What is infinity focus? How to use it? Is that the same as the hyperfocal distance?
Photography is a world with plenty of technical stuff you should be aware of and take advantage of.
For the purposes of astrophotography, we will make some sense of those terms and cover the importance and techniques of using focus to infinity at night.
How NOT To Focus To Infinity
There is a simple and reliable way to make sure you are NOT focusing to infinity with your photographic lens: turn the focus ring all the way to its hard stop.
Now that we have kicked this out the way, let’s see how we DO focus to infinity.
Where is Infinity Focus?
How can someone focus on infinity? We are used to focusing on a target: a person, a building, an animal. But where is infinity?
In photography, with “infinity,” we mean “very distant” objects:
- The Moon is orbiting at about 380 – 400 000 km away from us. The Sun is at about 150 000 000 km, and distant galaxies are millions of light-years away.
- A mountain range on the other hand, can be only a few tens of kilometers away.
As far as photography is concerned, though, all those targets are at infinity.
What Does Infinity Mean
The concept of “infinity focus” is related to that of “hyperfocal distance.”
Before continuing, let’s recap a couple of concepts and the related terminology.
Focus Distance. When you focus on a target at say 10m from your lens, everything that is 10m away from your camera is in focus: this is the focal plane. Everything closer or farther than the focus distance is, technically, out of focus.
Depth of Field. As we just saw, when you focus on a target at a certain distance, everything in front and behind it is not truly in focus, but the amount of focus blur gradually increases when moving away from your target.
This “blur gradient” creates a zone in front and behind the focal plane in which things still look reasonably sharp and in focus. How much this zone extends in front and behind the focal plane is the depth of field.
Hyperfocal Distance. This is that almost magical focus distance for which you have the widest wide depth of field. If we call H the hyperfocal distance and you focus your lens at that distance, everything from a distance of H/2 from your camera to infinity will be acceptably sharp and perceived as being in focus.
In other words, the hyperfocal distance is the shortest focus distance for which distant objects (like stars or distant mountains ranges and peaks) are reasonably sharp and perceived as being in-focus.
Focus To Infinity. When you focus to infinity, you are purposely focusing on distant targets to get them as crisp and sharp as possible, rather than setting it to having them reasonably sharp like when you use the hyperfocal distance.
However, focusing to infinity will still lead to a rather wide depth of field, particularly with short focal lengths.
Hyperfocal Distance Or Focus To Infinity for Astrophotography?
Landscape photographers are known to often use the hyperfocal distance a lot to bring focus to most of the scene they want to photograph.
Street photographers use hyperfocal distance too. They have to work quickly to capture a scene instantly or a situation that often presents itself out of nowhere, and focusing every time is a waste of time. So Street photographers are used to prefocusing their camera using the hyperfocal distance and let its “magic” do the rest.
Astrophotographers, though, need to focus to infinity. With the exception of star trails and starry landscapes, we don’t care at all about depth of field: all we want is to have stars, nebulae, galaxies, planets, and the Moon sharp and in focus.
Techniques To Focusing At Night
Focusing at night and in low light can be frustrating, as more often than not, your camera autofocus cannot lock on the target or will get very slow.
Switch To Manual Focus (don’t use Auto)
The best way to focus at night, particularly on stars, is to switch to Manual focus.
Some cameras, such as my Sony RX 10, have a physical switch allowing you to quickly select the focus mode: Auto, Manual and Continuous.
With interchangeable camera lenses, you may have a lens you can switch to manual by simply flipping a switch or sliding the focus ring towards the camera.
But with the vast majority of cameras, if you can change to manual focus, you will do it by changing the focus settings in a menu.
Using Live View To Check Focus
Back in the day of film photography and the first DSLR, you could only rely on the optical viewfinder for focusing.
I have the old Olympus OM-1 SLR and have added a split-screen with microprisms as a focusing aid device.
Those optical viewfinders are ok in good light conditions, but at night, they make focusing a very difficult task indeed.
Nowadays, the LCD in DSLR and Mirrorless cameras is not only there to let you preview your photos or for changing your camera settings, but it can also show you how your settings affect the exposure of what your camera is seeing: say hello to Live View.
And with Live View, you have focusing aid tools such as image magnification to magnify a part of the image and focus peaking, to highlight the sharp edges of the scene.
Those focusing tools are particularly useful to focus on the Moon and the Stars.
Choosing Your Focus: Adjust The Focus Ring
With manual focus, you need to physically manipulate the focus ring of your lens in order to focus.
Back in the days of film cameras, this was the only way to focus and lenses were built for giving you a great focusing experience. Back then, lenses had a long focus throw (the amount of ring rotation to go from the closest focus distance to infinity), allowing you precise adjustments, and the focus ring was smooth and offered a nice resistance.
Nowadays, with lenses built for autofocus, you can find those features only in high-end quality lenses.
