I suspect Canon will venture into the c. 40mpx market but not for any of the reasons yet mentioned. I think they will do it because it will sell lens. You put some of the older L-series lens (let alone non-L) onto a 40+ mpx body and you will soon be screaming for better lens.
And no I can't scientifically back that statement up but I experianced first hand the IQ "old" lens could produce on the 18mpx 7D when I upgraded to that 
I am not sure this makes any sense. Whether you stick a 300mm ii 2.8 L lens on a 5 MP body or 22 MP body it is a great lens. Stick it on a 40 MP body and I think the same result happens.
I can see IQ being a factor for some bodies (mirrorless more than anything) but for SLR's I don't think current lenses with higher MP count sensors (40+ as you alluded to) would alter IQ.
Am I wrong here?
Increased pixel density means the sensor is putting more stress on the resolving power and aberration correction of the lens, in other words: more pixels reserved for showing each and every bit of aberration. Furthermore, the lenses have a resolution limit expressed in lpm (lines per millimeter) or lppm (line pair per millimeter). Take a 36mm wide sensor and put 8000 pixels on the wide side, and your lens will need to resolve 1.425x as many linepairs as it would for a 21 MP sensor or the image will look softer. Someone else can probably explain it better, but the basic idea is: pixels / sensor size = pixel density. The bigger the pixel density, the smaller the pixels. What comes with smaller pixels you can look up elsewhere, I don't know how to explain it without writing a thousand pages on it.
LOL
I don't know how many times I'll have to debunk this myth. But here it goes again. First off, let's define a few things.
Lens resolution: The spatial resolving power of the lens (in lp/mm)
Sensor resolution: The spatial resolving power of the sensor (in lp/mm)
System (or output or image) resolution: The measurable spatial resolution of the images produced by lens+sensor (in lp/mm)
System resolution is the result of a convolution of what the lens resolves with the spatial grid of the sensor. Both components have an intrinsic blur. This blur is generally approximated by a gaussian function, a spot of light that follows some kind of bell curve (peaked in the middle, falloff as you move away from the middle of the spot). To actually compute the REAL system resolution of a lens and sensor, you would need to know the actual PSF or Point Spread Functions of both. That kind of information is difficult to come by, and greatly complicates the math to get a small amount of additional precision. We can approximate system resolution by using this function:
ir = (1/sqrt((1/(lr*2))^2 + (1/(sr*2))^2)) / 2Where:
ir = image resolution (output resolution, system resolution) lp/mm
lr = lens resolution lp/mm
sr = sensor resolution lp/mm
This function is a modification of a simpler function:
ib = sqrt(lb^2 + sb^2)Where:
ib = image blur
lb = lens blur
sb = sensor blur
To convert a blur size into lp/mm, you take the reciprocal and divide by two. If we have a sensor with 5µm pixels, its spatial resolution in line pairs is:
res = (1l / 0.005mm) / 2l/lp
res = 200l/mm / 2 l/lp
res = 100lp/mmIf we invert this:
blur = 1l / (100lp/mm * 2l/lp)
blur = 1l / (200l/mm)
blur = 0.005mmSo, to directly derive the measurable spatial resolution of an output image from the spatial resolutions of a lens and a sensor, we simply combine these two formulas. First, let's assume a diffraction limited lens at f/8. Since it is diffraction limited, the lens will be exhibiting perfect behavior, so we'll be getting 86lp/mm. We have a 5µm pixel pitch in our sensor...let's just assume the sensor is monochrome for now, which means our sensor is 100lp/mm. If we run the formula:
ir = (1/sqrt((1/(lr*2))^2 + (1/(sr*2))^2)) / 2
ir = (1/sqrt((1/(86*2))^2 + (1/(100*2))^2)) / 2
ir = (1/sqrt((1/172)^2 + (1/200)^2)) / 2
ir = (1/sqrt(0.0058^2 + 0.005^2)) / 2
ir = (1/sqrt(0.000034 + 0.000025)) / 2
ir = (1/sqrt(0.000059)) / 2
ir = (1/0.0077) / 2
ir = 129.9 / 2
ir = 64.95
The image resolution with a diffraction limited f/8 lens and a 5 micron pixel pitch is 65lp/mm. That is a low resolution lens. One which most people would claim is "outresolved by the sensor". Such terminology is a misnomer...sensors don't outresolve lenses, lenses don't outresolve sensors...the two work together to produce an image...the convolution of the two produces the output resolution, the resolution of our actual images, and it is that output that we really care about.
