Focusing on the Milky Way - Using Infinity Focus and Hyperfocal Distance to Maximize Depth of Field in Astrophotography

Standing Still

About this Post

This blog post is all about getting in-focus images of the Milky Way and your foreground subject in a single exposure, without using blending or other compositing techniques. 

Find Infinity Focus

During my astrophotography workshops the first topic we cover is finding the infinity focus on the lens we will be using during the class. Many lenses are marked for infinity on the focus ring, but this should be tested to ensure accuracy. If your lens doesn’t have a marking for infinity, take the time to determine infinity focus before shooting. 

To do this, manually focus your lens on the most distance object, such as a bright star (if it’s dark) or distant mountain ridge. If your camera has a “live view” feature, try using that and zoom in on your distant subject to aide in focusing. With un-marked lenses, use gaffer's tape or similar to tape your focus ring into place for the evening. Switch to manual focus if you haven't already.

Throughout your night photography shoot, instead of adjusting the focus on the camera or using auto-focus as you would in daylight, you will physically move the camera forward or back in relation to your subject to achieve focus. Since the focus is set to infinity, we know the stars will be in focus regardless of where we position the camera - that’s the goal with this method.

Now What?

To recap, so far we have our lens focused at infinity and we know that any photograph we take of the sky is going to feature stars that are in focus. But what about our foreground subject? If you are too close to your subject, it is going to be out of focus and you might not be happy with the results. The solution is to back away from your subject until the subject is in focus, and then take your shot. A wider angle lens will allow you to be closer to your subject and have both the subject and stars in focus. The more telephoto the lens, the further back you will need to stand to get both the subject and stars in focus. 

Math and Numbers

If you’re like me, you don’t want to spend too much time guessing where to position your camera to get the shot. Luckily, you can figure out where to position your camera to get your subject and stars in focus using a hyperfocal table, like the one in the PhotoPills app for iOS. Hyperfocal distance refers to the distance between a camera/lens and the closest object that is in focus when the lens is focused at infinity, at the given aperture setting for each different lens. 

Using the PhotoPills app, you can plug in your camera and lens info, and the table will give you hyperfocal distances at various apertures for that setup. You will know exactly how far back you need to be from your subject. Keep in mind, the hyperfocal distance will change based on the set aperture and lens focal length - this is crucial for astrophotography. 

Discussing Hyperfocal in Practice

• The lower the f-stop, that is, the wider open the aperture, the further the hyperfocal distance will be from the camera. For example, when using a 35mm lens at f/1.4 aperture, the hyperfocal distance is just over 94 feet. If I stop down that same lens to an f/2.8 aperture, my hyperfocal distance is much closer, at about 47 feet. But this lens can gather so much light at f/1.4 and that is a big reason why it is part of my kit. As a result, I commonly use this lens for larger, more distance subjects, so my camera can be far enough away (at least 94 feet) to get both the subject and the stars in focus while shooting wide open at f/1.4.
• The wider angle the lens, the closer the hyperfocal distance will be. For example, a 20mm lens at the same f/1.4 aperture as our 35mm lens in the example above, has a much closer hyperfocal distance of 31 feet; that’s one third the distance! When my subject matter is closer to the camera, a wider angle lens will give me greater flexibility in my composition. Are you starting to see why the variety of lenses in your kit can be important for night sky work?
• As a general rule of thumb, longer focal length lenses are best suited for larger, further subject matter, while wider angle lenses are best for closer, smaller subjects. This is how I approach most night scenes, in terms of lens selection. 

School's Out Forever

Notes/FAQ

• What if you were to simply manually focus on the subject? The stars would likely be out of focus and appear soft. If this is not the effect you desire, then learning the concept of hyperfocal distance will come in handy. 
• There will be situations that it is simply not possible or practical to use this method to capture the scene how you wish. You can opt to try your hand at focus-stacking, or you can simply focus on your closer-than-optimal subject and let the stars fall out of focus. I've done both and had fun doing so.
• Consider stopping down your lens to a smaller aperture to increase depth of field, and then use a light source to help add light to your now darker scene. 
• Approximating the hyperfocal distance using field techniques, such as focusing on the first third of your scene, may not be effective given our wide open apertures for night sky shooting. Proceed with caution.
• Once you get used to each of the lenses in your night kit, you will become more comfortable with the limitations of each. It will eventually become second-nature for you to guess the distance from your camera to the subject without measuring. You'll begin to know what lens will work best for the given scene. 

As with anything, practice makes perfect, and the best advice I can give you is to go out and shoot! Enjoy the stars.

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Want to learn, hands-on, how to photograph the night sky? Join me on a future workshop. 

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