Quick and Easy Method to Focus a Camera Lens on Stars

Here is a quick and easy method to focus a camera lens on stars in the night sky. If your eyes are not as good as they once were, this method might help.

The key to focusing a camera on stars begins with a few simple rules. First, set your camera and lens to full Manual mode and manual focus. Point your lens at a very bright star. Then use Liveview mode and your camera's focus magnifier to zoom in for a closer view while focusing.

That is the basic concept, but focusing on stars can still be hit and miss as it is difficult to judge how sharp the focus is or isn't on a tiny LCD. If you have an add-on large monitor, then it is easier, but most AP folks probably don't. So how can you make getting a sharp focus easier? Well you can try this inexpensive, quick, easy method. All it requires is a special type of lens filter you can find on eBay or other places.

What do you need?

You need to get an 8 point "star effect" filter like the one shown below. This will show light spikes from light sources like bright stars. I've had my Sunpak 8 Point Cross Star Effect filter for years, but I'm sure you can get one like it on eBay or any number of other sources.

Sunpak Cross Star 8 Point Filter

How do I use it?

1. Attach the 8 point "star effect" filter to the lens.
2. On the lens, set the aperture and focal length which you will be using.

Note: Changing aperture are focal length (for zoom lenses) will likely effect your focus. So if you change the aperture and/or focal length after focusing, you will need to refocus.

3. Point the lens at a very bright star.

Make sure that your camera settings allow the star to display brilliantly with a dark sky around it. This sharp contrast will help you focus.

4. In Liveview mode with the focus magnifier on the highest setting, adjust the focus until you see the sharpest (often thinnest) light spikes coming from the star.

Here is an image of my Sony a7 LCD showing the light spikes after I focused on Anteres. Notice how thin and sharp the spike appear. As you can see I used the focus magnifier at the highest setting of 11.7x zoom to better see what I was doing.

Sony a7 Liveview Screen Showing Manual Focus Magnifier

5. Take a test photo or two and verify the stars are in sharp focus.

Here is an image of my Sony a7 LCD in Playback mode as I view and zoom in on the test photo I just took. The stars are slightly elongated in this sample because I was using a shutter speed of 3.2 sec with a 135mm lens. For the next test photo I took I corrected the exposure to a 1 sec shutter speed to prevent star elongation.

Sony a7 Playback Screen Showing Test Image Zoomed in for Verification of Focus

The thing to look for is very crisp, thin, sharp light spikes as well as tiny points of light for dimmer stars. If the stars are elongated as they were here, the light spikes will look thicker, so adjust the exposure speed as I did. Here is a 100% crop from the second test photo I took with the shutter set to 1 sec.

100% Crop of Second Test Photo Showing Sharp Focus

In this image you can see that the light spikes are now crisp, thin, and sharp, dimmer stars are nice tiny points of light and Antares looks sharp. Antares is very bright, so don't expect a bright star to look as sharp as the dimmer stars because it has a lot of glare when viewed at 100% crop level.

6. Focus is accomplished, so carefully place a piece a tape across the focus ring on the lens to prevent accidentally changing the focus.

At this point I usually remove the 8 point "star effect" filter from the lens. But you can leave it attached to create the star spike effect on brighter stars.

Recap:

1. Attach the 8 point "star effect" filter to the lens.
2. Set the aperture and focal length at which you will be shooting.
3. Point the lens at a very bright star.
4. Use Liveview and focus magnifier to focus on the star.
5. Take and view test photos to make sure focus is sharp.
6. With focus accomplished, tape down focus ring on lens to prevent accidentally changing it.

That's it!

Download PDF file of article.

Nice info. I might try that trick myself.

It works really well. I had forgotten that I had the Sunpak filter as I long ago focused and taped down my AP lenses. Last week I bought a Sony a7, so I needed to refocus all my AP lenses for the full frame camera as the focus is different from an APS-C camera. I then remembered the Sunpak filter, found where I had stored it, and put it to good use.

I remember reading long ago how some old timers even used a small piece of screen wire (like is used on windows) to focus on stars. I never tried that though as this is easier to put in place!

Thanks for the nice tips, quite useful.

I am fortunate enough to use a Zeiss MF lens for Canon EOS that has a hard stop at infinity focus, so that infinity actually is, well... infinity:)

The same can not be said for all the AF lenses I have used so far...

As in the original post, I am also shooting with a sony a6000, the pancake 16mm, PLUS I also have the fisheye adapter too. Waaaay back in 2016 while in Norway, I snapped some night sky pixs for fun and got my first-ever meteor capture on film or virtual film, plus really sharp stars!! I DO NOT remember having any difficulty at all in focusing. Now trying to use the a6000 again to snap the Milky Way, cannot get everything in sharp focus, which is how I found the shootsforfun post about focus.

I am focusing on infinity and I get distant trees out of focus! I focus on those same distant trees, and the screen-displayed focusing range shows about 2meters!! The trees are actually about 69 meters and I am quite sure it's not 2 meters.

Anyone have an idea of how to focus manually using the screen-displayed virtual distance scale in the a6000?

The star effect filter idea is a nice one but I don't have that, and live-view on the a6000 isn't an option, but it is on my canon cameras.

Thanks in advance and stay safe out there, everyone!!

I remember seeing some "new" filter advertised by some of the astro youtubers that seemed to do exactly this.... but I thought that it worked on a different principle and not just creating "starlights" from point sources.

Easier to use a Bahtinov mask.  You can also 3D print them

Basically, this is a VERY old thread from 2014, so 7 years old now. I've long since given up on using gadgets and/or gizmos to attain a sharp focus on stars. I learned that the best way is to simply use your eyes and follow a few simple common sense steps.

Here are the steps I follow to focus a camera and lens on stars:

How to Achieve a Sharp Focus on Stars

For cameras with interchangeable lenses, never rely on the infinity mark for true infinity focus. It is ALWAYS best to focus using your eyesight. ALWAYS use manual focus. Follow the seven steps below to achieve a sharp focus on the stars.

Step 1:

Set your camera to Liveview.

Step 2:

Set up your camera to use focus magnification. When focusing on objects in the night sky always use the focus magnification to zoom in.

Step 3:

Initially set your lens focus ring to the infinity mark. This is to get "ballpark" focus. You will need to fine tune focus. If focus is too far off, you probably won't see any stars as they tend to get so blurry that they disappear.

Step 4:

Point your camera at a reasonably bright star. Why? Because they are easier to locate in your Liveview LCD, especially if focus is soft.

Step 5:

SLOWLY turn the lens focus ring until the star is as small of a point of light as you can get.

Step 6:

Look for dimmer stars nearby the brighter one. Why? Because dimmer stars require a better focus to be able to even see them. Even slightly soft focus can cause dim stars to disappear. So if you can make them display and make them as small of a point of light as possible, you will have achieved a good focus.

Step 7:

Take a few test photos and zoom in to verify focus.

A Few Additional Pointers:

• You will likely need to open the aperture up, perhaps even to the widest setting, in order to see better.
• Some people like to tape down the focus ring on the lens after achieving focus. You can. But if your focus ring is fairly tight and not prone to moving, then I wouldn't because taping can actually move the ring. Be careful and don't touch the ring after achieving focus.
• If you change aperture settings, then verify the focus to make sure it didn't change.
• Periodically verify focus during the evening.

Note that it is easier to focus with telephoto lenses than on wide angle lenses. You can only zoom in so far with focus magnification, and WA and UWA lenses are more difficult for our eyes to see the stars on during focusing. So expect to take more time with a 24mm lens than with a 400mm lens.