When the new Nikon AF-S 20mm 1.8G was announced (only a few weeks prior to Photokina 2014) it created quite a buzz on the Internet. And - as always with announcements nowadays - the first impressions were quite biased. Many (include myself) expected the price to be way higher, but therefore the optical and mechanical quality to be more in line with the 24/35/85 f/1.4 primes. What Nikon gave us instead was an unexpected addition to the highly regarded (especially in terms of price/performance-ratio) line of the f/1.8 FX primes, where we can now choose between 20/28/35/50/85mm focal lengths. The 20mm is not only the most recent but, also the most expensive of the bunch. Still, it is definitely more affordable than the aforementioned f/1.4 primes. Before the 20mm was released, the Nikon AF-S 24mm f/1.4G was the wide angle lens of my choice in the 20-28mm area. I actually bought it on "day one" since the day after the release I was going to go on vacation and I figured that would be quite the opportunity to "test" the lens.
Since I have used the 24mm quite extensively over the past few years and many of my favorite photographs were created with it, I am also going to show some comparisons between the 20mm and the 24mm because my guess is many folks that are/were in the market for the AF-S 24mm f/1.4G (but didn't pull the trigger yet because of the high price tag) may also be interested in the AF-S 20mm f/1.8G. But first: what do you need a fast wide angle prime lens for?
What is often read in the context of fast wide angle lenses is that no one needs them, because landscape and architecture work is always done from a tripod and stopped down (as was the shot above). To me, this is only partially true, because there are especially two types of photography where a wide focal length combined with a fast maximum aperture is somewhere between favorable and necessary.
One type is called "landscape astrophotography" or "astroscape photography" where you take a picture of the sky, most of the time including the milky way and some landscape in the foreground in just one shot. To get an impression, here are two shots taken with the 20mm 1.8G in this context:
Most important for these kind of shots is, how well the coma is corrected with the lens. To get things out of the way first: no, the coma correction is not perfect with the 20mm f/1.8G, but: it is not with any of the fast wide angle lenses available. Here is a comparison between 100% crops (lower left extreme corner) of the 20mm f/1.8G @1.8 and the 24mm f/1.4G @2.0 - the shots were taken using a tripod from the exact same position, so the content inside the frame is not identical because of the different focal lengths:
The second type of photography where a fast wide angle lens shows its strengths is often referred to as "environmental portraits" but to me anything qualifies here, where you have your (small) subject close to the camera but still want to include a wide landscape (or something like that) in the background:
I won't give you a conclusion on what lens to buy (by the way: for the time being I will keep both, the 20mm f/1.8G as well as the 24mm f/1.4G). Decide for yourself what's best for you by the pictures you can find here or anywhere else. But honestly: if you already have any of the recent (and very good) wideangle zooms and you don't need the fast aperture of the 20mm, then just don't buy it. The differences stopped down (especially compared to the very good AF-S 18-35mm 3.5-4.5G) are minor in my opinion. If you have any questions feel free to contact me via my flickr-profile.