To put it bluntly, the Olympus 25mm is one of those lenses that you don’t need to worry about sharpness at any aperture. It is very sharp right from f/1.8, across essentially the entire frame. While the center sharpness isn’t as blisteringly crazy sharp as something like the Olympus 75mm or the 42.5mm Nocticron, it is plenty good enough for any purpose you could think of, and this quality continues to the edges.
Stopping down can yield very slightly higher resolution than wide open, but we’re splitting hairs there. Pick the aperture for the depth of field you want or the amount of light you have and let the lens do its thing. Take a look at the image to the left, taken at a relatively close focus distance at f/1.8. Click to enlarge and see the excellent levels of detail this lens can produce at f/1.8.
Olympus has done a relatively good job with the bokeh on the 25mm f/1.8. For the most part, out of focus areas are rendered very neutrally, with smooth backgrounds and evenly lit specular highlights. When focusing further out, backgrounds can start to get a little busy, but this has more to do with the amount of blur rather than the way the lens renders. There is some green fringing visible in the background at times, though it’s generally not too distracting. In all, a good performance here.
I will say, while the difference in amount of background blur isn’t staggeringly different from something like the Panasonic Leica 25mm f/1.4, I did find the 25mm f/1.8 crossed that fine line in my mind where I wished for more blur. This line will come at a different point for every shooter, but often I was wishing for just a little more separation than I could achieve with this lens, especially when shooting a little further away from my subject. I don’t generally feel this way when shooting the Panasonic 25mm f/1.4, so it seems that for me, the line is right in that area.
Color, Contrast and Chromatic Aberration
Overall image contrast with the Olympus 25mm f/1.8 is on a very high level, right from f/1.8. In fact, there’s almost no discernible difference in contrast from f/1.8 to f/4. Likewise, colors are rich and vibrant. Nothing to complain about here in that department.
There is a small amount of lateral chromatic aberration visible in the right circumstances with this lens, though it’s on a low level that is extremely easy to correct. In most cases, I didn’t see any. The lens does display a small amount of longitudinal CA, but even this is quite well controlled.
Distortion, Flare and Vignetting
The Olympus 25mm f/1.8 has a minor amount of barrel distortion that can be corrected easily and will only be visible in the most stringent straight line situations. The lens works well against bright light as well, maintaining good contrast and producing a minimum of ghosting when the sun or another bright light source is in the frame, or just out of the frame.
If there is one weakness to the 25mm f/1.8, it’s in the vignetting department. I’d imagine Olympus sacrificed corner illumination to keep the size small. At f/1.8, there is significant corner darkening, and stopping down only slightly alleviates the problem until you get to the small apertures. It’s not until around f/4 that the vignetting drops to negligible levels. Still, this is generally not an issue for most shooting.