I took the pictures below with a five-year old 6 Mpixel Canon 10D and a brand new 21 Mpixel Canon 5D Mark II. Both images were made using a 24-105 f/4L standard lens. Both images are shown here at 100% and both crops are taken close to the center of the image. The image was taken at f/4 – which is not particularly optimal as the lens is pretty good, but not ideal. Residual chromatic aberration was manually corrected (in Lightroom 2.2) in both images using the edges of the roof to see the aberrations clearly. The images were set to same color temperature. It is hard to tell whether all other in-camera settings are comparable. For now, I am just hoping that the default settings allow a reasonable first comparison.
The test thus stress the more fundamental implications of the difference in sensor size (and potentially in in-camera processing) rather than any system implications like having to change the zoom settings or move the camera position to get a similar field of view with the same lens.
So what can we learn from this test?
- As was to be expected from the camera specifications, the 5D Mark II has a slightly higher pixel density. It corresponds to an 8 MPixel version of the 6 MPixel Canon 10D (say the Canon 20D or 350D or 30D) and can thus capture a bit more detail. This fundamental difference in image quality is, however, visible but not too dramatic.
- Fine details are more visible on the 5D Mark II. This is best seen in the wings of the gryphons. This might be due to the difference in pixel density, but it might also be due to differences in spatial filtering inside the camera (the optical anti-aliasing filter or the demosaicing and sharpening algorithms). It can be argued that if one camera does less sharpening than the other (at these settings), this can be compensated by further sharpened in software.
- Obviously the 5D2’s full frame sensor captures a lot of image (see below) which is simply mechanically cropped in the 10D because the sensor is 15×23 mm instead of 24×36 mm. We can use these extra pixels to do a number of things:
- We can simply create a 60%×60% larger poster with slighty better per-pixel sharpness. Never mind what can be seen on the image: you just get more sharp pixels as far as the sensor is concerned.
- Or we can use those extra pixels to sometimes crop the full frame image to improve composition or to give extra “digital zoom”: if you throw away more than half of the pixels, you still have the quality of say a Canon 20D or 30D.
Interestingly Nikon allows you to do this digital zoom trick in the Nikon D3, but Canon doesn’t support it in their high-end cameras. This is because Nikon needs to support DX (small sensor) lenses on their FX (large sensor) bodies. In the Canon world this trick is not supported/recommended: EF-S (small sensor) lenses are made to be physically incompatible with full-frame bodies (supposedly to prevent damage to the mirror).
- Finally, as a variation of the first option, we could try to take the same picture from the same location using the larger sensor. To get the same reduced field of view of the smaller sensor camera, you need to use a longer lens (38mm lens) on the full frame camera. If we assume the lens has the same (pixels/mm) sharpness as the original 24mm lens, and can more or less sustain that sharpness across the wider sensor, you get 21 Mpixels instead of 8 Mpixels (Canon 30D) with roughly the same per-pixel quality. Unfortunately, in reality, you won’t get all of that improvement, because the smaller sensor is using the part of the image that has the best quality: the center. So the extra pixels further from the center are (especially on wide-angle lenses) of lesser quality than the ones which the small sensor gets. Thus, in this example, you can clearly already see some vignetting in the corners of the image (despite having partially manually corrected this in Lightroom).
What can be improved in the test?
It would be nicer from a didactic perspective to use a Canon 20D/30D/350D instead of the 10D. That gives the exact same pixel density of 2.4 Mpixel/cm2 as the 21 MPixel full frame sensor.
And it would be better to use a tripod rather than relying on the image stabilizer. And take the images sooner after each other to avoid the difference in shutter speed. Or to at least have had more light to minimize the impact of the shutter speed in the first place (these pictures were taken in the late afternoon on a very cloudy winters day). Obviously a few images were taken to make sure that the phenomena were reasonably repeatable.
More relevantly, it would help to use an even better lens while avoiding the maximum aperture to get lens limitations out of the way as much as possible.