I have addressed the night focus bugaboo before. It was a singular topic of a post (I can’t focus at night) and it has been a running thread including a sub topic of problems with long exposures.
But I finally broke down and tried a strategy invented by astronomers, specifically a Russian astronomer named Pavel Bahtinov
Here is Jupiter and it’s four moons as seen through a Bahtinov mask in a 2 second exposure. Notice how the spikes are all radiating from the center. If you look closely, however you’ll see that the horizontal central spike is just slightly below the center. This indicates the focus is slightly off.
How do you get the above spikes? You use a mask that is placed over the end of your lens, lens hood (or if a telescope your dew shield). The mask looks like this:
To use you just snap a photo, zoom in as much as possible on the LCD to check the alignment of the spikes. Tweak the focus and repeat until the spikes are all well aligned. Not hard and doesn’t take very long!
You can read my review of this mask here. The cost, including shipping is about $20.
If you’re bold, you can create a template and cut your own mask. I say bold because there is a LOT of cutting needed. I gave up after trying to make just a few dozen cuts in some thin black plastic.
If you’re interested in other focusing techniques at night, you can read Jerry Lodigruss’s column on the subject. It’s geared toward astrophotographers, but don’t let that stop you from applying the principles to terrestrial sources too.
Will it be a little clunky to carry around yet one more thing? You betcha. Especially since you may be able to get autofocus to work just fine on either Jupiter or the Moon. But those bright light sources aren’t always available – and autofocus is not always accurate, either.
The Bahtinov mask should work even if your only source of light is a distant street lamp.
No sooner did I post this, than “Sky at Night Magazine” posted a “how to make a Bahtinov Mask” article.
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