For an experiment, I asked Clinton (chunky) to come up with a down-scaled Bahtinov mask that I could slip onto a 50mm daylight filter, so that I could focus my DSLR's standard lenses properly for widefield. The plan was to be able to loosely screw the filter/mask on to my lens, focus, take some 3-sec shots (oh for live view) and then remove it after achieving focus - all without losing the focus I'd just attained.
He obliged (thank you Clinton) and sent me a laser-cut sticker cut to 51mm OD in double-time by post. It just sits inside the rim of a standard 50mm filter and is adhered to the glass. I removed the backing paper and kept it - I made another mask in its own right with it. As another experiment, I cut out and continued the slots towards the center of the backing paper to eliminate the central obstruction and see if it makes a difference - which is all just development!
Both worked fine - the original as well as the re-cut version. I had the filter on my 105mm Nikkor f/2.5 and obtained the following focus patterns. Once I had it in perfect focus, unscrewing the filter carefully didn't rotate the focus assembly so it stayed put. Lovely.
Being such short focal length, the centrally-obstructed mask is quite noticeable on a 105mm lens (dim), but less so on the 50's and down. A secret is to open your aperture right up to ensure that the iris in the lens is lrger than the obstruction.
One thing for Tak owners - the 50mm Bahtinov filter mask fits on the front of your Taks "man hole cover" of a lens cap. The removable centre step-down aperture accepts a 50mm filter and is threaded to suit. I didn't know that until now. This means that a 50mm Bahtinov mask mounted in a filter should do both your lenses and telescope!
I haven't tried it yet, admittedly, but i can't see why it won't work.
The diffraction pattern is chunkier than normally seen through a device with longer focal length, but still very recognisable. Being that these masks are designed (optimally) for a fixed focal length, I think it goes to show how flexible the design is, in that they manage to work at all over a range of my lenses.
Pic 1: The mask mounted in a 50mm filter, about to be attached to my 105mm Nikkor prime and 350D.
Pic 2: In focus
Pic 3: Out of focus
50mm DSLR Bahtinov mask
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