How does the lens diameter influence photo quality?
I have tested two different 50mm lenses in my camera. One was a Nikkor 50mm ∅52mm. The other one was a Sigma 50mm ∅72mm. I took some pictures with both lenses using the same setup for aperture and shutter speed, but couldn't notice significant differences in the quality of the pictures.
So, how does the diameter affects the photo quality, if it does? What advantages would the ∅72mm lens have over the ∅52mm one?
Note that part of the difference is that Sigma seems to have put the filter completely on the outer edge of the lens while Nikon did not. Look at the filter size and diameters of these 3 lenses: http://www.neocamera.com/search_lens.php?mount=nikon&focalwide=50&aperturemax=1.4%3Amax&prime=1&by=feature - Nikon F/1.4D: 52mm filter / 64mm diameter, Nikon F1.4G: 58mm filter / 74mm diameter, Sigma: 77mm filter / 85mm diameter. I would not read much into it, you would have better luck getting useful information out of the MTF charts.
For a prime lens (especially in that focal length range) I'm more interested in size and weight, and the Sigma is significantly larger and heavier than is the Nikkor. Both being f/1.4 lenses, they're comparable in light gathering ability (of course one could really be an f/1.38 and the other an f/1.42, but that's marginal).
It's not just about maximum aperture. Even in two lenses with the same focal length and max aperture, one could have a larger diameter. The larger diameter could be because of using larger lens elements, which could have advantages with regard to sharpness and light falloff at the edges of the image circle. Some lenses may even project a larger image circle than is strictly necessary. These difference would likely be more apparent at larger apertures (especially wide-open), if they are there at all.
Having said that you can't automatically assume the "larger" lens will always be better optically.