Regarding the FA*24: first of all, I don't think anyone's saying that this lens can't produce good images, or even that it isn't a good lens, just that it doesn't qualify as "legendary glass." I agree with that. It is "historical glass," being one of the first mass produced lenses with an aspherical element, using I believe a resin mold technique that never caught on but which gives the lens a bit of warmth. As far as I can tell, cost-is-no-object Leica didn't release an ASPH lens until 2 years after the FA*24 was released. But I'm not even sure about all of that and such details only matter to collectors, so, onward.
While this post talks about the FA*24 in some detail, I'm really going after what attributes make up a fine lens.
Originally posted by Digitalis You can see the dark centred bokeh circles on specular highlights - which is why the FA*24mm f/2 produces such harsh bokeh. As I recall the Japanese call it "neisen bokeh" - meaning double lined blur. The Ideal bokeh disc has a evenly lit centre which gradually darkens towards the edge.The FA*24mm f/2 it works the other way around slightly darker centre - with bright green/magenta fringed edges.
Yes and no. The nisen bokeh is caused by, as you say, bright outlines of bokeh discs, which is overcorrected spherical aberration. But from everything I've seen, the FA*24 has only moderate overcorrection compared to most Pentax lenses, and is no worse than the "reference class" bokeh of the 50/1.4 lenses. I would be willing to wager that the FA 31 adds more color to its bokeh than the FA*24, too, but I went with a red-dot on my 35mm lens so I can't say that for sure.
What the FA*24 suffers strongly from is a contrast between sagittal and tangential resolution. This exacerbates the bokeh outlines for parts of the disc. Here's an example (shot on film).
Look at the disc bokeh in the trees, upper left corner, where you see the sagittal edges (lines from the center to the edge) are brighter but where the tangential edges (lines equidistant from the center) are much better. Together, they create wedge-shaped ugliness, but this shows that the problem isn't purely circular in nature.
The near focus, light color branches are instructive. Toward the center of the frame, there is no double-lined effect, but this arises with some vengeance with the branch extending to the upper left corner (sagittal) about halfway from dead center. Yet on the right side, nearly the same distance from the center, is a branch with what looks like undercorrected (that is, desirable) spherical aberration, but this one is angled away from, not towards, the corner, which is to say that it is positioned tangentially.
I point this out because a legendary lens should have good spherical correction as well as having sagittal and tangential resolution in parity with each other. For example, the FA 43, despite its colored bokeh fringing and lack of edge resolution, has rather harmonious resolution that leads to pleasing rendering.
Originally posted by twitch Here's an example of what I didn't like, a crop (about 60% of the frame) of a FA*24 shot
Curiously, I think that shot shows what I like about the lens. High contrast yet soft. Lends punch to the overall image without pulling the eye away with unnecessary detail.
We can agree, I think, that the FA*24, though not outright bad, doesn't resolve fine detail well enough to be "legendary." That said, the visual effect of an image--which to me is what makes a successful picture--depends on a first impression made by contrast, which tells the story, and only then uses fine detail to keep the mind interested. As the person who was standing behind the shutter when it released, I don't need to be reminded of the broad strokes in the image, so contrast doesn't seem as important; but for someone viewing the picture for the first time, that's the first thing they see. And the FA*24 gives that in gobs.
So, yes, I think Pentax could design a better 24mm lens, and ought to for any 135-frame camera they release. However, given the options for a much larger f/1.4 lens, much slower f/3.2 lens, slower and optically inferior zoom, or many-fold more expensive alternatives, the FA*24 hits quite a sweet spot in the current lineup. What makes the 24 so desirable is how unique it is: there are lots of very good 35s, 50s, and 85s, but only a slow progression in the area of 24s. While I fully intend to keep my FA*24, I would vote for giving us a similar, more modern lens rather than remaking this 22 year-old design.