I have two very different 20mm prime lenses in my collection.
The first is a Nikon 20mm f4 manual focus lens. This lens is known as the “Galen Rowell” lens as it was a favourite of the noted landscape and wilderness photographer of the same name. The best thing about this lens is that it is absolutely tiny. From lens mount flange to the front of the barrel it is only 36mm long – less than an inch and a half – and looks dwarfish on a D700. How I wish my D90 would meter with manual focus lenses because, if it did, it would live on there permanently.
My particular lens is a ‘K’ type non-Ai lens, made around 1976, that was later converted to Ai.
Lens performance is solid. At f4 its sharp across the board with little light fall off. This is unusual for a wide angle, but probably due to its conservative f4 maximum aperture. Sharpness gets better stopped down, with peak performance at f8 and f11.
|Nikon D700 with Nikon 20mm f4 at f4, and at 30cm minimum focus distance|
The other 20mm lens I have is a Sigma 20mm f1.8. This is one of the fastest wide angle lenses you can buy and allows you to hand hold in extremely low light levels. However, it is big, about the same length of my Nikon 18-135mm DX zoom, but a bit wider around the barrel, then protruding out to take huge 82mm filters. Some reviewers haven’t given this lens a spectacular write up, with soft corner performance (true), BUT it does have one very distinct ace up its sleeve, and that’s close focusing. It can focus to less than 20cm away from the film/sensor plane, which means at the closest focus point, objects are only 7cm away from the front of the lens. With f1.8 and fast aperture, this can give spectacular results with out of focus blur for a wide angle, which certainly cannot be repeated with the Nikon.
|Nikon D700 with Sigma 20mm f1.8 at f1.8, and at 20cm minimum focus distance|
So, I decided to do a side by side comparison test, both at maximum aperture and minimum focus distance on my D700. The differences are stark. The Nikon lens is not particularly close to the subject, but with the Sigma you’re right on top of it. In fact, this shot wasn’t quite at minimum focus distance as the petals were already about to touch the lens. Sure the bokeh isn’t creamy smooth, but it’s not an 85mm f1.4 either. Worth noting though that the centre of the flower centre is still sharp.
So, for these reasons, both have a safe home in my lens drawer, even if it might look like I’m doubling up.