Sigma AF 20mm f/1.8 EX DG ASPHERICAL RF

Quality low-light, wide-angle, full frame lens. If you want a fast aperture (f/1.8), wide-angle (20mm), full frame sensor capable prime lens you have virtually no other options under $1,000. Quality build--a bit heavy with a fully mechanical auto focus/manual focus (de)coupler. Feels solid holding and focusing. Fast auto focus. OK sharpness wide open @ f1.8. Good center sharpness @ f4.0 and up. Low distortion and CA. Extreme close focus ability with decent bokeh. Get to know its limitations and you can excel at photo taking with this specialty lens.

Surprising Sharpness Considering Its Reputation I bought this as an additional secret weapon for weddings and industrial hand held, low-light shooting. This is the latest version of this lens (there is an older version that is not "DG"). This lens is either loved or hated apparently due to the Sigma curse of poor quality control. I've been lucky with Sigma's so far, but I've had friends that had to send their lenses back. To Sigma's credit, they always seem to return a well adjusted and properly operating lens. This copy is pretty soft in the center at F/1.8 which is about what you'd expect. However -- and to me inexplicably -- just a 1/3 stop closure to F/2 clears things up immensely. I've shot it on my D200 and my D7000 and it works well on each, though I had to dial in correction for a bit of back focusing which is only really apparent on very shallow depth of field shots. Shooting it on DX (1.5x crop, the edges are soft until you start to hit F/2.8 or so and not really getting sharp until F/4 -- but by F/5.6 the lens is extraordinarily sharp. There's little out there at this wide an angle that is as affordable as this lens. For many shots, that gauzy softness -- particularly for weddings -- might be actually usable. Some carefully applied masked sharpening can work wonders to make a nice looking shot. Soft focus and noise clean up better than motion blur. One of the wonders is how close this thing focuses. This is one of the lens strengths/weaknesses. Some said the lens just wasn't as sharp when wide open at close focus as it was nearer infinity. My copy seems to be the opposite of that. Wide open shots at f/1.8 a few inches from the lens appear sharper than the shots further away. Could be an anomaly and it's not dramatic but that's what I think I'm seeing. Overall, a surprisingly useful lens at F/2 where I will choose to shoot it for very low light performance. This lens, paired with the good high-iso noise of my D7000, can make some shots possible that I normally would not bother with knowing that there would be no usable quality. Although it seems these lenses can be a "crap shoot", I recommend getting one if you need something very fast and fairly wide. For this price you can certainly find something you can use it for when you have no other options. If, like me, you have old Nikon film cameras then you will like that this lens has a real aperture ring. Since the lens is full-frame compatible it can be used as an ultra wide lens on full frame and film cameras -- through frankly with it's edge sharpness issues on DX I would think using it on film/FX would require careful framing and limit your applications at large aperture. My version is for Nikon AF and I presume the other versions are similar in this respect: there's no motor in the lens. It is entirely dependent on the focus drive motor in the camera which means slightly slower focus performance and a bit more noise. Still, I don't consider this lens excessively noisy or slow but if you're used to a professional AF-S or USM style lens this is a step back. Not bad for a motor driven AF lens from Sigma though. A word on build quality: this is a Sigma EX (top-of-the-line) lens and it feels solid. Certainly it's huge for a prime with its nearly 4" length and 82mm filter. It has the hokey focus clutch like a Tokina which I'm not fond of, but I don't focus this one manually anyway so who cares. Just make sure the focus ring is pushed forward towards the objective when you use it.

Surprisingly good for the price! Very bright lens at this focal length. Surprisingly sharp, even though the edges lose some sharpness. So, it may not be the best lens for film or full-frame digital cameras, but with the crop factor of many digital SLR's, the edge sharpness in negligible. Still, given the price, this is an excellent lens for all formats. One thing that could be better is focusing speed. It's kind of slow, so I recommend manual focusing, especially in low light conditions.