This, of course, depends entirely on what you like to shoot. As we know, the bar where we think of image quality has been rising over the years ad technology advances. We have small sensors now that would embarrass much larger sensor from 5 years ago. For example … shoot the CX 1” sensor on the Nikon V1 at a soccer game in the evening and you will be very pleased with the results if you compare them with the results from the D70 generation of cameras. The high ISO required for the action is going to allow much crisper images and of course noise is far less obtrusive on the modern sensor, despite its 2.7 crop factor versus the D70’s 1.5 …
The larger the crop factor, the more depth of field you see at the same framing size. I won’t go into details here, but suffice it to say that smaller sensors use shorter focal lengths (wider angles of view) to get the same framing of the subject, which has the effect of putting more of the subject in focus. This brings backgrounds into play and can be a real problem for those who like that nice blurred background …
But worse than that, smaller sensors bring with them smaller amounts of area and thus gather less light. The square of the crop factor is the area difference (or close enough) and that means that the m4/3 cameras gather 1/4 of the light because of their 2x crop factor. Thus, they are 2 stops behind full frame in noise and other image quality considerations. The laws of physics are not to be denied on this point, yet we see the silly prediction all the time that smaller sensors will “catch up” to larger sensors one day …
Well, not in this universe
However, what people probably mean (were they engaging in a bit of critical thinking) is that the “excellent image quality” bar has been met by smaller and smaller sensors with each generation of sensors. Here is where I think we sit today …
Note: This diagram illustrates my “excellent” bar for various sensor sizes … this is not exclusionary, in that I shoot inappropriate sensor sizes and live with the results quite often :-) … but it is what I use as a guide as to the size of system I will consider “all purpose” …
Any camera except for a smart phone can produce a terrific image in sunshine and open shade. I think I would consider the CX system to be the smallest sensor that can reliably shoot in deep woods, which are a lot darker than people who do not walk in the woods might think. ISO rises pretty quickly …
Indoors, studio lighting allows any sensor again, except perhaps for the phones. I know that some pros have used phones as a gimmick and they do ok with them, but the limitations are extreme with these sensors and the ergonomics od a phone, so I would never consider it appropriate to use them except for snap shots when they are the only camera at hand. Indoor lighting is where the rubber meets the road, though … for a long time I thought that FF was the only size that could handle indoor shooting. But over the years I got more familiar with modern sensors and have settled on m4/3 as the last sensor to cross the indoor ambient light bar.
Perhaps CX will get there soon … it looks pretty promising, but my experience with the J1 was that it is more work than I want to deal with to shoot it in ambient light, so I can wait. Besides, the m4/3 system embarrasses the Nikon 1 system where completeness and compactness of lenses is concerned. So I am happy right where I am for now.
However, remember that I was a system that does it all. If you are a street shooter, then you might follow different reasoning. If you are an indoor sports shooter then m4/s with contrast AF will not cut it … you should be looking at the dSLR phase detect AF and that means minimum APS-C. Of course, I would probably go D600 and FF just to get much better focal ranges on pro lenses and of course to get that extra stop of noise control and thus shutter speed.
In other words … YMMV.
