i'm not a sigma user, but im not sure how we can single them out as 'specialty' cameras when specifically juxtaposed against a 28mm fixed lens $4500 camera. sure the sigmas are in and of themselves good at some things, not good at others. but whether one 'likes' 28mm or not, one must admit it itself is a 'specialty' fl, no? i mean by definition 28mm is really good at some things and terrible at others, just like the sigmas are reputed to be.
It's a fair point - certainly any fixed lens, fixed focal length camera is a "specialty" camera in a sense. So the Coolpix A, GR, X100, etc, etc, are specialty cameras in that sense. And by definition, any camera that costs nearly $5000 is a specialty camera just by virtue of how few people will ever be able to afford it, or at least justify the cost of it. So, sure, when you combine a fixed lens/focal length with a high price tag, you're dealing with a double specialty. I put the RX1 into more or less the same category at it's new pricing, maybe not so much at current used prices. And almost every camera is a "specialty" camera in a sense because each has some things it does well and some things it does much less well. The RX100 and G7X are specialty cameras - the only thing they do better than a host of other models is fit in a pocket. This seems to matter to a good number of people, but it's a specialty feature for sure.
That said, once we're talking about fixed focal length, fixed lens cameras, any of the cameras noted above are capable of doing nearly anything one can reasonably do at their respective focal lengths, maybe with the exception of super fast auto-focus. But the Sigmas are in a whole different league in terms of specialty. Each is also a fixed lens/focal length camera, so therefore with the same inherent limitation as any of the cameras noted above. But they also can't really do much in lower than excellent light. It can hang in there at ISO 320, but you really want to be shooting as close as possible to base ISO. Yes, if you know what you're doing in post, you can isolate the blue color channel and get decent B&W shots up to 1600, maybe 3200 in a push, but using a single color channel is a pretty limited version of B&W and you need to know what you're doing to get there. It's incredibly slow shot to shot, so even firing off 3-4 shots in a row - not burst mode, but just back to back to back, you're shooting blind to the extent you can even do it. People say the Fuji cameras and the DF "slow you down and change your approach", but really that's just an option with those cameras (as it is with any) - you can shoot as fast or as slow as you want to with those cameras. The Sigmas force the issue to an extent that significantly limits the types of shooting you can do with them. It's possible to shoot on the street with them, but you better nail the first shot - you don't get to fire off another couple as the scene evolves. I wouldn't even think about shooting any sort of action with one. Although I've seen some great portraits from them, it's only a particularly sharp detailed type of portrait that may not work well generally. And, finally, processing the files adds a really cumbersome step to anyone's work flow unless they're content to ONLY use Sigma's SPP program, in which case you're significantly limiting your processing options.
The Sigmas are AMAZING landscape cameras, architecture cameras, flower cameras, etc, in very good light, if their rendering is to your liking, and you can process the files effectively. The detail in the files is simply mind-boggling. I had the DP1M for a couple of months and it was incredibly great at that which it did well. Although even within it's specialty there are tradeoffs. Lenses that are designed for the ultimate in sharpness (which the Sigma would have to be since it's spending it's life in front of that sensor) tend not be great for bokeh, and the bokeh from the DP1M was pretty nasty looking to me - I have a few semi macro shots where the flower or spider's web look incredible, but the OOF background is just awful. Each of the other cameras under discussion has much more "balanced" lenses, good sharpness, good bokeh, pretty good at everything (with the RX1 being pretty
great at everything). The Sigmas are insanely great for limited uses but those uses are so limited relative to the other cameras at it's focal length can that I consider them much MORE specialized cameras than the others.
Almost any camera is better at some things than others and can be said to be somewhat specialized and maybe each of us considers "specialized" cameras those that don't do well at what we do well? But the Sigmas, IMHO anyway, are far more specialized than most.
-Ray