No your right that some lenses are better than others in terms of quality. What I meant was that at working macro apertures (higher f numbers) most lenses are diffraction-limited anyways. That is if you have two modern macro lenses and you stop them both down to f/22 or higher the most likely result is that both lenses will be diffraction limited and produce the same overall result. The same does not hold true at less extreme apertures like say f/8 where most lenses are at their optimium working apertures.



Yes. Macro lenses generally employ lens designs that allow them to work better at higher magnifications than general purpose lenses. This allows them to generate higher quality images in the macro world, especially at their optimum apertures.



Well diffraction comes into effect on the other side of the scale- the high f numbers. So you will find most lenses don't allow you to stop down the aperture past say f/22, even though it is mechanically possible to go higher they don't want you too because you will get unsharp results. Just for a historical aside, Edward Weston had one of his lenses modified to go to f/256, he then complained to Ansel Adams (in a letter) of unsharp results with the said lens. Ansel Adams then explainined that modifying lenses in such a way wasn't a good idea because of diffraction problems.

I'm not sure what effect the maximum aperture (lowest f number) has on lens design. Generally the faster the lens the harder it is to make though.




Simply put it's because if you take two lenses and frame a shot at the same magnification (in this case 1:1) then depth of field is identical. In other words the picture you setup with both lenses would be identical, same framing, same subject size, same aperture. So depth of field is also the same. The only thing that affects Depth of field in this case is the aperture you use and/or the Image Format (digital vs film, cropped digital vs full frame).