Continuing the exercise in StupidCrap™ from last time, let's take a look at the 23mm f/1.4 as compared to the 18-55. You may remember I jumped from 18mm to the 35mm. The reason was that I made the conjecture that the much maligned 18mm f/2 was similar in it's shortcomings to the 23mm f/2 but for some reason it's gets a pass while the 18mm somehow has the reputation of the worlds worst lens (exaggeration). I still don't understand why this is the case. Both are similar in my experience in terms of performance and perfectly reasonable for anything but the most critical applications.
I saved the 23mm f/1.4 because unlike the two just mentioned it's one of the Fuji primes that is clearly in a different class without a whole lot of room for interpretation. This should be pretty quick. Below is a comparison in the center of the frame.
The 18-55mm on the left and the 23mm on the right. With my torture test target you can probably see the clear difference in performance without even opening up the screenshot and seeing all the pixels. These are at f/4. I won't bother to show you the 23mm at larger apertures but the performance is similar, especially in the center. I am shooting crazy close here and larger and larger apertures give a huge difference in lack of DOF.
Next up we'll look at the extreme corners. Unfortunately I flipped the comparison so the 18-55 on the right and the 23mm on the left at f/4.
Even more than the center comparison you can probably see the difference between the two lenses without any sort of careful examination. This is the giant difference I've seen in the larger Fuji primes. The f/2 WR versions and the smaller Fuji primes exhibit a small difference in the center as compared the 18-55mm but in the corners not so much and in many cases a bit worse. That's not the whole story though if you take a look at the MTF charts on Fuji's site in that the smaller Fuji primes (and the larger ones to some extent) are a bit more traditional in fall-off toward the edges/corners where the 18-55mm dips in performance and then gets better toward the corners. I went through that way back when discussing the 18mm a while ago.
Lastly let's take a look at how the focus fall-off looks close up between the zoom and the big prime. On the left is the zoom, the 28mm f/1.4 on the right.
Obviously this would be an entirely different thing if we let the prime lose at maximum aperture but why bother. I've talked to a few people that do not like the look of the OOF areas of the 18-55mm. Myself? Well I think every X Series lens I've used falls on the pretty darn good side of the line when it comes to OOF and transition areas relative to some of the horrors I've seen with zooms and wides. You can find a background that will make any lens look horrible. The examples I've seen that people have sent me look to be anecdotal in the sense that they would probably make the primes look bad as well at a similar aperture and distance. Sure one particular prime or another within the same focal length range might look a little less bad if you look really hard but I doubt it would look night and day. I just haven seen a Fuji X lens that falls on the horrid OOF rendering side of things and the 18-55mm is no exception.
The bottom line is the the 18-55mm is pretty good and has some atypical MTF performance that should be considered (especially wide open or close) vs most of the primes but I would consider it on-par with the smaller primes so far for most picture making scenarios. The smaller primes do seem to exhibit a bit more contrast and bite overall. The 23mm f/1.4 is in a different class and you pay for that with a larger lens. I usually opt for the smaller lens myself but I think I'm keeping the 23mm f/1.4 and going with the X100 series for my "smaller" 23mm option.
One more lens to look at in the 18-55mm range, the 56/1.2 which is next up. Please realize that I'm only testing these well within what anyone would consider extremely close circumstances but that's where I use them. At the end of all this I'm not at all recommending or asserting that the absolute "best" IQ should at all be the factor when choosing what glass you might want. In fact I hope to remove some of the anxiety by showing that none of these lenses are perfect and the 18-55mm is pretty good, so are the small primes even though I think there's been a trend to over-hype some of the smaller cheaper Fuji's when it comes to relative IQ (yes they are pretty good).
In reality it's the size and the faster/quieter motors that make the case for these guys as well as the lower price. I'm not that price sensitive when it comes to any of these lenses as I look at all of them as a bargain compared to the prices of full-frame or medium format glass and I tend to choose the smaller lenses. A good example is the 14mm f/2.8, I've already determined I don't want the 16mm f/1.4 in my wide category I rather the 23/1.4 and don't want anything in the very wide category that's as large as the 16/1.4 is. In fact I use my 18/2 far more than either of the other lenses.