel, anything beyond the point of your subject remains in focus until infinity.
Theoretically, everything remains in focus and that is why we call it as ‘Infinity Focus’. You don’t have to adjust the focus of your lens every time you compose for a picture.
When to use infinity focus?
Infinity focus is typical to focus on far away subjects like stars, Milky Way or Astrophotography, where you want to make sure the subject appears sharp in your photographs. Shooting star trails is the best example for using infinity focus. Instead of relying on autofocus, you will get better results with manually focusing on infinity for far away subjects. Photographing Fireworks is another good time to use infinity focus.
How to achieve infinity focus?
Good old days manual lenses had hard mechanical stops on focus scales of the lenses. So it was easy to set the lens on infinity focus even in the dark by just turning the lens to a mechanical stop.
With the advent of autofocus lenses, these infinity stops started to disappear. Setting up an infinity stop to a lens needs a lot of calibration for the lens manufacturer.
With autofocus though it is possible to shoot at infinity, (later on that) so none of the manufacturers cared about giving infinity stops to autofocus lenses back then.
Although, today we find infinity stops on many lenses, not all the lenses have the infinity stop marking on them.
Two effective ways to set your lens to infinity focus.
1. When your lens has the infinity marking on it
An infinity stop is marked with an inverted 8 on the focus scale as shown in the below picture
You simply have to choose to set your lens to infinity by moving the focus point to this mark. Don’t forget to take test shots before you go for the final one. The exact position of infinity focus differs from lens to lens, so it always better to know your lens and it’s exact infinity mark position where it works the best.
2. When you do not have an infinity mark or your lens
This is a simple manual method. Set your lens to autofocus and focus on a distant bright object. Maybe on the brightest star or a far away light source. The moon serves the best to set autofocus.
Once you have achieved that focus, gently turn your lens to manual focus without touching the focus ring.
You now know that you have a distant object in focus because you have already achieved that with autofocus. Turning to manual mode after achieving correct focus with autofocus means the focus will remain intact until you disturb the manual focus ring on your own.
Without disturbing the focus ring, take test shots of your distant objects. There are 99% chances that you will get your far away subject in focus.
I have used this method for star trails, the Milky Way and it works perfectly.
Although my lens has an infinity mark on it, I prefer using the above autofocus method most of the times. Since the autofocus lenses are not calibrated manually these days for infinity focus, the lens performance at infinity focus is affected by many variables such as the temperature of the lenses, the moving parts inside a lens.
Zoom lenses have more moving parts and the focus point may get affected at every focal length. Lens mechanics also vary with each camera manufacturer. A canon motor in a lens may behave differently than a Nikon motor.
So it is best to know your lens in advance and test it before you go out for the actual shoot to get the desired results. Most of the times, this method with using autofocus and then switching to manual mode works well.
The only care to be taken is to do it gently. Once you are in the manual mode and you do not touch the focus ring, you are sure that your lens will not refocus again and will be steady at the same focus setting (at infinity) throughout your shoot.