Great writeup, Ray. Very insightful points, and great photos as always!
I think this point needs some qualification:
In general, when comparing two formats with similar sensor technology and similar base ISO values*, the larger one will have an image quality advantage under two circumstances: 1) when light is plentiful such that both can be used at base ISO; and/or 2) when a more shallow DOF than the smaller format can deliver is acceptable. Note that I say "acceptable", not "desirable".
Say for example you are shooting the X100S at f/5.6 because you need as much field as that camera gives you at f/5.6 and that you're forced to shoot at ISO 1600 because you need the shutter speed that ISO 1600 gives you. Sure you can shoot the RX1 at f/5.6 ISO 1600 and marvel at how much nicer and more malleable the files look, but if you really needed that DOF, then your RX1 has to be set to f/9, and if you needed that shutter speed, your RX1 will now use ISO 4000.** Now is that ISO 4000 RX1 file still more lovely and malleable than the ISO 1600 X100S file? That depends on the specific sensor technologies of those sensors, and it may well be, but it's no longer going to very noticeable to most photo enthusiasts.
My point is that we tend to always think of the ability to go more shallow as a bonus but under any shutter speed limited, light constrained circumstances where you are unwilling to go to more with a more shallow DOF than the smaller format can deliver, there is no image quality advantage inherent in the use of a larger format.
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One comparison you didn't mention unless I missed it is size - Are these cameras similarly coat pocketable? If so, does that still apply with the EVF on the Sony? X100S strikes me as a bit easier to grab and go, but I haven't spent a lot of time with either, and I'm pretty size sensitive (eg, my E-PM2 feels more coat pocketable with the P14/2.5 than it does with an O17/1.8).
Amin,
I don't know (and honestly don't want to know, because then I'd have to think about it!) exactly what the factors are, but in low light in particular, the RX1 strikes me as being at least one stop of ISO "better" than any other camera I've used, being at least as clean at 12,800 as any APS sensor I've used at 6400, and that's before down-sampling, just working with the files at 100%. Its better yet down-sampled to a matching resolution. And when trying to recover shadows and/or highlights, it seems to have more latitude before the quality starts to break down. Maybe resolution is part of that too, not just sensitivity levels - I really don't know. Now, if I'm wrong about this, PLEASE DON'T TELL ME!!!! Honestly, once I've bought a camera, I'd like to maintain whatever illusions I had to develop in the decision process as long as possible! In my experience, my IMPRESSIONS are (and that's all I'm offering up here are shooting impressions - I leave the technical reviews to those who both better understand and enjoy the details) are that the X100s and the Nikon Coolpix A are about equally good (but somewhat different) at 6400, which may give the Nikon the actual edge
IF there's some level of ISO inflation going on with Fuji. The RX1 trumps either of them by at least a stop, unless my eyes just aren't seeing clearly (which the ISO thread on X-Spot indicates may be the case). In terms of the "necessary aperture", that most often applies to my shooting when I'm trying to maintain the
smallest possible aperture when trying to continue to use zone focus effectively in dwindling to dwindled light. At which point the higher ISO capability seems to be the controlling factor, along with the sensor size. If I'm shooting at a 28mm equivalent with an APS sensor, the lens is actually about 18mm and will produce more DOF than the actual 28mm at full frame. With m43, since the actual focal length is 14mm, greater DOF yet at any given aperture. A really good 1" sensor that's good at 3200 with an actual, what, 7mm lens, may be the low light street shooting nirvana for me, but so far its only turned up in the RX100, and that camera wasn't any sort of sweet spot to me. But the RX1 at 35mm will never be a low light street camera for me - I didn't intend it to be. I'm just generally looking at file quality and the ability to push and pull and find subtle details in very low light files shot at high ISO, usually pretty close to wide open. I believe the RX1 does that better than anything I've shot with. If this is true, I'll be happy to hear more of it. If its not, I really don't want to know that...
As for size, I haven't really tried to pocket either one. I know from owning the X100 that it was coat pocketable but I generally used it on a neck or sling strap of some kind, and I've done so with the X100s so far as well. The RX1 has a somewhat smaller body in all dimensions but a far larger lens which greatly increases the overall depth of the camera. I don't know which of my coats it might fit in the pocket of, but I'm not planning to use it that way regardless. If I end up with a Nikon "A" or the new Ricoh GR???, that will probably serve as my pocketable camera (coat or shirt or occasionally maybe pants) and the LX7 fits comfortably in a coat pocket too, but these other cameras are larger than I'd choose to carry that way, even if its doable.
-Ray