Sometimes it’s worth reminding ourselves how little megapixels matter with some subjects—limited depth of field can eliminate nearly all of the potential resolution of an image. This example was shot with the Canon EOS 5D-IR (infrared conversion) using the EF 100-400/f4.5-5.6L IS USM at f11. Color as opposed to infrared would make little difference. Click to see the whole frame from which the crops below were taken.
(To conserve viewing space, the railroad tracks were rotated to a horizontal position). Even at a greatly-reduced size, the foreground and background blur are evident—and this is at f11! Results are worse at f16 due to diffraction (I tried both apertures). Click for actual-pixels, or see below.
Below is an actual-pixels crop. Only a very narrow band of the entire frame is actually in acceptably sharp focus, no more than 300 pixels (out of 2912), probably no more than a 100 pixel band is at peak sharpness. A 6-megapixel camera (instead of 12) wouldn’t produce an appreciably different result.
Photos like the example make a good case for a reduced-frame sensor, such as found on the Nikon D200, where the smaller sensor allows use of a shorter focal length lens at the same subject-to-camera distance . See Depth of Field—The Shorter Focal-Length Advantage.