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).

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.

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!