As the digital world moves towards ever more absurd megapixel counts, one inconvenient truth will (very) slowly sink in: the depth of field required to exploit the sensor resolution becomes so narrow as to render the megapixel count meaningless unless technical excellence is achieved: a dualing photographic Scylla and Charybdis will suffer only perfect technical execution for tomorrow’s high-resolution sensors to be worthwhile. In this case, Scylla and Charybdis refer to depth of field and diffraction. Stop down too much and diffraction kills contrast and resolution. Stop down too little and there is inadequate depth of field.
It is only at the exact plane of focus that the highest resolution can be obtained, a direct result of the size of the “circle of confusion” in relation to the photosite size. Any subject that is not pseudo-planar (all subject matter at the same effective distance) simply cannot be imaged at anything close to the resolution the sensor might offer. Landscapes (for example) are often pseudo-planar, though many landscapes have near/far details which cannot be imaged sharply at the same time (this is why tilt lenses and view cameras offer a great advantage).
Stopping down expands the zone of sharpness, doubling it every two stops (eg f/4 to f/8). But if that zone is 1/2" to begin with at f/2, it’s still only 1" at f/4, 2" at f/8 and 4" at f/16! Such numbers are quite realistic for many subjects; this is why precise focus is so critical with high-resolution cameras. It’s also why Live View can be critical for some subjects—missing focus is a non-starter.
Let’s look at the sunflower image below, taken with the 21 megapixel Canon EOS 1Ds Mark III and the Leica 90/2 APO-Summicron-R ASPH, a brilliant lens with few peers (see review). The sunflower image was taken with focus on the center of the emergent flowerhead.
Canon EOS 1Ds Mark III + Leica 90/2 APO-Summicron-R ASPH @ f/8
The detail level is outstanding, but it is limited to a narrow zone at most 1" deep (2.5cm). And yet this is at f/8! The sunflower is approximately 5' tall; this is not a macro shot.
Observe the actual pixels crop below. The leaf is only a few inches in front of the crisply-rendered stalk, yet it contains no fine detail. Stopping down to f/16 would increase depth of field, but it would also degrade the finest detail noticeably—see Diffraction.
Bottom line: future DSLRs offering 28 or 36 or 43 megapixels will not deliver anything close to that resolution except with outstanding optics shot at f/4 - f/5.6 with perfect focus. Even the existing 21MP Canon EOS 1Ds Mark III makes this limitation very clear. Hone your skills now if you’re lusting after more resolution.