Originally posted by normhead Most of the improvements in better glass is better control of CA and purple fringing... if it does that, it will be better and definitely worth buying for some. With my older glass, if I get purple fringing on a bird shot, no problem, the birds will back at the feeder tomorrow.
But I've never owned a lens that never gives you CA or Purple fringing (well, maybe my DFA 28-105 but I haven't used it that much yet.)... so is that even a thing?
From what I see resolution isn't increasing much, but better control of CA should produce much better micro-contrast.
I think, the moment you tell a lens designer he's not allowed to go beyond, say, a 52mm filter, you've limited the amount of glass he can put in to bend the light enough to get the scene in without distortion, to keep up resolution (especially away from the dead centre), the maximum aperture, reduce coma and control aberrations both chromatic and spherical.
It's why I have both the DA21 and Samyang 24mm f1.4, for instance.
Now, with the FA*85 vs the upcoming DFA, the old lens had a 67mm filter size and is about 600g, with screw drive, no weather resistance.
This DFA will be a modern, big lens. The Sigma equivalent has an 86mm filter size, nearly double the weight. Compared to the old FA*, it also has an aspherical rear element (my guess is one moulded/ground part, not hybrid) and two elements with rare elements in the glass to lift their light bending power.
This is what you want to reduce your purple fringing.
IQ aside, the new DFA should have a fast in-lens motor and be weather sealed. It'll be a very different animal from the FA*85.
No one says you have to buy it, the whole Kenspo vs Normhead thing.
I've got an FA 77 Ltd already, for instance, which is like a cut-down version of the FA*85. (Hirakawa had previously designed the A*85, IIRC)