I've been using the DA21 MTD (not the HD version) for a while. In the recent years, it has been used for principally one single purpose : night photography, especially cityscapes. And for that it shines, so much so that it's my go-to lens for such applications.
However, I recently acquired a K-1 and was wondering if there was a full frame compatible lens capable of delivering similar results while taking advantage of that high resolution sensor. The 21 has superb colours, good sharpness without being the "best", some distortion that's hard to note in real life, but mostly it creates superb starbursts. That's its main perk:
I chanced upon a FA28mm and decided to acquire it and compare both. Considering that the DA21 is APS-C only, I set my camera to APS-C in order to compare apples with apples. I looked at resolution, starbursts, and a few other things. Here goes (right side is always DA21). Click for high-res.
Focus point crop at F3.2
Same at F8
Focus point crop at F5.6 (different frame)
Near the focus point at F5.6
Out of focus area at F3.2
Center of the frame, wide open for both lenses
Same at F8
Bottom edge wide open for both lenses
Same at F8
Corner wide open for both lenses
Same at F8
The conclusion of that short series is that, apart from the focal length, there isn't much to differentiate the two lenses. They are quite comparable, which shows how capable the 28 is. Based on this, either could serve me well.
Now for some starbursts. Here is the scene I used:
Here is a crop at F4:
Smaller crop at F4
Same type of crop at F8
Disregard the fact that the 21 was dusty (which I saw after the test).
While the 28 performs decently, the DA21's advantage is pretty clear. In particular, the odd number of aperture blades helps create more defined rays of light.
The DA21 is smaller, lighter, has a semi-built-in hood and a metal body. It has quick-shift. The FA28 is still very well made in itself and both are durable.
This comparison makes deciding difficult. I haven't found a lens compatible with FF which delivers the kind of results the DA21 (and DA15 for that matter) can offer. I have heard that the F28mm is not as sharp as the FA28, but that its 5 aperture blades deliver pleasing starbursts. It's mighty rare, however (in fact, the listing I saw was for the F28, but I got the FA. No complains of course). The FA31 would yield the same FOV as the 21, but its 9 aperture blades mean that the aperture is more or less a circle, so no starbursts there.
I'm interested in opinions!