Updated: Mar 25
Focusing a camera lens in the dark represents the most challenging operation for amateurs and professional astrophotographers and night photographers. Traditional Bahtinov masks have been in use for astrophotography in relatively large aperture telescopes. This type of mask delivers a diffraction pattern, creating a set of three fine spike lines around a bright star. Once the three lines are aligned in a single spot, the focus is achieved.
There are many options in the market, and it is easy to get lost due to the obscure propaganda from the sellers and misleading experiences from different users. I do manufacture, sell, and actively contribute to open-source focusing masks projects. This article provides an in-depth analysis of the suitability of each type of focusing mask for camera lenses in order to avoid common frustration. I have used a Canon 100D (crop sensor) and two lenses to conduct these tests, the stock 18-55mm and 75-300mm.
About Bahtinov Mask
Several types of masks have long been used as focusing aids for astrophotography. The distinctive pattern delivered by the popular Bahtinov mask was invented by Russian amateur astrophotographer Pavel Bahtinov in 2005.
When the stars are not in focus, the default pattern would be an Airy disk as a result of a circular aperture. If the Bahtinov mask is in place, the pattern exhibits asymmetric spikes representing the transform of the mask pattern’s spatial frequency and orientation.
What you really need to know
You will find several Bahtinov masks in the market defined by the mask’s size (aperture). Unfortunately, to generate a useful pattern, the shape of the mask ALSO depends on the focal length and the Bahtinov factor.
The Bahtinov factor is defined as:
BF = f/s
Where "f" is the focal length and "s" is the size of the slit+space. To get enough resolution, the Bahtinov factor must be at least 150, ideally 200. Obviously, the more lines, the better.
That means that the real limiting component is the focal length. The shorter it is, the smaller the slits and spaces have to be. As cuts get too small, you can multiply the "s" number by 3 and use 3rd order spectrum, as it has no visual difference compared to the 1st one.
The following image represents a tri-Bahtinov mask I created for a 75mm lens with BF=200, using the 3rd order spectrum. The slits are ~0.6mm wide, which sits almost at the limit of 3D printing.