And back in those days, all lenses had engraved a scale of distances that was actually much more accurate than that used today in only some lenses.
In the beginning of the article, I made a clip to show how not to focus to Infinity: you do not turn your focus ring all the way until the hard stop.
This is because autofocus lenses need to focus “past infinity” in order to find the sweet spot for the focus. If you hit the hard stop, you are focusing past infinity.
So, can you set your lens on the infinite symbol? As a coarse way to focus I’d say yes, but if you expect to do so and get a precise focus on stars, you will be disappointed.
The use of the infinite symbol is not precise enough: with some lenses, you have to align your focus with the middle part of the infinite symbol; others have a line indicating the symbol you have to align to.
There are better and more precise ways to focus to infinity, particularly for the stars and the Moon.
Aim At A Bright Object
Nowadays, even in the countryside, there are a lot of man-made lights around, particularly if you are not in a truly wild environment.
Wind turbines, street lights, lighthouses on the coast, radio antennae all have some sort of light you can focus on.
Pop your camera on a tripod, magnify the view in the Live View and gently turn your focus ring until your distant target looks sharp.
Focus on Star
To focus on stars, magnify in your Live View a bright star and consider these telltale signs for good focusing:
- The better your focus, the smaller the stars are
- The better you focus, the more faint stars will become visible on the screen
- The better you focus, the less chromatic aberration you have around your stars
You could also use a Bahtinov mask in front of your lens.
Focus On The Moon
To focus on the Moon, magnify in the Live View a region of the Moon near the edge of the Lunar disc or, better, near the Lunar Terminator (the zone of transition between the illuminated and dark hemispheres of the Moon surface.)
Here light conditions are such that you will be able to see a lot of details on the lunar surface: use the craters to guide your focus.
Two Tips For A Better Focusing Experience
Tip: Use a piece of gaffer tape to lock focus in place
When you do astrophotography, because you may find yourself out there during cold and humid nights, you may experience lens fogging.
To avoid the problem, you should use a lens heater that wraps around your lens, near the front element.
Coowoo is a reliable brand of lens heaters for photographic lenses you can get from amazon.
To avoid moving your focus ring while installing the lens heater, tape the ring down with a piece of gaffer tape.
This tape does not leave glue residues on your lens and can be easily removed.
Tip: use a guiding scope mounting ring
Particularly with long telephoto lenses and with high magnifications for the Live View, focusing will introduce a lot of camera shake.
This makes it difficult to focus, as the image constantly wobbles and shakes. A neat trick I took from videography is fitting a guiding scope mounting ring over the focusing ring and using it as a focusing bracket.
Using the ring makes manipulating the focus ring smoother, more precise and causes less image shake.
Frequently Asked Questions for Astrophotography Focusing
How do you check infinity focus?
The metal body of your lens/telescope slightly shrinks or expands with the decreasing or increasing of the ambient temperature.
In turn, this causes the focus to shift: you may start your session with the best focus and then discover at home that half of your images are blurred.
Take the habit of checking your images from time to time and slightly adjusting the focus if needed.
For this, it is enough to review and magnify your last photo on the LCD screen of your camera: if it looks soft, refocus.
How do you focus to infinity without an indicator?
While more important for setting the lens to the proper hyperfocal distance, for astrophotography, the scale of the distances is not reliable enough to focus to infinity.
Telescopes, in fact, have nothing like this.
Sure, the infinite symbol can give you a quick way to get in the ballpark of your focus, but nothing more. The same goes for the hard stop.
Despite this, because you are relying more on what you see on the camera Live View rather than what is written on the lens itself, it is not a big deal and the focusing experience for astrophotography is the same as with any other lens.
Except for one thing: with lenses without the hard stop, you cannot mark the position where the focus on infinity is spot on. Instead, if you have a lens with a hard stop, you can simply mark the position for infinity focus to get consistent focusing results in a much faster way.
This is particularly useful if you are using legacy lenses or lenses that need an adapter to be mounted on your camera body. If the adapter does not perfectly reproduce the proper distance between the lens and the camera sensor, your focus shifts.
Some adapters will not allow you to focus to infinity, while others will affect how close you can focus. Either way, the scale of distances indicated on the lens will not be accurate and reliable anymore.
But, again, this is more a problem for when you want to set your lens to the hyperfocal distance.
Do all lenses have infinity focus?
Except for the case of using adapters or some accessories such as close-up lenses, all lenses can focus to infinity.
Infinity focus is not quite the same as hyperfocal distance, although the two concepts can be related.
If you are a landscape photographer and use lenses with accurate distance scales printed on them, you can use hyperfocal distance to get reasonably sharp pretty much everything in your image, from the foreground to the horizon.
But if you are into astrophotography, you really want to have your focus spot-on for the stars and the Moon, and for this, you will be using infinity focus.