With ultra-wide aperture lenses, the biggest unknown will be wide open sharpness. Most of these types of lenses are brilliant stopped down a bit, but most also suffer from a glowy softness at the wider apertures. The most shocking thing about this lens, is that it is sharp right from f/0.95. The central part of the frame has very nice resolution at pretty much any focus distance from about 2 feet to infinity. To see this for yourself, take a look at the crop at the right (click for full size), which is a 100% crop of the center right of an image taken at f/0.95. Wide open, there is a little lower contrast, and slightly lower resolution that at smaller apertures, but it’s definitely safe to say that the lens delivers at f/0.95. Stopped down to even f/1.4 brings that level of sharpness up a notch, with absolutely razor sharp central region and good corners. Stopping down to f/1.4 also increases resolution quite a lot at minimum focus distance. By f/2.8, the lens is absolutely tack sharp across the entire frame.
Contrast and Color
As mentioned above, contrast is somewhat moderate at f/0.95, but not bad. At f/1.4 and smaller, contrast is outstanding with this lens. There is a vibrant presence in the images with deep, rich color to accentuate the contrast. There is a real beauty to the way this lens draws, at least in my opinion, and I was continually impressed with the look of the images I was getting.
When you’re using a lens with an aperture wider than f/1, naturally bokeh is going to be on the top of your list of key characteristics. The lens is capable of providing very nice subject separation at closer focusing distances, and as with many other lenses, the character of the bokeh changes a little depending on your focus distance. At closer focusing distances, the bokeh out of the Nokton is absolutely stunning. It’s beautifully creamy and still somewhat unique, with some cats-eye specular highlights near the edges, which is common for lenses with really large apertures. The shot at the left is sort of a ‘bokeh painting’ taken at night, with the lens focused to about 1.5 feet. Focusing to MFD yielded one big blurry blob.
At further focus distances, the Nokton displays a busier character that is typical Voigtländer. If you’ve owned any Voigtländer lenses in the past, you know that some of them, such as the Leica M mount 40mm f/1.4, have some very unique out of focus renderings. Many people love it, some people hate it.
At medium focus distances, the specular highlights will start to gain a green ring from longitudinal chromatic aberration, and the character of the bokeh becomes a little more nervous overall. I still like the effect rather well, and it works for me, but take a look at the samples both on this page and at the end of the review to evaluate for yourself.
Chromatic Aberrations and Flare
If there’s one weak area with the Voigtländer 17.5mm f/0.95, it’s with chromatic aberration. This isn’t unexpected with an f/0.95 lens, and it certainly shows up, most notably in the form of purple fringing on high contrast boundaries. Street lights at night, chrome, white clothing against a dark background…all of them will show purple fringing when in focus, and red or green longitudinal chromatic aberration when in front of or behind the focus point, respectively. These can be fixed with software like Lightroom 4.1, though in some cases, you may need to do some bigger cleanup. The degree of CA lessens as you stop down the aperture, and is mostly gone even by f/1.4, though it takes even smaller apertures to completely eliminate it. Still, in field use, I didn’t find that it ruined images, and when shooting portraits or the like in most situations it doesn’t even show up.
The lens is pretty resistant to flare. When a light source is in the frame, there is little to no flare. The worst flare performance comes when a light source is just outside the frame, which can cause a veiling loss of contrast near that edge. Still, very impressive performance overall here.
Field Curvature, Distortion and Vignetting
Aside from some fringing at wide apertures, the Nokton 17.5mm has one other big weakness, and that is field curvature when focused at a distance. Field curvature occurs when a lens has a curved field of focus rather than a flat plane. On the Voigtländer 17.5mm, this exhibits itself rather severely when focused near infinity. If you focus using the center of the frame, the curvature is such that objects between 5 and 20 feet on the edges will be sharp, but further objects get soft. If focusing on a distant landscape, for instance, the edges of the frame will be soft, even when stopped down, where the near field objects are very sharp on the edges.
This didn’t really appear in closer shots, even when shooting a flat subject like a wall. At closer distances, the field curvature is negligible, and the lens is razor sharp across the frame when stopped down a bit. I did find that you could minimize the impact of the field curvature when your whole subject is far away by simply using one of the edges (about 1/4 of the way in from the edge) to focus. This kept the edges sharp and the depth of field kept the central region acceptably sharp as well.
There is a little bit of barrel distortion, but nothing that is generally distracting. There is one sample image on the next page that appears to have heavier distortion, but it’s an illustion caused by a pole that is actually leaning. The lens exhibits very noticeable vignetting wide open and even at f/1.4, but I actually enjoy the look it lends to the image. It disappears in general field use by about f/2.