I was looking at an article on Olympus' micro Four-Thirds 7-14mm f/2.8 lens. As the ultra-wide-angle end is at 7mm, they've had to use a pronounced convex front element, which of course, doesn't allow a filter. It should be obvious that drastic measures are needed to correct for distortion.
Somehow, many people, who probably won't be buying the lens anyway due to the price, think it's a simple matter to design such a lens to accept a filter in front because on formats with larger sensors, the equivalent focal length lenses can accept a filter.
Those lenses don't reach 7mm at the wide end. There are other lenses from other optics makers which have similar convex front elements, but this is lost on those who just want a fantasy. "Ultra Wide Angle lens for US$500? Sure, we can do that. Oh, and you want it pocket sized? Easily done." :-D
Similarly, the Panasonic 20mm f/1.7 pancake lens doesn't have fast auto focusing, even in the second generation lens. Why? There isn't enough room for such a motor in a compromised (pancake) design. The Leica 15mm f/1.7 designed by Panasonic should be really quick since it is a better shape and has more (relative) room.
I can't imagine why people can't understand that optimal (is there ever a perfect?) optics require room. If you put things into a compact package, there are many compromises. We've seen this from what Olympus and Panasonic took from Four-Thirds to micro Four-Thirds.
Panasonic's micro Four-Thirds 7-14mm f/4.0 and 8mm fisheye lenses mimic Olympus' Four-Thirds lenses. They seem very well built, although they lack weather-sealing. The trouble is that they made them so compact that there are many optical problems, many of which are ameliorated by software. Similarly, Olympus' 12mm f/2.0 lens fixes things in software to allow such a tiny design to work. (I'd rather it be the size of the 75mm f/1.8 and eschew any software fixes.)
Many people have complained about the size of the 12-40mm f/2.8 lens. I find it extremely small. They find it extremely large. The difference is that I've been using the 14-35mm f/2.0 lens with a 77mm filter size and the 14-54mm f/2.8-3.5 with a 67mm filter size, both excellent lenses. The 12-40mm fits in-between the two in ability and price, but only uses a 62mm filter size.
There are still compromises, I'm sure, but I haven't found them. The fact that Olympus went to 12mm from 14mm says a lot. There is a lot of extra work to get 12mm working correctly, especially in a zoom. That the lens isn't huge also says a lot, especially when its zoom range is bigger than that of the 14-35mm lens. I expected the lens to have a 67mm or even a 72mm filter size. It is much more compact than I would have expected.
I'm waiting to see the weather-sealed lenses from FujiFilm--the 16-55mm f/2.8 and 50-140mm f/2.8. What I thought I saw was that the 50-140mm lens had a smaller filter size than the 16-55mm lens. That makes me wonder about compromises made to the 50-140mm lens to allow it to be more compact than it should be. Nikon's 70-200mm f/2.8 lens has a 77mm filter size, though they don't have an APS-C sized version for an easy comparison. However, if the 50-140mm lens is responsive and sharp, I don't think most people will notice a problem.
Am I more practical about these things? Is it really difficult to see that larger lenses can have better optics because there is more room for the correct geometry? When will companies produce a line of (almost) no-compromise lenses for small format camera bodies, when pigs fly?