A few days ago the specs were leaked for the next Canon small format camera, the EOS 7D. Since then the rumor mills have been running rampant with the news. What raised some concern for me was that the camera would carry an 18 megapixel APS-C sensor. More pixels in a tiny sensor does not bode well for noise.

However, I’ve been saying for quite some time, that Canon’s solution to the pixel problem was simple, make the sensor bigger. Their APS-C format sensors measure 22.2 x 14.8mm, 1 to 1.4mm smaller than the APS-C format sensors in Nikon, Pentax and Sony cameras. It stands to reason that if you want to keep increasing the resolution and you can’t make the pixels incredibly more efficient, you just make the sensor bigger.

On the efficiency side, Canon reached the limit for micro lens size with the 50D and its gapless lenses. That rules out one possible mechanism for improvements. Making the sensor more efficient is another, and Canon seems to make progress here each generation, though it’s almost never revolutionary.

That leaves increasing the sensor size. The question isn’t so much can EF-S lenses support it; it’s clear from looking at images of adapted EF-S lenses that their image circle can usually cover a 1.3x APS-H sensor, something in the 1.5x range shouldn’t be a problem. The real question is can the mount support it.

In that vein I’ve done some quick measurements of the 2 EF-S lenses I have access to and it seems that the rear element protrudes ~10.16mm from the front surface of the lens mount.

Turning things around, the front edge of the mirror on the Rebel XTi in the up position, rests about 10.16mm behind the front surface of the lens mount, equal to that of the EF-S lenses measured. However, the front edge of the mirror on the 40D sits further back, at about 11.5mm. That seems to indicate that there is at least a 1.3mm margin to increase the depth of the mirror box (or at least the size of the mirror) with out interfering with the use of EF-S lenses.

Now the question is how big of a sensor does that result in and is the resolution consistent with the published rumors of the 7D?

If we figure the sensor can increase in size by 1.3mm in both directions, it will measure 23.5 x 16.1mm. If we keep the same 5.6 micron pixel size from the 50D we end up with a ~18 million pixel sensor.

Tangentially, it also occurs to me that the 7D is also a branding change. Presently the non entry-level APS-C 1.6x cameras are branded in the x0D range, which is very much what the 7D looks like. There’s still room to continue that numbering scheme for at least 4 generations. However a departure in sensor size may warrant a departure in naming as well.

So while it’s still a rumor, a larger sensor in the APS-C 1.5x format at 18 million pixels seems to hold up under some back of the envelope scrutiny. Will Canon take this approach? That remains to be seen, but with the expected announcement coming in just a couple of days, we don’t have long to wonder.

Updated 9-1-09: It appears that all this speculation was for naught. The EOS 7D turns out to be a nearly 18Mp sensor in the standard Canon APS-C 1.6x format. However, it still seems that should Canon wish to do so at some point in the future, produce a 1.5 crop body their EF-S mount may just support it.