So, let's assume we now have a diffraction limited f/4 lens. Our lens spatial resolution is now 173lp/mm. Quite a considerable improvement over our f/8 lens. It is actually double the resolving power of an f/8 lens. Same formula:
ir = (1/sqrt((1/(lr*2))^2 + (1/(sr*2))^2)) / 2
ir = (1/sqrt((1/(173*2))^2 + (1/(100*2))^2)) / 2
ir = (1/sqrt((1/346)^2 + (1/200)^2)) / 2
ir = (1/sqrt(0.0029^2 + 0.005^2)) / 2
ir = (1/sqrt(0.000008 + 0.000025)) / 2
ir = (1/sqrt(0.000033)) / 2
ir = (1/0.0057) / 2
ir = 175.4 / 2
ir = 87.7
Our image resolution with a diffraction limited f/4 lens is 87.7lp/mm. That is a 35% improvement. In this case, most people would say the "lens outresolves the sensor". But again, that is a misnomer. The two are still working together in concert to produce an image. The results of the image have improved. Now, lets say we still have our f/8 lens, and we now have a sensor with half the pixel pitch. Were using 2.5 micron pixels. Same formula:
ir = (1/sqrt((1/(lr*2))^2 + (1/(sr*2))^2)) / 2
ir = (1/sqrt((1/(86*2))^2 + (1/(200*2))^2)) / 2
ir = (1/sqrt((1/172)^2 + (1/400)^2)) / 2
ir = (1/sqrt(0.0058^2 + 0.0025^2)) / 2
ir = (1/sqrt(0.000034 + 0.000006)) / 2
ir = (1/sqrt(0.00004)) / 2
ir = (1/0.0063) / 2
ir = 158.7 / 2
ir = 79.4
Our image resolution jumps to 79.4. Well, supposedly, the sensor is "far outresolving the lens" at this point...and yet, the spatial resolution of our images has still improved considerably. By over 22%, to be exact. The fact that our sensor is capable of resolving considerably more detail than our lens does make the lens the most limiting factor...however it does NOT mean that using "the same old crappy lens" is useless on a newer, higher resolution sensor. Our results have still improved, by a meaningful amount. It is not necessary to build a new lens to take advantage of our improved sensor.
Lets take this one step farther. We are using our same f/8 lens. It isn't a great lens, it's decent, for it's generation. At f/4 it is not diffraction limited, but it performs pretty well. Let's assume it is capable of resolving 150lp/mm instead of 173lp/mm. If we run out formula again:
ir = (1/sqrt((1/(lr*2))^2 + (1/(sr*2))^2)) / 2
ir = (1/sqrt((1/(150*2))^2 + (1/(200*2))^2)) / 2
ir = (1/sqrt((1/300)^2 + (1/400)^2)) / 2
ir = (1/sqrt(0.0033^2 + 0.0025^2)) / 2
ir = (1/sqrt(0.000011 + 0.000006)) / 2
ir = (1/sqrt(0.000017)) / 2
ir = (1/0.0041) / 2
ir = 244 / 2
ir = 122
Wow. Our crappy old lens which isn't even diffraction limited at f/4, combined with our greatly improved ultra high resolution sensor, is still giving us a lot of bang for our buck! Our image resolution is up to a whopping 122lp/mm! That is an improvement of over 53% over our f/8 performance. Well, let's say we finally break down and buy a better lens, one that is diffraction limited at f/4:
ir = (1/sqrt((1/(lr*2))^2 + (1/(sr*2))^2)) / 2
ir = (1/sqrt((1/(173*2))^2 + (1/(200*2))^2)) / 2
ir = (1/sqrt((1/346)^2 + (1/400)^2)) / 2
ir = (1/sqrt(0.0029^2 + 0.0025^2)) / 2
ir = (1/sqrt(0.000008 + 0.000006)) / 2
ir = (1/sqrt(0.000014)) / 2
ir = (1/0.00374) / 2
ir = 267.4 / 2
ir = 133.7
Hmm...well, things haven't changed much. Relative to our older lens, we now have 133lp/mm. Unlike the previous jump of 53%, we have now gained a 9.5% improvement in resolving power. Ten percent improvement isn't something to shake a stick at, but our previous older lens that isn't diffraction limited at f/4 still performs remarkably well on our ultra high resolution sensor. To eek out any more performance, we would have to get a lens that was diffraction limited at a wider aperture. At apertures wider than f/4, optical aberrations begin to dominate, and achieving significantly improved results is more difficult. Additionally...you only get the improved resolving power at apertures wider than f/4...if you regularly shoot scenes at diffraction limited apertures of f/4 and smaller, then the only real way to improve the resolution of your photographs themselves is with a higher resolution sensor.
Pushing sensor resolution to obscene levels is a lot easier than pushing lens resolving power to obscene levels. Upping sensor resolution is the far more cost effective means, and therefor the one that tends to appeal to the masses (regardless of whether they know why.)
