sangjiny wrote in post #10499762
Something I found on the net.
1. Glass lenses are ground and polished using abrasives and the surface shapes required are sherical or, in some cases parabolic in section (some very expensive lenses). To produce images achieving maximum resolution (sharpness) the lens surface must be accurate to very high precision for the lens to deliver full resolution - small fractions of a wavelength of light. The grinding and polishing processes are only assured of producing lenses of the desired acuracy for circular lenses; it is extremely difficult though not impossible to achieve this accuracy for other shapes.
2. Lenses have two sides and to work properly, the spherical surfaces of both sides must be concentric with the lens disk, co-axial and free of "wedge" (one side thicker than the other). By starting with an oversized blank, opticians use a few tricks to trim the blank down to the desired diameter and achieve these requirements - however these tricks can only produce a circular lens because it similar to spinning the lens on a lathe. Lathes cannot produce any other shape.
3. The most desirable properties of a lens are its ability to form sharp images without artifacts, and light gathering power especially in dim lighting.
Both of these properties are maximised by circular lenses, only someone absolutely ignorant of optics theory would attempt any other shape.
There are also a number of aberrations present in the images formed by all lenses and these are generally axially symmetric for circular lenses. Non-circular lenses may produce distortion, coma and astigmatism that are not radially symmetric, which is a very undesirable state of affairs where accurate imaging is required eg in an enlarger, photocopier or aerial camera. I'd add there would also be quite a few angry SLR users if these objectives were not adhered to.
4. There is one special image defect - diffraction - that is particularly caused by non-circular apertures - the diffraction spikes often seen in photographs around bright points or the sun. A circular lens aperture does not produce diffraction spikes. A lens aperture with corners - triangular, square, hexagonal, etc will produce diffraction spikes - even the iris in a camera lens will do this. Again, you would have to be a really dumb optical designer to deliberately introduce such a defect in a lens by making it square.
5. Cost. To make them in any other shape a larger circular lens must be made first and then cut down to the desired shape which is inevitably more expensive and time consuming than using the circular lens as-is.
6. Inside camera lenses, especially zoom lenses, some elements must rotate as you focus or zoom them. Rotating a non-circular lens is going to be tricky if you are also trying to control the orientation of the aberrations and diffraction spikes at the same time.
7. Lastly there is one damn good reason to stick with circular lenses - they are very easy to mount securely in a lens housing with screw-in retaining rings that form an effective seal keeping dirt and moisture out. Non-circular lenses are much harder to mount securely - requiring mountings that are have compound curves (eg spectacle frames).
Something I found on the net.
1. Glass lenses are ground and polished using abrasives and the surface shapes required are sherical or, in some cases parabolic in section (some very expensive lenses). To produce images achieving maximum resolution (sharpness) the lens surface must be accurate to very high precision for the lens to deliver full resolution - small fractions of a wavelength of light. The grinding and polishing processes are only assured of producing lenses of the desired acuracy for circular lenses; it is extremely difficult though not impossible to achieve this accuracy for other shapes.
2. Lenses have two sides and to work properly, the spherical surfaces of both sides must be concentric with the lens disk, co-axial and free of "wedge" (one side thicker than the other). By starting with an oversized blank, opticians use a few tricks to trim the blank down to the desired diameter and achieve these requirements - however these tricks can only produce a circular lens because it similar to spinning the lens on a lathe. Lathes cannot produce any other shape.
3. The most desirable properties of a lens are its ability to form sharp images without artifacts, and light gathering power especially in dim lighting.
Both of these properties are maximised by circular lenses, only someone absolutely ignorant of optics theory would attempt any other shape.
There are also a number of aberrations present in the images formed by all lenses and these are generally axially symmetric for circular lenses. Non-circular lenses may produce distortion, coma and astigmatism that are not radially symmetric, which is a very undesirable state of affairs where accurate imaging is required eg in an enlarger, photocopier or aerial camera. I'd add there would also be quite a few angry SLR users if these objectives were not adhered to.
4. There is one special image defect - diffraction - that is particularly caused by non-circular apertures - the diffraction spikes often seen in photographs around bright points or the sun. A circular lens aperture does not produce diffraction spikes. A lens aperture with corners - triangular, square, hexagonal, etc will produce diffraction spikes - even the iris in a camera lens will do this. Again, you would have to be a really dumb optical designer to deliberately introduce such a defect in a lens by making it square.
5. Cost. To make them in any other shape a larger circular lens must be made first and then cut down to the desired shape which is inevitably more expensive and time consuming than using the circular lens as-is.
6. Inside camera lenses, especially zoom lenses, some elements must rotate as you focus or zoom them. Rotating a non-circular lens is going to be tricky if you are also trying to control the orientation of the aberrations and diffraction spikes at the same time.
7. Lastly there is one damn good reason to stick with circular lenses - they are very easy to mount securely in a lens housing with screw-in retaining rings that form an effective seal keeping dirt and moisture out. Non-circular lenses are much harder to mount securely - requiring mountings that are have compound curves (eg spectacle frames).
wow that really covers it 
