Focus On Stars - Wide Angle Bahtinov Mask | Review

June 17, 2021  •  1 Comment

If you've read my previous article on the Kase SharpStar Filters, then you know a Bahtinov Mask is a great way to focus at night!  However, there's one problem - these filters don't work well with wide angle lenses (35mm and wider). The wider the focal length, the less visible the diffraction spike becomes.  This is where Gábor Takács' Focus On Stars filter comes in.  

Gabor does a great job of explaining things on his website, so I'd recommend reading through it.  The short version is that a traditional bahtinov mask does not have enough small slits to work with a wide angle lens.  The Focus On Stars filter is comprised of dozens of tiny bahtinov masks combined into one.  Now, even with a 14mm lens, you'll be able to clearly see a diffraction spike!  This should make focusing with a wide angle lens much more precise.

 

Filter Options

Gabor currently offers two different sized filters - 100mm and 150mm.  The 100mm should work well for lenses that have a traditional filter thread on the front (like my Tamron 24-70mm).  In this case, I used my Lee Filters 100mm filter holder and adapter ring.  However, my Nikon 14-24mm lens does not have a filter thread, and requires a special 150mm holder.  I use the ProGrey G150Z filter holder, which does a nice job.  You can read my full review on that here.  After you've decided which filter size you want to go with (depending on your lens selection) be sure you have the proper sized filter holder!

On my first night with the filter, I was pleasantly surprised!  I could actually see sharp, defined diffraction spikes with my Nikon 14-24mm lens.  This would have been impossible with the Kase Bahtinov Masks I reviewed earlier this year.  

This image has been heavily cropped to show the visible diffraction spikes at 14mm.  Only the brighter stars will show a nice 3-line diffraction pattern, which is ideal for achieving perfect focus.

 

Using the Filter

To be honest, I was pretty skeptical that this filter would even work.  I figured that you could not get diffraction spikes at 14mm, due to the way bahtinov masks are designed.  So I must say, I was pleasantly surprised when I finally got to test out the Focus On Stars filter.  Using my Nikon D780's Live View display, I was able to find a bright star, zoom in on it, and begin adjusting my focusing ring until the diffraction spike looked nice and sharp.

In the 3 photos below you'll notice that one is front-focused, one is back-focused, and the other has perfect focus.  The diffraction spikes clearly show whether or not the stars are sharp.  Without this filter, perfect focus would be much more difficult to achieve.  All you can see are round, spherical stars and the challenge is to determine when those stars are as small as possible.  That's the old-fashioned way of focusing, and a lot of folks have trouble with it.

You will need to position a bright star properly to see the three lines of the diffraction spike.  Gabor suggests "moving the filter up and down and/or rotating the camera horizontally".  If you have Live View turned on, you should be able to see when the filter/camera is moved to the correct angle and the three lines become visible.  It would help to have the lens focused near the "Infinity" marking during this process.  Now you can make very small adjustments to the focus ring until you see the diffraction spike lined up perfectly.  (Click here to see an enlarged version of the photo below)



This comparison shows the difference between a sharp and blurry star.  The center photo has a sharp, defined diffraction pattern, indicating perfect focus.  Meanwhile, the outer two photos have a misaligned center line, which means the focus is slightly off.

  

As I am writing this, I remember a night out in the Colorado mountains.  I was shooting with a friend who had a similar setup to me - Nikon camera, and a Nikon 14-24mm lens.  As we got everything setup for a great nightscape image, I noticed a strange problem with his lens.  No matter how he turned the focus ring, the stars never got fully sharp.  They were always blurry.  If we could just turn the focus right a bit further to right, then the stars would have finally gotten sharp.  But that was impossible.  Unfortunately, he wasn't able to take a single sharp photo that night.  And frankly, I was surprised he hadn't noticed this problem sooner, or done anything about it.

The reason I bring up this story is because it may happen to you.  If you find it absolutely impossible to get sharp stars, even with the Focus On Stars filter, then it's possible your lens needs to be sent in for repair.  Don't hesitate to reach out to your lens manufacturer, along with some test photos, to see about getting it fixed.  

 

Focusing in the Dark

The main problem I have with the Focus on Stars filter is that it darkens the Live View preview significantly.  I use a Nikon D780 which has one of the best Live View displays on a DSLR.  Normally, I can see tons of stars in my Live View display.  However, when I attach the Focus On Stars filter, I can't see much of anything in Live View.  If you have a newer mirrorless camera, you should be okay.  The mirrorless cameras tend to have a spectacular Live View display.  In fact, I was able to see the Milky Way in real-time on a friend's Canon EOS R!  If you have an older DSLR, or you just have trouble seeing the stars, then the Focus on Stars filter may darken your Live View preview too much.

With that said, here's what I would do.  First, scan the sky overhead and find the brightest star.  Once you find it, move your camera and lens up to that point in the sky.  Take a quick test photo to verify it's in the frame.  Once you can see the bright star in the frame, center it up.  Next, attach the Focus on Stars filter.  Turn on Live View and zoom in.  You should see the diffraction spikes around that bright star.  If so, begin adjusting your focus ring until the 3 spikes line up properly.  

