Canon currently produce two camera bodies that have quite a high degree of similarity, based on their general specifications.  The two bodies are the Canon 7D, which is an APS-C sensor camera, that has 18mp of resolution, 8 fps drive and the Canon 1Dmk4, which has a larger APS-H sensor inside, with 16mp and 10 fps drive speed.
I often am asked as to what some of the main differences are in image quality between these two cameras.  With this in mind I set up the two cameras to shoot some comparative images from both in order to compare some of their characteristics.  In two earlier comparisons I shot the two cameras at ISO 400 and at ISO1600, in good light.  I used the same lens and focal length for both of those comparisons and I cropped the 1Dmk4 file until the subject size matched that of the 7D.  Those two posts make interesting reading in case you have not read them and a good basis for comparison with the results of this post.
For this comparison I took a completely different approach to comparing image quality.  In order to equalize the difference in subject size resulting from the different sensor sizes in the two cameras, I used a longer focal length on the 1Dmk4.
I used a Canon EF 70-200 f/2.8L IS II lens, mounted on a tripod.  Only the camera body was changed each time.  No sharpening was applied other than a default setting of 25 in Adobe Camera Raw.  Raw images opened in Photoshop CS5, downsized and saved using File Save For Web and Devices at 190kb file size.
Canon 1Dmk4, 70-200f2.8.  1/40 at f5.0, Iso 3200.  Focal length 130mm.  As framed 16mp.
Canon7D, 70-200f2.8.  1/50 at f5.0, Iso 3200.  Focal length 102mm.  As framed 18mp.

The images were taken in very low light, as can be seen from the exposure details.  For the first two images I shot at iso 3200 on both cameras.  The frame includes some darker areas where noise tends to be seen more easily as well as a detailed subject in the silver can.

The first image is that of the Mk4, with its 1.3x crop effect, taken with the lens focal length at 130mm.  The image directly beneath it, and above this text, is that taken with the 7D, with its 1.6x crop effect, taken at 102mm.  The size of the subjects in the frame are pretty similar, which is a result of shooting the Mk4 at 130mm and the 7D at 102mm ( I was aiming at 100mm).
I had to add just a little (0.3) positive exposure adjustment to the 7D image in LR to get it to a similar brightness level.
Viewing at the small size that I am able to post at on the blog doesn’t really reveal too much difference in visible image quality between the two cameras.  At this point the two images above represent downsized versions of a 16mp capture on the larger sensor (27.9 x 18.6) of the 1Dmk4 and an 18mp capture on the 7D’s (22.3 x 14.9 ) smaller sensor.


Canon 1Dmk4, 70-200f2.8.  1/40 at f5.0, Iso 3200.  Focal length 130mm.  Cropped to less than 2mp.
Canon 7D, 70-200f2.8.  1/50 at f5.0, Iso 3200.  Focal length 102mm.  Cropped to less than 2mp.
I then cropped the same two images.
In order to make it a bit easier to compare the image quality more easily,  I made heavy crops.  I tried to keep the subject size the same.  Megapixel size of each image is roughly 1.5 to 1.7mp before downsizing.
This crop allows the detail in the image to be more accurately observed, even after downsizing for the blog image parameters.
Comparing the quality of the two iso 3200 images side by side shows that the Mk4 image has a lot less visible noise than the 7D image.  It shows less visible colour noise in the khaki background, and it shows far less noise in the dark wood below the can.  Looking at the silver tones on the can itself, it can be seen that noise in the 7D image is giving it a slightly grainy look.
The Mk4 image is showing less colour noise throughout the image, and is also retaining detail and fine contrast far better, easily visible in the printed letters on the can.
Canon 1Dmk4, 70-200f2.8.  1/20 at f5.0, Iso 1600.  Focal length 130mm.  Cropped to less than 2mp.
Canon 7D, 70-200f2.8.  1/13 at f5.0, Iso 800.  Focal length 102mm.  Cropped to less than 2mp.
I took more images for comparison, from the same position and with the same lens settings, but this time with the 1Dmk4 set at iso 1600 and the 7D at iso 800.
Compared in this way the 1Dmk4 and the 7D images appear quite similar in quality and noise, with the 1Dmk4 perhaps showing a very slightly superior image.
These numbers translate into a raw noise advantage of roughly one stop for the 1Dmk4 over the 7D.



The roughly one stop advantage of the 1Dmk4 when compared to the 7D using the method outlined above, of using longer focal length to reach a similar subject size, is not unexpected.
The technology applied to these sensors is from the same era, and both use Canon’s gapless microlens technology to maximize light gathering.
The differences mainly come about from the larger individual megapixels that are found on the 1Dmk4 sensor.  Each pixel is 5.7 microns whereas the pixels found on the 7D are 4.3 microns.
The larger pixels seem to better at gathering light, at the expense of subject size.  If you are able to make up the difference in subject size, either by moving closer to the subject, or using longer focal length then the camera with the bigger pixels will deliver higher-quality images, all other things being equal.
After extensive use of both of these camera bodies, my own findings in actual shooting conditions pretty much supported what comparing these images shows in this post.  It is also important to remember that the images above are converted directly from raw captures with no processing done.  They would benefit from noise reduction and sharpening processing steps.
I did not calibrate the two camera bodies to the camera lens.  I shot indoors in very low light.
No sharpening applied to these images, but for my purposes, in raw format, I felt that both lens and camera combinations were working well enough for me to have confidence in the results.