Since this thread is about primes, wanted to share a few more examples that I took tonight. The first series is about DOF and cropping. The second series is about how focal length influences composition. Will end with a couple of comments about lens suggestions.
As in my previous post, all of these taken with a full frame camera (Nikon D810) on a tripod. ISO set at 64. WB normalized for all examples. All of these on a full frame sensor, none cropped in camera for a crop sensor.
Series #1: different focal length lenses all taken from the same spot and then cropped in LR to give a similar FOV. All lenses shot wide open. The point of the series is to show the effect of focal length on DOF. For the shorter focal lengths, the subject distance is pretty far away, so with a closer subject one could obviously achieve a greater degree of subject isolation.
35mm lens uncropped taken at f/1.4
Nikon 300mm @ f/2.8
Nikon 135mm @ f/2
Nikon 85mm @ f/1.4
Nikon 35mm @ f/1.4. This is a fairly extreme crop that resulted in significant image degradation.
Series #2: effect of different focal lengths and subject distances while keeping the subject the same size in the frame. I was attempting to show how you can achieve a relatively constant subject size within the frame with different focal lengths by changing subject distance. This was a PITA to shoot and it isn't perfect as the composition changed as I moved the tripod closer as I changed lenses. The composition changes radically as focal length changes (and thus shooting position has to change) to keep the subject the same relative size in the frame. Choice of focal length isn't just about "filling the frame," it's also about the relationship of all the elements within the frame. Notice how the relative size (and thus visual importance) of the glasses in the foreground changes as the focal length gets wider (assuming I wasn't shooting wide open and they were in focus).
Nikon 135mm @ f/2. Furthest away for this series.
Nikon 85mm @ f/1.4 a bit closer.
Nikon 24mm @ f/1.4. Closer still.
Nikon 20mm @ f/1.8. Closest.
Lots of factors to consider when choosing lenses. The suggestion of the 24-70 f/2.8 zoom isn't a bad one--it's a great lens. Bulky though and not as fast as shooting primes. For what I like to shoot, 35mm on full frame is my preferred focal length (which would be a 24mm lens on a crop sensor). Mentioned in a post above and I won't argue.
As I said in my first post (and others have echoed), take some time to figure out what *your* preferred focal length is. Shoot with your kit zooms and see where you tend to fall out. Think about that as the focal length you should choose for your first prime.