If your camera's Live View isn't bright enough to see the diffraction spikes, then you have a couple options.  First, try to find out if your camera has an "Exposure Boost" function.  Most modern cameras now have this feature, which will significantly brighten the Live View, allowing you to see better in low-light. For those on Sony, you should look into "Bright Monitoring".  For those on a Nikon D750 or D780, you'll want to go into the "i" button menu, while Live View is turned On.  Then, scroll through that "i" menu until you see "Exposure Preview" and turn that On.  Now, as you increase the Shutter Speed up to 30 seconds you should see any stars in your Live View preview get brighter!  Increasing the ISO and opening up the Aperture should also help.  Alternatively, you can try setting the camera to the Video Mode, increasing the ISO, and seeing what's visible during the Live View preview.

If the Live View preview is still too dark, and you can't see anything with the Focus On Stars filter attached, then try looking for a distant light on the horizon instead.  As long as the light is ~50+ feet away, it should work.  If you have a friend, you could even have them run out into the foreground  with a headlamp and then focus on that.  

No Filter
Filter

This Before and After comparison shows just how much the Focus On Stars filter will darken your image.  This can be a serious problem for those using older DSLRs, which don't have the best Live View.  Even on my new D780, I had trouble seeing diffraction spikes in Live View.

 

The Focus on Stars filter will work differently depending on your camera, location, time of year, and light pollution.  For example, if you have some distant lights on the horizon, you should have no problem finding and focusing on those.  On the other hand, if you're out in the desert with no lights around and no bright stars, you might not be able to see much of anything on your Live View.  This will make focusing with the filter more difficult.  However, now that summer is here, Jupiter will be shining brightly for most of the night.  The planets are some of the brightest objects in the night sky, and are my preferred focusing assistants.  

Of course, there's a simple workaround to the Live View brightness problem.  Take a photo, zoom in, and check the stars.  If the diffraction spike isn't sharp yet, make a small adjustment on your focus ring (pay attention to which direction you turn the ring).  Now take another photo.  Compare the two photos and see if the diffraction spike got smaller or larger.  If things look worse, you turned the focus ring the wrong direction.  Try turning the other way and compare.  Depending on your lens, this might not be that easy.  Many modern lenses have very "fast" focusing rings.  In other words, even the slightest adjustment can cause the stars to blur out substantially.  This will really complicate this trial and error method of focusing.  However, this is the way I focus my SpaceCat telescope with its bahtinov mask.  I take short test photos, make small adjustments to the focus ring, until the diffraction spike is perfect.  It takes more time than using Live View, but I can still make it work and get razor-sharp images!

 

Unfocused
Focused

Here you can see the difference between a focused and unfocused image, with the help of the Focus on Stars filter.  When the focus is perfect, the center line is right in the middle!

 

Final Thoughts

I've been doing astrophotography extensively for about 5 years now.  When it comes to wide-angle Milky Way photography, I've never really had a problem getting my stars sharp and focused.  This is for a variety of reasons - good eyesight, nice Live View display, lots of practice, and meticulous double-checking.  Therefore, I was fine with focusing the old-fashioned way.  Turn on Live View, zoom in, put the focus ring near the Infinity symbol (I've memorized the exact spot that gives me sharp stars), and make small adjustments on the focus ring until the stars are as small as possible.  That's it!  Pretty easy for me.

However, I've also spent the last 4 years working with students, and one of the most common problems I see is that they cannot get reliably focused images.  This is often just due to laziness / carelessness.  The student might think the stars are sharp, but they don't spend the time to really zoom in all the way and verify things are 100%.  Or, they start with their lens at 24mm and get everything focused.  At that point they take a test photo and realize they are zoomed in to 24mm, and not at 14mm like they want to be.  So they zoom out to 14mm and forget to refocus the lens!  Other times, the student simply doesn't have good enough eye-sight to verify the stars are as small as possible.  Even with a good Live View display, they won't be able to tell when the stars are perfectly sharp or not.  Therefore, the student would be greatly helped with this filter!  The unique diffraction spike will clearly show whether the lens is focused or not.  It will also force them to slow down, check, double-check, and triple-check that the diffraction spike is actually sharp.  After they've gotten the focus perfect, they can remove the filter and begin taking their test photos.

My final word is that the Focus On Stars filter works as advertised.  You can see diffractions spikes all the way to 14mm!  If you're tired of getting home and finding out that all of your photos are blurry, this might be a worthwhile investment.  My only real problem with the filter is that it significantly darkens the Live View display, which may make focusing difficult for those with older DSLRs.  Keep in mind, you will need a filter holder.  I use the Lee 100mm system for most of my lenses, and the ProGrey G-150Z system for my Nikon 14-24mm lens.  However, if you don't want to deal with a filter holder, you could potentially just hold the filter up to your lens during the focusing.  Then, use your free hand to turn the focus ring until the diffraction spike looks good.  

Click here for more information on the Focus on Stars filter. 

 


Comments

Chris(non-registered)
Does holding my 100mm Bahtinov Mask in front of my Nikon 14-24mm lens really work to help me focus? It doesn't matter how far the filter is from the glass?
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