Monday, October 14, 2013

Stopping Down and Stacking Moon Photos: Experiments at f/5.6, f/11, and f/13

Recently, I came across a site that talked about how stopping down the f-stop can help increase sharpness. I wasn't sure if this applied to astrophotography or lunar photography, so I asked the r/astrophotography community on Reddit. I got some helpful replies back that talked about the "Lunar 11" or "Looney 11 Rule," and a specific suggestion for stopping down 2 stops from wide open (so from f/5.6 to f/11 in my case).

sample moon photo at f/5.6 f/11 f/13
Summary sheet of my moon f-stop experiments with Canon Rebel XT

I gave it a try last night, and took a wide variety of exposure times at f/5.6, f/11, and f/13 to compare. I actually took test shots at every f-stop to experiment, and chose the best settings to go out and shoot stacks for my test.

variety of moon exposure photos
127 moon photos on my computer with different combinations of f-stop, ISO, and time

After selecting the best exposure times and f-stops, I went out to take a series of images to stack at each setting. I auto-focused on the moon in between each exposure change. I took 30 consecutive frames of each and loaded them into Registax. I manually chose 5 alignment points on each set, and used the same alignment points for each. I adjusted the first Wavelet slider to exactly 18.0 for each one (I don't know what the Wavelet units are, but if you use Registax you know what I'm talking about). I didn't do any post processing (other than stacking), so that means no additional sharpness, contrast, or color changes after using Wavelets. I tried to be as consistent as possible for a fair comparison!

Conclusion: I can see a little improvement after stopping down to about f/11 or f/13. I think the differences would be more noticeable if I had higher resolution photos (I'm shooting at 8 megapixels). It's really hard to tell any difference - but then again it doesn't take any extra work to shoot in f/11 rather than f/5.6 except I've sorta already learned the right kind of settings to guestimate ISO and exposure time at f/5.6 so I'd have to re-learn it at f/11. Sigh. This is a lot of effort for a little improvement, but I guess that's the name of the game!

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