Original Publish Date: Sep 20, 2010
Updated: April 27, 2016
A common and vexing question is “how do I focus when it is too dark for the camera to auto-focus, and too dark to manually focus?”
There are five strategies that I commonly employ, but before I describe those strategies, it is important to understand a little bit about what focus is and is not. There is no such thing as perfect focus. How to achieve focus will depend on several factors especially the focal length of the lens, the lens speed, the clarity of your viewfinder, the accuracy of the markings on the lens (if any), whether your camera has live view, and whether there is any bright object that can be used to focus.
It might sound silly, but you should start by thinking about what you WANT to be in focus. Usually for a star shots everything should be in focus from the foreground to the stars in the sky. The stars do NOT have to be perfectly focused, however – more on that in a moment.
Why does focal length matter?
More than anything else the focal length of the lens dictates the depth of field. A long focal length (200 mm for example) results in less depth of field while a short focal length allows a greater depth of field. The depth of field matters quite a lot if you are trying to get a foreground object against a background of stars. The longer the focal length the farther away your foreground must be to keep both the foreground and the sky in focus. The ideal point to work with is your “hyperfocal distance”. Keep your foreground at or beyond your hyperfocal distance and all will appear as sharp as the lens and conditions will allow.
Hyperfocal distance decreases as you stop down and increases as you open up the aperture. Shorter focal length lenses have shorter hyperfocal distances. Indeed a 17mm lens at f/5.6 has a hyperfocal distance of about 15 feet (2.8 meters) while a 200 mm lens at f/5.6 has a hyperfocal distance of 1,250 feet (381 meters). That is quite a difference! These numbers assume you are working with a 1.6 crop factor. When using a full-frame (35mm) sensor the distances drop to 1.72 and 238 meters respectively.
But… How DO I Focus?
Ok, with those details out of the way the question remains: how do I get good focus when it is too dark to see to focus. Here are the methods I recommend. Start by turning OFF auto focus. I am embarrassed to say how many times I have carefully focused only to discover that I did not turn off autofocus!
- Do not wait until dark to focus. Focus on your foreground when there is still enough twilight to see clearly. Put gaffers tape, or a lens band across the rings so that focus does not change accidentally.
- Use “Live View” and zoom-in to sharpen focus if your camera allows it and there is sufficient light.
- If not enough light, find a bright light source in the area and focus on that. The moon, is a good choice (though a closer focal point may work a little better). Try a nearby streetlamp, sign or other bright object.
- Bring your own BRIGHT light source and use it to focus on something at or beyond your hyperfocal distance.
- Take high ISO images to confirm your focus and then make minor tweaks to the focus. The High ISO image will be grainy but it will also help you verify what is within the frame of your photo. Tweak the focus and shoot again.
Before you invest a tremendous amount of energy getting the sky in focus consider that the stars are point sources of light anyway. Perfect focus at infinity will improve the contrast of the stars against the night sky, but sharp foreground against a slightly out of focus sky is preferable to a sharp sky and out of focus foreground… at least in my opinion.
One last comment about focusing at night. You will find more success focusing most manual lens and high-end lenses than autofocus lenses – especially the less expensive ones. Why? Autofocus lenses are cheaper and easier to make if the lens does not stop at the infinity setting and if focus can be achieved with less movement of the lens elements. Autofocus lenses, after all are designed to focus quickly. A manual (or higher end) lens might allow a half or full turn for the focus adjustment range while an autofocus lens may have a mere 1/16th of a turn to tweak with.
Topics such as this are covered hands on and in more detail with our students in our Star Circle Academy Workshops.