I have had the Nikon 50mm f1.8 (the old version) for a few years now and I never really used it much. I had bought it for my D90 but I was never really happy with the images from it. Plus, when the 35mm 1.8 DX came out I found myself using that far more as it was a better all purpose focal length on a DX (cropped) format camera. I just stepped up to a D700 however and it was the only fx format Nikon lens that I had so I had no choice but to give it another go. I was quite surprised with the results. For some reason it's much better on a full frame camera then it is on the D90. As with most cheaper lenses it's a bit soft at maximum aperture. At 1.8 it's a bit noticeable and you get glows and ghosting around highlights, but at step it down and it gets better. At 2.8 it's very sharp. Way better than I was expecting in fact.
D700 50mm 1.8 AFD ISO 200 f/2.5
As you can see contrast and sharpness are really good with some nice natural fall off that gives the image a sense of scale.
Colour rendition is not an issue either although I've found that you can get a blueish tint in some lighting conditions. Wide open there is a degree of fringing too. What is noticeably lacking though is chromatic aberration. There is some very slight CA at 1.8 but once you stop down it's more or less gone. Not bad for a very cheap lens.
D700 50mm 1.8 AFD ISO 200 f/2.5
Depth of field is great, as you would expect on a full frame camera. This shot again was at 2.5 and there is still great separation with the background, even stopped down, while foreground is sharp. Focussing is reasonably fast, and pretty much bang on.
I could go on and on about charts and scopes and graphs showing fall off, but I think that you'll get a far better impression with some pictures instead:
D700 50mm 1.8 AFD ISO 200 f/4
Note that the vignetting on this was added in post for creative effect. There isn't major vignetting on this lens, and it's quite pleasing when there is. It's not overpowering by any means.
D700 50mm 1.8 AFD ISO 200 f/2.8
So, Again you can see the lovely fall off here and nice, pleasing Bokeh
Again, a nice smooth image. Not bad for a very cheap lens.
The question arises then, how does this compare to the new one. Unfortunately I don't have one to compare it too, but from what I've read it's better wide open, which is this lens's Achilles heel. On the other hand the 50mm 1.8 AFD is cheaper and if you keep it at f 2.5 or higher it will be fine. If you need a FX lens and don't have much money (having spent it all on a camera), you can probably pick one up second hand for under $100. It's certainly a descent starter lens, and I'm very picky about image quality.
As for me I won't upgrade to the new 5mm 1.8, but I am considering stepping up to a 50mm 1.4.
