Lenses with a focal length of 21mm and less are generally called "superwide" by most photographers. Rightly so. The step from 24mm to 20mm is a big one. For several years, 28mm was the widest lens I owned. When I acquired a 24mm, the change was easy - in fact, it was perfect (for me). The 28mm had frequently not been wide enough for my taste. As I began using the 24mm, the feeling that I needed something wider (a feeling which I'd had many times when equipped only with the 28mm) simply disappeared. 24mm was easy to adjust to and ideal for me and my shooting requirements for more than 10 years. But then I started feeling I could go wider - hence the 20mm. But with this change, I had trouble adjusting.

Numerically, 20 is not that far from 24. But the difference in angle of view is great. A 24mm lens is easy to handle - few photographers have difficulty making use of it. There's something about going wider, though, that changes dramatically. Twist a 20mm lens on your camera, look through the viewfinder, and suddenly feet begin appearing in the bottom of the frame. Utility wires jump into the top of the frame without warning. Skies suddenly become huge, sometimes featureless expanses. In short, ultrawides require much more care than more typical wide angles. Generally, you need to get very close to some foreground element to make memorable ultra-wide pictures.

If you're happy with your 28mm, your 24mm or your 35mm, don't think you have to get anything wider. Stick with what you're satisified with and shoot pictures. Consider an ultrawide only after you've desperately wished you had one several times.

Once you have an ultrawide, be it a 20mm or a 17mm or a 14mm, devote a few rolls of film to experimenting with your new optical tool. Get to know it before you embark on a long trip. Because ultrawides take a bit of practice.

Now, what about this particular Nikkor?

The Nikkor 20mm f4 is one of the smallest and lightest Nikkors ever produced. In my experience, only the 45mm f2.8 GN was smaller. The tiny size means it takes up very little space in a camera bag, which means it travels with me all the time. It's true that f4 isn't very fast. But the short focal length means 1/30th of a second is a perfectly useable handheld shutter speed, so the maximum aperture isn't really an issue to me.

Like every manual-focus Nikkor I've ever touched, build quality is excellent. The sturdy metal barrel is tough. The lens was manufactured for a couple of years before the AI (auto-indexing) feature was introduced. As far as I can tell from my research, the optical and mechanical qualities of the lens were unchanged when Nikon switched to the AI system, except for the AI aperture ring and lug.

When I use it wide open, sharpness is acceptable. From f5.6 to f11, it is terrifically sharp. At f16, it's smallest opening, sharpness is lessened due to diffraction - but depth of field is immense. Include the sun in the frame, and ghosts (multiple images of the shape of the diaphragm) appear; but ghosting disappears as soon as you move the sun out of the frame.

I love this lens' tiny size, and, as long as I close it down one stop from maximum aperture, I'm mighty pleased with the sharpness it produces.