Using a Bahtinov Mask to Focus Photo LensesThere is no such thing as one focus on infinity
There is no Such Thing as Focus on Infinity
If you pointed your camera into the night sky and forgot to turn off auto-focus, your camera will report not being able to focus. And if it did, something is wrong . Even though there are solutions providing auto-focus functionality, the actual challenge for beginners is to realize that there is no focus on infinity as such. Actually, better: there are many focus points for infinity… You barely notice this in the daylight photgraphy. In astrophotography, conderable time is spent to get the focus right even though the starts in a very large distance and apparently do not move…
If you read some books and sources on the Internet, you can read about different ways to find and fix the focus:
- Use a bright, distant object around you to focus in advance
- Fix the point of focus with a piece of duct-tape on the lens
- Use the magnification feature in the live-view and focus more precisely
- Take take make a test shot and keep focsuing until the scene is in focus as needed
- Watch the change of star sizes while turning the focus ring – the smaller the stars, the sharper they are
All of this works, but it is tedious, takes a lot of time, and de-facto stops working completely with a longer focal length, say, 200mm and longer. Or maybe, I’m not patient enough for this…
Fortunately, since the year 2005, there is a tool known as Bahtinov Mask. Mr. Bahtinov was definitely patient enough to find a simple, almost a dump solution. This solution has been discovered by a broad community through a post on the Cloudy Nights forum in 2008 by another guy. The only problem was that Bahtinov’s main challenge was focusing of real, large telescopes, and not photo lenses.
What is Different with Bahtinov Mask on Regular Lenses?
A Bahtinov Mask is made of three sectors of bars. Two 90 degree sectors cease at 20 degree to the third 180 degree sector:
Bahtinov’s formula for the bars in all sectors is:
Focal Length / (Range 150-200) = Bar Thickness and Bar Gap in mm
There are 27 pages of discussion on Cloudy Nights and another 16 pages on the Australian Ice In Space forum, plus different variations of the mask, like the Tri-Bahtinov mask helping in collimation of reflector telescopes.
All in one, if you have a focal length of 100mm, the bars and gaps should be around 0.5mm thik. This is the math side of the things. On top, the Bahtinov effect vanish a lot with shorter focal length. Stars are getting smaller. Plus, there is propotionally less and less light collected at this focal length.
Better Designs of Bahtinov Masks for Focal Lengths of 400mm and Shorter
I initially bought the above mask on eBay. There are many others sold on Internet and in local shops. Most of them are almost the same. I kept struggling with it even on 400mm where only a very bright stars produced spikes long enough to see and use them even on 10-times magnification in the live-view on Canon. It was even harder with full frame cameras and close to impossible with 100mm and shorter.
It takes quite a time to find good designs which actually work much better in the popular focal length range of 100-400mm. The major problem is that the most shops specify masks by the their diameter or by the distance of the mounting holes. They do not specify which focal length was used to calculate the pattern. If you would calculate the mask pattern for 400mm using the Bahtinov factor of 150 you will get a totally different result:
The mask on the right side works much better!
There are other approaches to achieve better results, incl. shorter focal lengthes. All approaches have one aspects in common: make the bars thiner and the distance between them smaller. One approach is to engrave a fine Bahtinov pattern with laser on acrylic glass. This makes the bars very tiny and addtionally increase the amount of light passing throught the mask.
There are products implemented this way:
- SharpStar provided by Lonelyspeck
- Skylabs Enhanced Bahtinov masks
- Artesky focusing mask sold by Astroshop.de
The last one seems to be the most accessible solution for me at least. It works best so far with all lenses I tried. The above link points to a 60mm version, where 60mm is the size of the actual mask shape, and NOT the focal length it supports. I did a real test of various masks and will post the results some time soon.
Bahtinov Masks and Filters
Very fine masks, especially ones which are laser-cut or engraved have an interesting effect with CLS, UHC or other deep-sky enhancement filters. They visualise shifts in focus points for different parts of spectrum with color diffraction patterns closely surrounding the actual focus point. On my Astromonik CLS CCD filter, there is a there is a phase with magenta colored diffraction rings around stars. It is followed by the actual focus point, and then the turquoise colored diffraction phase comes. You can use this effect to make sure you reach a focus point where no or less colored halos are caused by these filters.
DIY a Bahtinov Mask
If you have an access to a 3D printer, the best choice would be to print a Bahtinov mask fitting your needs. It will still take a few attempts to find an optimal combination of all parameters. Here is a list of a few references:
- https://satakagi.github.io/tribahtinovWebApps/ – provides the classical form as well as the “Tri-Bahtinov Mask”. You enter parameters and download the generated SVG file. The SVG can be imported into TinkerCAD or Autodesk Fusion 360 and then used to create a model for printing
- https://www.easyastrobox.com/?page=tools – claims to replace the the old https://astrojargon.net/ generator and provides two kinds of generator as well. For some reasons it fails to handle focal lengths shorter than 400mm. For longer focal lengths, the generated pattern has very wide bars… Not sure it works welll
- https://astrojargon.net/ – After a long time being offline, the site is back with a Windows app as a generator. The page also refers to the GitHub project by satakagi which is listed in the first point. The app and some additional infos are available from GitHub directly: https://github.com/FarmerDave/AstroJargon