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The $1M question is what the photography market leaders — Nikon and Canon — are planning to do about mirrorless. Not counting Nikon 1; I mean mirrorless that can rival and surpass DSLRs. Canon does have EOS-M, which at least is based on a healthy sensor size, but I don't see much enthusiasm around it. People still talk more about Nikon 1.
Based on this interview and this other interview from DPreview site, I guess that the manufacturers are going to launch mirrorless cameras based on their mainstream lens mounts. It makes sense, because the lens availability and the health of the used lens market are key factors. Having a market of cheap and/or exotic and/or special-purpose lenses is paramount.
See my case: I bought my first DSLR four months ago; my wife calls me Mr. Krabs because I hate to spend money; and yet I already have three Nikon F lenses. (Used to have six, but have sold all that sat unused.) I wouldn't like to lose my investment on these.
As much as I envy the size and prettiness of most mirrorless models, and while I agree that mirrorless is the thing of the future, I won't marry to an obscure lens mount that only has 3 or 4 expensive options and might be abandoned by the manufacturer anytime. (See what happened with Samsung NX. And I almost bought an NX200 because it was indeed gorgeous.)
"Ooooh, but a mirrorless camera based on DSLR mount would be big and heavy because of the flange focal distance!"
First, let's clarify what is flange focal distance (FFD): it is the distance from the mount's flange plane to the sensor plane. For example, Nikon F is 46mm. FFD is not the focal length, and actually the focal length is just a theoretical value. A lens of "100mm" means that its image magnification is equivalent to a 100mm-deep pinhole camera, but the lens may be of any size.
FFD is somewhat related to focal length. If you replaced your lens by a pinhole, then focal length and FFD would be equal. Due to this property, lenses with focal length similar to the flange distance (e.g. 50mm lenses for DSLR) are simpler and cheaper.
DSLR mounts, Nikon above all, have long FFDs because there must be space to accomodate the mirror between flange and sensor. Compact and mirrorless cameras can be much thinner because there is no mirror, and the mount FFD can be shorter.
Figure 1 is from an article that reviewed the Samsung NX300, similar to the NX200 that I almost bought. One picture in the article incidentally shows the difference of camera "depth":
Shorter FFDs have another advantage: they simplify the construction of wide-angle lenses. The "natural" lens for a DSLR is around 50mm (normal) while it is around 25mm (wide angle) for a mirrorless.
Having said that, I think that a longer FFD is not an impediment to adopt traditional DSLR mounts in mirrorless cameras.
Certainly a mirrorless built around Nikon F must be longer, lens cap to back, but it does not need to be heavier or bulkier because of that! The mount point could protrude a bit from the body, so the body itself is kept skinny, and that's it.
For example, the difference from Samsung NX mount FFD (25mm) to Nikon F (46mm) is not that big. I played a bit with the Figure 1 and added some length to the mirrorless lens, trying to match the FFD of both, to see how it would look like. The result can be seen in Figure 2.
The lens now looks big and long when compared with the mirrorless body, but it is not that bad. This elongation would be a small price to pay for a mirrorless camera that can share lens with SLRs and DSLRs. I don't see why Nikon or Canon shouldn't try this way.
People that defend mirrorless sometimes get carried away in their arguments. They talk about size, but most mirrorless lenses are not small at all. Apart from the pancake wideangles, most lenses have the same size as DSLR equivalents. The volume occupied by a mirrorless coupled to most lenses is basically the same as a DSLR.
Lens size is a byproduct of sensor size, not of FFD. Full-frame mirrorless cameras need big lenses, there's no way around it. There are lens systems that are indeed smaller, like EOS-M, Micro Four Thirds and Nikon 1, but they are built around small and invariant sensor sizes (APS-C, 4/3" and 1", respectively).
And people want full-frame even in mirrorless. Sony is delivering just that, and is selling well. (Sony managed to have two different mounts, but they are trying to amend that.)
Full-frame and APS-C deliver a shallow depth-of-field with reasonable apertures. Smaller sensors can deliver the same "bokeh" but need lenses with very large apertures, which is another challenge of lens engineering.
I said that wide-angle lenses are easier to built with shorter FFDs. This is true in principle, but a naïve design would deliver light rays too obliquely at the extremes of the sensor frame. Good lens implement telecentricity to avoid this problem, but then the design gets expensive again.
Micro Four Thirds mandates telecentricity in the standard to mitigate this problem. Smaller sensors (like Nikon 1 and, again, Micro Four Thirds) are less affected by this problem. The worst case is a full-frame with very short FFD.
Given all these facts, in my opinion the current DSLR mounts would bring more advantages than problems into the mirrorless world.
First off, I think that current mirrorless models are too expensive.
I really like the Samsung NX and all those 'retro' models from Fuji and Olympus. But I bought a Nikon DSLR. Why? Because it was cheaper than the mirrorless models; I felt safe by going Nikon; I can find inexpensive good lenses for it; easy to sell Nikon/Canon used equipment if I buy something that I don't like or if I need quick money.
I don't understand how a device that is simpler, with less moving parts, often with smaller sensors, trying to conquer the market, can be so expensive. I understand that R&D costs needs to be covered... but my money won't go to the development of yet another incompatible lens system.
Are folks charging so much for a pretty design these days? Apple does that, but my productivity on a Mac or on an iPhone is multiple times of the PC or Android. A camera that is slightly smaller does not buy me the same gain.
If the price drops a little, I see a future for "small sensor" mirrorless like Nikon 1. I mention Nikon 1 because it did one thing right: its small sensor brought truly small lenses. I'd buy a Nikon 1 as a second camera. People that want a camera that is better than a smartphone, but not as big and heavy as a DSLR, would buy (and indeed are buying) Nikon 1 and Four Thirds. Many folks out there see the full-frame shallow depth of field as a liability, because it makes the casual shooting and video recording more challenging.