2004.12.12
I was having concerns about how to get proper focus on an autofocus camera lens that doesn't have a hard stop at infinity. I read on this website about building a Hartmann mask for manually focusing accurately to infinity and decided to give it a shot.
I cut a circle out of a cardboard box big enough to fit down inside the lens hood on my 50mm lens and then cut two 5/8" holes in it. I carried this along with my photo gear down to the Overlook last night to test it out and hopefully get a good show of pre-peak Gemenid meteors.
For the focus test, I aimed the camera due east such that the stars would be traveling up. This meant that the proper orientation of the holes in the Hartmann mask was horizontally opposed. I set the exposure time to BULB and turned the manual focus ring all the way to infinity to the point where the internals stop moving (though the ring is free to continue turning). Then I lined up one of the ridges on the focus ring with the "current focus position" marking on the camera and opened the shutter.
I let it expose the star field for about 30 seconds, then covered the lens with a black glove. Then I moved the focus ring one knurl away from infinity (the lens focuses past infinity, so starting at the far stop, I know I want to go back a little ways). I waited 30 seconds and then removed the glove and exposed the star field for another 30 seconds. I repeated this process until I had exposed the stars at 7 different focus points.
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A closeup of the Hartmann mask focus test. The star advances upwards in this image,
so the first focus point was at the bottom. The most in focus star trail is the sixth
one, which means five ring knurls back from the infinity stop.
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I then closed the shutter and viewed the image in playback. As expected, the double star images come slowly together and then begin to diverge again. The sixth exposure (which occurred after the fifth focus adjustment) was the most in-focus. The fifth exposure was much closer to in-focus than the seventh. My best guess was that the infinity focus point could be reached by running the focus ring out to infinity until the ring turns but the helical focuser does not, then continue turning it until a knurl ridge is lined up with the marker, then turn back for about 4.75 knurls. This puts it almost at the 6th exposure point, but slightly on the side of the 5th exposure point. And the knurls on the focus ring are really close together so 0.25 of a knurl is about as much accuracy as I can hope for. I reset the focus to this point and then set up for long-exposure meteor photography.
The odds of getting a meteor in frame for anything short of a heavy rain of meteors are very modest. Reeves calculated a probability of getting one bright meteor on film with a 50mm lens about every 5 hours of open shutter. Well, needless to say, I got lucky. The following was my first exposure of the night:
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A Gemenid meteor graces the very corner of my photo. Closeup in the center for detail.
Clicking on the image will bring you to a larger version.
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Of course I took many exposures over the course of about two hours and this is the only meteor I caught... and it is only in the corner... but whatever! It was actually a very bright and long meteor, and this is only the very beginning of it. But I'll take what I can get. Also, I was quite pleased with the focus results from the Hartmann mask.