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Photography Tips

Focusing on the Stars

by
Timothy Farmer
July 29, 2024
Tips and tricks for getting sharp astrophotography images.

Getting good, sharp astrophotography can be a little tricky.Most cameras auto focus will not work reliable, so we manually focus (a few of the newest mirrorless cameras report to being able to autofocus on stars but I have no real world, hands on experience where I find this to be true). We will go through step by step on how to manually focus for a DSLR and Mirrorless.This works if you are using a star focusing filter or not. At the end I will talk more about filters, hoe to use them, and put up a few links to where you can buy them.

Let talk about the differences between the DSLR and mirrorless bodies for a second. I have been shooting mirrorless now for a few years and one of the big advantages is the ability to turn up your ISO and see better in low light. This makes focusing easier for sure. The DSLR you are looking at reality if you’re only using the view finder. If you can do live view, then your pretty like the mirrorless cameras.

Quick note on lenses:

Wide angle lenses naturally have a beater depth of field and are easier to focus than longer lenses. Most Milky Way landscapes are done with super wide 15mm to 50mm. At the 24-50mm range focusing is more critical but even at 15mm or wider you still need to be close. Starting off with the wide-angle lenses might bring you quicker good results.  

DSLR Focusing.

Method 1: Prefocus during the daytime and locking or taping your lens.

This is probably the easiest of all methods but most time consuming because you must set up your camera hours before photographing the Milky Way. Set you your camera and focus on something far away like a mountain or trees several hundreds of feet way. Take a piece of tape and tape your lens so you can’t move the focus ring. This can be done anywhere; you don’t need to be where you are finally going to shoot. The draw backs are you can’t shoot anything with that lens until after you shoot the Milky Way and if there is a great temperature differences from when you focus and when you shoot, the lens may have changed focus due to thermal contraction. Technically you can do this with a mirrorless camera but there is now need to.

Method 2: Using the distance scale on your lens.

A lot of older lenses have a scale on them. Focus on infinity during the day and make note of where that is on the scale. At night,just go to that point and you should be in focus. Also, very easy but you need a lens with the scale on it. Another nice thing about this is you can easily check the focus during you shoot.

Method 3: Focus and check

Focus on a star as best as you can then do a test shot and pixel peek to make sure it is sharp. During the focusing part of this, if you have live view turn it on, then zoom in on a bright star and focus until it becomes the smallest point possible.

Method 4: Using live view if your camera allows it

Not all but most DSLR allow you to put your camera in live view. This is basically how we do it with mirroless cameras. See the next section for details.

Mirrorless focusing:

Since mirrorless cameras all feed their viewfinders and rear screens from the sensor, you are basically always in live view.

Step 1. Set camera up for exposure simulation (most people shoot in this mode anyways_

Step 2. Set up your base exposure according to your lens and camera.

Step 3. Locate a bright star and zoom in as much as you can. On my camera this is 10X.

Step 4. Adjust the focus so that star is as sharp of a point as possible.

Step 5. Start shooting

Tips:

Find a star towards the center of the image because some lenses with distort stars as you get close to the edge. When you are finished,double check to make sure nothing shifted. I always recommend you turn your screens as low as you can to help you night vision, but if you’re having trouble seeing the stars, turn your camera to exposure simulation and turn your ISO up beyond where you are planning to shoot. This will make the stars brighter and easier to see. If you go too high, your image will have a lot of noise. I personally find viewing at the same exposure that I’m shooting at is plenty to see the bright stars through the camera. It is always good to do a test shot and a little pixel peeking before you start and on your last image just to make sure nothing shifted while you were shooting.

Here is an example of a soft focused and sharp focused star.

Bahtinov Mask aka a Star Focusing Filters:

There are several designs from screw on, square, or slip over the lens. They all work the same. You put it on your lens and focus until the lines line up evenly. They work equally well for DSLR and mirrorless though I find with the mirrorless I do not need them, but if you’re having trouble getting sharp focus, I would recommend trying one out. I personally like the square ones because I’m afraid if I unscrew a screw on one, I may knock the focus, but to be honest, I never have.

I personally find the Bahtinov is the best but since I shoot a mirrorless camera I use that method and I normally don’t have any issues with focusing.

Talking about some problems you may come across. The main one is going to be condensation on your lens. I just had this problem last month when shooting at Echo Bluffs. When it is very humid like it often is in Missouri during the summer, the dew falls, and your gear will get wet. This is a problem on the lens. The dew will cause your images to be soft, like shooting thew a diffusion filter. The best way to deal with this is to have a lens or dew heater on your lens. This keeps the front of you lens warm enough so condensation will not form. If you try to just wipe you lens clean your will have trouble keeping it clean enough during you shoot and it’s hard to see it when the dew is starting to form.

If there is any chance of dew forming, make your life easier and just put the heater on. They only cost a few bucks and can save a shoot. I forgot mine but lucked out when the dew only started forming at the very end of my shoot. I lost a few images. When you are looking at your rear display you will not notice it. It will look like the image is sharper because the bright stars will start to stand out even more. We call this star bloat. You can also get star bloat when processing your image if you stretch your histogram too much.

On last tip. Practice. When you are out shooting, try different methods and see which works best. Review your images before packing up.Take extra images. Be critical on the stars in the center but also look at the stars at the corner so you can see if your lens distorts them (it probably does) but realize you are going to be more critical than most people looking at your photos. They will just be blown away with how cool they look.

And have fun!