Sigma AF 20mm f/1.8 EX DG Review

General
The Sigma AF 20mm f/1.8 is a full-frame compatible wide-angle lens (we're in ultra wide-angle territory, in fact), that on paper appears as a very attractive solution when you're on a budget. 20mm and f/1.8? Sounds great, right? Is there a catch? Well...Read on.


Pros/Cons
+ good value. 20mm, f/1.8, for not much money
+ mechanical quality. Sturdy, it inspires confidence.
+ aperture ring (=can be used with manual film cameras, too)

- poor image quality wide-open (but read more on 'Final Verdict')
- AF/MF clutch is nice, but not as nice as instant override
- corners never get great, even stopped-down

Intended Users
Great for:

• if you need 20mm and f/1.8 and you're on a budget, this is pretty much your only choice
• low light street photography - when it's dark, corners won't be as visible
• live music/performance photography - again, if the light is low (and you can get close to the action), corners don't matter.

Not for:

• landscapes in good light. The corners are not great even when you stop down. 
• overall, highly textured scenes. The center is quite good between f/4 and f/8, but not as good as you'd expect from a fast prime.
• if you can give up autofocus, you have many better options

In low light, corner performance becomes rather irrelevant

Final Verdict
I really wanted to like this lens, but, it's a so-so situation. Basically, it boils down to one thing: scope. If you plan to use this as a landscape lens (and, let's face it, many prospective buyers would want to do just that), this is not a great option. With highly detailed textures and in good light, you would be much better served by an older AIS prime - such as the 20mm f/2.8. It's manual focus, it's over a stop slower, but once stopped-down, it behaves well.

However, there is also another possible audience for this lens: live music/performance (from just under the stage) or street photography in low light. The dynamics here are different. Not only do situations like these often require autofocus, but low light means the corner performance will be mostly masked. If you recognize yourself as someone interested in such photography (and you are on a budget), then I think the Sigma 20mm f/1.8 is worth a try.