The X100's Fujinon 23mm F2 lens, being non-interchangeable, lies at the heart of the camera's imaging capabilities. It's a relatively complex design - 8 elements in 6 groups, including one dual-sided glass molded aspherical element - and Fujifilm says it's specifically matched to the sensor, which uses offset microlenses for better light-gathering towards the edge of the frame.
Although we're not able to bring you our usual studio lens test widget in this review, after looking carefully at many hundred of shots we're confident that we have a pretty detailed assessment of the lens's strengths and weaknesses. Our judgment is that it's a very fine lens indeed, capable of resolving a large amount of detail with a relatively low level of aberrations.
At long focus distances, the X100's Fujinon 23mm F2 lens performs very well. At F2 it's impressively sharp in the center of the frame, and this sharpness holds up pretty well to the 'short' edges of the frame. Beyond that the corners get progressively softer, with a drop in contrast and visible smearing due to halation. The corners progressively improve on stopping down; at F4 they're critically sharp. Sharpness then holds up well through to F11; at F16 the image is visibly softened due to diffraction.
This is shown in the rollover below - note that here we're using raw files converted using Adobe Camera RAW 6.4 with no correction for chromatic aberration.
Wide-open sharpness is also highly dependent upon focus distance, decreasing progressively as the subject gets closer. At intermediate distances - say 1-2m, where you're likely to be taking pictures of people - the lens is perhaps just a little soft and low in contrast wide open, but not problematically so in our opinion (it's more than made up for by the ability to shoot indoors without flash, and throw the background somewhat out of focus). Within the camera's 'macro range', images get very soft indeed at F2, with significant halation due to spherical aberration across the frame - we'll look at this in more detail later.
|F2, approx 1m subject distance||100% crop|
The X100's lens shows very low levels of chromatic aberration, and what little there is gets automatically corrected by the camera during JPEG processing. If you process the raw files using Adobe Camera Raw or SilkyPix some colour fringing from lateral chromatic aberration can become visible, but it's easy enough to correct when needed. In the sample below there's a little green/magenta fringing towards the extreme corner of the frame in the uncorrected ACR output, which has clearly been corrected in the JPEG output and can likewise be readily removed in raw processing.
|F8, JPEG||100% crop from camera JPEG|
|100% crop, RAW + ACR||ACR with -10 red/cyan correction|
Longitudinal chromatic aberration (colour fringing in front and behind the plane of focus) can also be visible if you go looking for it, but is rarely any kind of problem in practice. Again the camera's JPEG processing appears to suppress it by desaturating the colour fringing.
|F2, JPEG||100% crop from camera JPEG|
|F2, RAW + ACR||100% crop, RAW + ACR|
Distortion is exceptionally low - you have to go out of your way looking for it to see any at all. For the record, though, there's very slight 'moustache' distortion that can be visible if you place a straight line right across the long edge of the frame - this can be seen in the example below. This sort of complex distortion will generally require relatively sophisticated correction software if you want to remove it completely; the good news is that you'll very rarely need to, if ever.
The X100's lens shows impressively low levels of light falloff, aided by the use of offset microlenses on the sensor. By our measurements the extreme corners are just a stop darker than the center of the frame at F2, falling to 0.7 stop at F2.8 and a completely negligible 0.3 stops at F4. The falloff pattern is also very broad and even across the frame (as opposed to falling off a cliff at the corners), which means you'll very rarely be aware of it at all. This is illustrated in the rollover below.
To put this in context, the Leica X1 shows one stop of falloff at F2.8, and the Panasonic 20mm F1.7 shows 1.7 stops wide open. About the only near-competitor which can match the Fujinon in this respect is Samsung's excellent 30mm F2 for the NX system.