Originally posted by thibs
c'mon, considering their aperture, the FA Limited ARE compact.
As for pancakes, FA Limited were never supposed to be pancakes AFAIK.
The 43 is relatively compact, but heavy. The 31 is quite large. The 77 is on the smaller side for a 135 near-85mm lens, such as the M85/2. But it is hard to compare film era lenses to digital.
The DA 21, 40, and 70 are true pancake lenses and very lightweight. The 15mm gets as close to a pancake on APS-C as one could hope for an UWA. The DA 35 Macro is not really a pancake, but pretty small compared to the competition and other macros in particular and designed to take advantage of in-body SR. It is designed to macro resolve.
The FA's are of a standard file-era prime design template. They are AF lenses with large apertures and helicoids and prices. Same for the FA 24/2 and FA*85 (I've owned the former). The 50mm's are pretty compact, but they are for every brand. It is just inherent in optical design for 135 and APS-C that from ~35-70mm you can get a fairly compact design with reasonably fast apertures.
But, if you you want faster glass at wider or longer FL's you will get much larger and especially heavier lenses. Frankly the DA 21 is not a f/3.2 piece off glass but more like an f/4 (where it gets nicely sharp, and is stellar from f/4 to f/5.6).
These are all just inherent design trade-offs. Obviously Pentax looked at future sensor ISO performance and the trend towards smaller cameras and and went with screw-drive primes of a very small size and mass without super-fast apertures. They have superb flare control, focus very fast, are great across the frame, have minimal coma and distortions, and are easy to carry. Mine all fit in a pencil case. To get there you lose some shallow DOF headroom but get durable build and quick-shift focusing (likely at the expense of WR).
The DA 35/2.4 and DA 50/1.8 also continue this philosophy of being very lightweight and compact, especially compared to Canikon and Sony. The Pentax design philosophy is more about size and weight and less about maximum fast aperture. If you have a grip about this, switch brands.