I think Astrostu is asleep so I'll link you here:
http://www.thephotoforum.com/forum/beyond-basics/147712-lunar-moon-photography-guide-astrostu.html
This is Astrostu's very detailed and excellent guide on luna photography. If you have any questions after reading the PDF in that post, then read the PDF again. There's nothing he doesn't cover
Thanks for the plug, Garbz. You'll get your kick-back next month ...
I actually read this post a day ago (when there were like 4 replies) and I didn't think my guide would cover what he wanted to do. I inferred he wants something like these two:
The top is the crater Bullialdus within the mare Nubium and Humorum. The bottom is the famous crater Tycho in the lunar highlands. I wish they were sharper, but such is the seeing here in Boulder (sucky).
However, now it seems like he wants photos like this:
If that's the case, then yeah, my guide will help with the technical aspects, but it doesn't recommend a focal length. What you (the original poster) really needs to do is specify what you want to accomplish. What's the smallest feature you want to be able to resolve (see) in your photos? If you want to be able to discern Tycho crater in your photos (it's around 85 km in diameter, I think), then that would mean that Tycho's at least 4 px across. With the moon having a diameter around 1738.1 km (equatorial radius), then the moon itself needs to be 4*1740/85=82 px across. That's fairly small.
In the above full-moon image, Tycho (the crater near the bottom with the ginormous rays) is about 20 px across (I just counted). To get that, you want a moon that's 20*1740/85=410 px across. That's about 5.7" at 72 ppi, which is smaller than that 8" moon in the above photo. But let's work with it.
Now let's say your camera is 8 Mpx. That's usually a vertical resolution of about 2300 px. So you want the moon to be about 18% (let's just round to 20%) of that field. That means you need a vertical field of view of 0.5°/0.2 = 2.5° (the moon is about 0.5° in diameter). That will then require a lens that is about 550 mm in focal length. (Calculated by using field of view = 360/pi * atan( (mm size of sensor) / (2 * (focal length of lens)) ).)
Maybe now you're getting the idea of why I didn't reply right away ... I'm sure I'm giving you much more information than what you wanted, but that's basically because you are not asking a very clear question. "I would like to be able to take photos of the craters on the moon" can mean very different things.