[Parallax]

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Seriously, as with most things, the raw materials are a minimal cost and the increase in production cost would likely far outweigh the savings.
As others have said there would be no benefit optically, and quite probably a detriment.

[owl]

I guess that viewfinder lenses are moulded as square. Square apertures are made all of the time, but round would be fine as well.

Round is easier to handle. I want my lenses round.

Good to ask basic questions and challenge conventional wisdom.

[KevinR]

Lots of practical reasons why a lens is round, but most are simple engineering and practical convenience. With zoom and focus requirements, many parts in the lens need to turn, and things just works so much better when the outer carrier is round. Round is also a very simple form to make, and is structurally remarkably stable. Parts can be threaded together rather than flange bolted as required if square. Forming and machining techniques are cheaper if based on round shapes as turning is a common and cheap technique. The mounting flanges would be a bit awkward if not based on round twist lock. So round it will be. As far as optics are concerned, they could just as we'll be any shape and would still function. Although bokeh might be a bit strange with square aperture blades, this is one area that would be a lot simpler if only square form used...

=vonBaloney;2906429...
A better question might be, "Why isn't the sensor round?" ....

Or to my mind, "why isn't the sensor square?", would be a more pertinent question. For a little more silicone wafer you can save the cost of a vertical grip... But I think the mirror and body conformance with old standards sadly prevents this approach.

[ElJamoquio]

I've often thought about a round sensor. Doubt it would happen... Something like putting a 44x33 sensor in a FF body seems more likely. That would get you every aspect ratio from 5/6 to 3/1* (slight portrait to wide panorama) without moving the camera.

Although the larger rectangular sensor seems MORE likely, neither is very likely.

*actually, near infinite... but I doubt people want a picture 6000 pixels wide and 1 pixel high.

[dadipentak]

Because a square lens wouldn't roll off a table and shatter into a zillion pieces when it hit the marble floor. Oh wait...

[Not a Number]

Forgot about the aperture, but it doesn't really matter that it's not square; it just needs to be able to fit into the same package.

It would have to be square to when wide open or at least much larger than the diagonal of the lens elements. Yes you can fit a square peg into a round hole if the diameter of the hole is larger than the diagonal of the square. It may not be an exact fit however.

[ElJamoquio]

It would have to be square to when wide open or at least much larger than the diagonal of the lens elements.

It would need to be the same size as it is now. Sometimes that's large, but usually it's pretty small. It usually looks bigger then it is because of the optics involved.

[lytrytyr]

Or to my mind, "why isn't the sensor square?", would be a more pertinent question. . . . But I think the mirror and body conformance with old standards sadly prevents this approach.

Not necessarily:

Kodak Instamatic Reflex 126 film 28x28mm original box manual, Xenar 45mm f/2.8 | eBay

[Luuk]

thanks to all the helpful replies. I think I have
a better understanding now of it