Originally posted by HoutHans
Bahtinov masks work brilliantly, but not that good on shorter focal lengths. The number of slits you need depends on the focal length (try one of the online mask generators) and for such short focal lengths you'll need a very fine mask, probably out of reach for a diy solution.
I have created Bahtinov masks and the fineness of the slits that give ideal results really gets fine with short lenses so at the 16mm focal length I think you would need some very good machine tools to make one. A little under 1mm is about as fine as one could reasonably expect to create and that is the size you would want for a 300mm lens for good results. I have a 3D printed mask like this for the 300mm lens and I can use it on shorter ones but it is almost completely unusable below 100mm and at 50mm I end up with 7 very faint very tiny very close together dots that are almost on top of each other so I am at best making an educated guess on focus correctness. You may be able to do a laser print of one fine enough onto a transparency but I'm not sure that will work all that well but it would be better than nothing
I have used
this generator in the past to create a SVG file and to convert it to a STL file for 3d printing
I used this to extruded. In looking at the generator the slit sizes for various lenses I see:
500mm - 1.7mm
400mm - 1.3mm
300mm - 1mm
200mm - 0.7mm
100mm - 0.3mm
50mm - 0.2mm
35mm - 0.1mm
28mm - 0.1mm
16mm - 0.1mm
It looks like the generator has a resolution of .1mm and we hit that limit at 35mm but extendign things out you would probably want a slit size of something like 0.05mm for a 16mm lens.