Originally posted by Bui Is the F version also plastic? At least it looks so in my eyes. Also I知 wondering why the F, FA and DFA WR have the same optics, but the latter is much more compact and lighter, yet still offers more features (WR, quickshift, hood)?
The F and FA are quite metal, and of a very good build quality.
If I remember correctly, the FA 100 and DFA 100 have a different optical formula, the FA 50 and DFA 50 the same.
The DFA 50 and 100mm (the ones with aperture rings), are very plastic and built with double heliocoids, which keeeps the design compact, but as drawback an extra increase in play.
If you notice problems with these plastic lenses will depend on what you use them for.
If you shoot flowers and insects, or have an APSC sensor size, the drop in image quality goes probably unnoticed.
On my K1 and using it for reproduction, it is quite obvious, especially the portret mode shots show the most drop in resolution out of center.
Putting the camera on a reproduction stand, with the lens pointing down, so gravity does not pull the optics out of alignment give the best results.
The problem is not unique to the DFA 50 macro etc. Lots of optics with a lot of play in them suffer from the same problem (remember the DA 16-45 ?
Pentax DA 16-45mm 1:4 ED/AL review: Digital Photography Review)