I think for APS-C lenses, it is high possible to build 16-50 F1.8/50-150 F1.8 in the same weight of a FF 24-70/70-200 F2.8 lens. The only reason the manufactures do not have make them is that they thought APS-C camera market is low/middle-end markets, which targets typically do not want to spend $2000 on one len.
Sigma made the first step to deliver 18-35 F1.8.. The most important reason they made 18-35 instead of 16-50 is to control the price& weights, otherwise it will be a $2000+ monster where the APS-C buyer will merely spend that much money on a single len.
At the prime side, Pentax for sure can deliver some fast APS-C lens such as 24mm F1.2/35mm F1.2/50mm F1.2/85 F1.2 (their size will still be smaller than FF lenses as 85 F1.2 APS-C only lens will surely be smaller than a FF 85 F1.2). The price will also be lower and at the $1000-$1500 level instead of $2000+ level.
However, it is just a dream as Pentax apparently sticks with the portability on APS-C region and does not have strong motivation to challenge itself due to small market share. And as far as we know, lot of pentax prime lens are in fact ff compatible lens, which simply means that they are lazy to design as a lot of lights are wasted!
To be more precise of this above idea, I just pick up Fuji as example: Fuji has a new released
XF 56/1.2 . It is a prime portrait-length lens that provides a 35mm-equivalent focal length of 85mm. Comparing to
1025g and $2200 pricing for Canon
85mm 1.2, Fuji 56/1.2 is $999 (new price) and only 405g. Apparently APS-C 85mm 1.2 can be much lighter and weighter than the FF equilvalent under the same max aperture. Fuji has already released
23/1.4 (301g, $749 in BH) and 35/1.4 (184g, $449 in BH). In Pentax's side, a close look up of prime lenses gives
21/3.2(119g, $696 in BH (HD version)), 35mm/F2.0 (196g, $399 in BH) and *55/1.4 (376g, $796 in BH). So in princple, pentax can easily make
21/1.8, 35/1.4 and 55/1.2 with slightly increase of weight and
maintain at the same price range. Apparently pentax looses the ground for portability comparing to mirrorless (MF, APS-C and FF) as the world of digital photography is evolving. For example, the full load weight of
Fuji X-T1 + 23/1.4 is
740g while the pentax
K5-ii (I will not use K-3 here) + 21/3.2 is
859g. Many young peoples, who love portability but have no preference between DSLR and mirrorless, will not consider the pentax. If Pentax/Roich will stick to APS-C line, they should seriously consider how they can hold its own place under FF and mirrorless. It is a possible crisis to Pentax. One of possible reasons is that they have anticipated that FF will gradually dominate even
the consumer level DSLR market, (which is highly possible to occur in next 3-5 years), they have to put their resource and priority to prepare and release FF or FF mirorrless camera/lenses.
Okay, the topic moves to Pentax Full frame... :-( Anyway, my point is that the whole prime lens of Pentax are FF competitible more or less, it is a huge waste. Even if Pentax released FF camera, they should seriously consider release fast but inexpensive APS-C lens (APS-C version 30F1.4, 55F1.2, 85F1.4 et al) . If they can control the prices below the current ridiculous high HD DA lens, there will be a really sale boost and benefits also our pentaxians... Not to mention fast APS-C zooms.... Fast with compactness, that's the place APS-C lens can win FF lens.
Last edited by starjedi; 03-05-2014 at 08:23 PM.