The biggest perigee moon of the year is happening this weekend and I'm planning to shoot it. I've taken some simple shots of the moon in the past, but this time I'd like to do it right. My plan is to incorporate the silhouetted branches of a longleaf pine (the subject of an ongoing project of mine) in front of the moon. I realize I need to be way back from the moon to get them even remotely close to the same focal plane (within the lens, not the universe!), but I have some questions:
1. Roughly how far from the tree do I need to be, assuming I'm shooting at 300 (for a wider shoot) or 600mm (300 f/2..8 IS II + 2xIII)?
2. Forgive my extreme ignorance on the subject, but I assume the so-called moon illusion that makes it appear larger near the horizon is just a psychological phenomenon, not something visible in-camera, right?
3. How much does humidity degrade this type of shot? It looks like it will be somewhere between 65-85% , which is actually a bit low for this time of year.
Any advice you might have is appreciated
Without question, use the longer scope. The moon is only about 30 arcminutes in size, and even at 1200mm (600/4 II + 2x), it still doesn't fill the entire frame on my 7D (1200mm vertical AoV, the smallest AoV on a 1.6x APS-C sensor, is 43 arcminutes so still lager than the moon.) You want as many pixels as you can on subject so you can maximize detail.
Humidity is hardly a problem unless it fogs up your lens. If you can keep a very low power blow dryer on hand, you can combat fogging due to humidity.
The biggest issue is seeing, or atmospheric turbulence. That is going to be the single largest factor that will affect your ability to resolve good detail. Seeing ranges from extremely good (practically no turbulence at all, details from distant stars come through so well that you can clearly see the airy disk diffraction pattern under high enough magnification) to utterly horrible (you can't tell that a star is actually a round object, it looks like a boiling spotted blob that is about 5-10x larger than the star really is). This seeing affects the moon...if you zoom in really high in live view at your longest focal length (i.e. use the 600mm option for sure), you can actually see a rippling effect along the edge of the moon. That is atmospheric turbulence warping the edge.
With a long focal length, you can kind of "cut through" some seeing and resolve enough detail to be useful. The other option, which is what a lot of high res moon imagers use, is to take video frames of the moon, at a high frame rate, and stack them together. That averages out the turbulence, and you end up with mostly real detail. There is the potential for artifacts to occur from stacking like that, glows or halos around mountains (where the bright reflection of a mountain peak might be framed against deep shadow behind), but overall, using video allows you to get much better detail of well lit areas than a single frame (unless you have a LOT of focal length, and a lot of skill.)
Regarding the size of the moon at the horizon. There might be a very slight amount of refraction enlarging it, however overall the apparent size of the moon at the horizon, vs. when it is overhead, is primarily a perceptual thing. If your aiming for detail, you want to image when the moon is overhead...preferably at the Zenith, but within the overhead 45 degree angle of view where atmospheric thickness is thinnest (minimizes the impact of seeing.) Imaging the moon at the horizon is bound to greatly increase seeing issues, so it isn't good if you want to resolve detail.
If you want a nice, moody moon shot, the best time is when it's on the horizon...just rising, with a really long focal length, or shortly after it has risen with the moon partially obscured by clouds. The moon at sunset, with pink, red, and orange clouds around it, is nice as well. A crescent moon with foreground trees silhouetted against it is another nice moody shot. I've been photographing the moon for years, and have photographed it in almost every scenario possible. The biggest factor, really, is focal length. You want to get up to 1200mm if you can (for APS-C), or even 2000mm (FF) to really pack on the detail. At 300mm on APS-C, the moon is actually going to fill only about 1/50th the area of the frame...it's going to be pretty small. At 600mm on APS-C, the moon is going to fill about 1/12th of the frame.