Smaller camera sensor = better image quality?

Just read the tutorial "Digital Camera Sensor Sizes: How Do These Influence Photography" with interest.

I read in another article called "The truth about 'digital' lenses!" that discusses the issue of smaller sensors with corresponding smaller lens. They basically say a digital camera using a smaller lens will reduce the amount of marginal light rays hitting the sensor. It goes on to say that this has the same effect as stopping down the aperture which will essentially improve the image quality. It seems to me this would also be an advantage of a digital cameras with the APS-C size sensor over full frame, besides lighter lens and lower costs. I am still trying to learn about digital photography so I would appreciate it if anyone could comment on this.

What that article speaks of is just another way of describing corner sharpness, which is indeed improved on a cropped sensor and gets better when the lens is stopped down. What I feel might be misleading is that, for the same lens, the center sharpness/resolution of a lens in line pairs per mm is identical on both a cropped and full frame sensor. No advantage of a smaller sensor is the true equivalent to stopping down the lens throughout the image--only in the corners.

In fact, the image will actually *appear* slightly sharper on the larger sensor because one is magnifying the image less. This is because lenses always perform better for larger scale textures (such as 10 lp/mm) compared to smaller ones (such as 30 lp/mm). If you look at the MTF chart for any lens it will show this result, and there is plenty of literature out there on how to interpret these charts if you perform a google search (sorry, not on this site...yet).

Hope this clears it up some and does not confuse further... :)

I myself shoot with a 350D, but I'm still curious about whether or not there is a loss of available-light capability with the 5D relative to the 20D/350D. Would be interested to hear any comments on the following:

For achieving the same depth of field on the 5D as on the 20D, 50 mm at f/2.8 on 20D is equivalent to 80mm at f/4.5 on the 5D or a loss in light receiving capability of 2.56. On the other hand, the 5D has increased ISO capability. Comparing sensor size and number of pixels, pixel area on 5D has increased by a factor of 1.64. Assuming square pixels, and defining pixel size as the dimension of the pixel, pixel size increases by a factor of 1.28. Pixel SNR increases as square root of pixel size (http://www-isl.stanford.edu/groups/e...tions/C072.pdf) or by a factor of 1.13 which doesn't quite compensate for the loss by stopping down for an identical depth of field. But will it really compensate for the 2.56 factor in loss of light by stopping down?

I suppose I neglect here, that for the same print size the larger sensor will have to be enlarged less. Does that have any bearing on the relative performance of the two formats? Will post-processing an image for noise on the 5D and reducing a 12MP to 8MP make it superior in image quality to that of a post-processed for noise 8 MP 20D image?

In other words, do you lose in available-light capability using the 5D or is it exactly compensated for by the increased ISO sensitivity plus increased pixel number?

Theoretically, they are exactly compensated (finer noise (due to better resolution) plus better ISO noise cancels the necessity of smaller apertures and higher ISO speeds). In the real-world the answer is more complicated because it depends on Canon's ability to minimize noise.

If they made a big advance, then it very well might be better overall. The 5D has a bit better noise performance, but I would say not enough to compensate for the much smaller apertures required. Just like depth of field and diffraction limit completely cancel such that the total resolving ability is pretty much constant regardless of sensor size (ignoring effects of the lens and extremes). Overall I do not think there is any disadvantage-- just more flexibility with DOF/ISO and the ability to use a wider range of lenses (literally).