Shape of Lenses -- Why are they always circular?

[DragonSpear]

TL;DR Summary

I am a bit confused about the consistent circular shape of lenses

In a magnifying glass, the lenses are always circular. I don't understand why because no matter what shape the lense is, it doesn't deny its physical properties and refraction would still occur, so why are they so consistent on making circular lenses.

I predicted that circular lenses may be cost-efficient and easy to produce but if that's the case there must be a non circular magnifying glass out there but I haven't found one on the net so I deduced that my hypothesis was probably wrong and now I need an answer.

 

[phinds]

In a magnifying glass, the lenses are always circular

No, they are not. Not even close.

 

[kuruman]

No, they are not. Not even close.
View attachment 327132

This is not to say that there is no utility to circular design. Manhole (personhole?) covers are circular for a good reason. Considering how heavy they are, it's much easier to drag them back on compared with, say, square covers that have to be oriented correctly before placing them on.

 

[Ibix]

This is not to say that there is no utility to circular design. Manhole (personhole?) covers are circular for a good reason. Considering how heavy they are, it's much easier to drag them back on compared with, say, square covers that have to be oriented correctly before placing them on.

Also, they don't drop down the hole if you put them in with the wrong orientation.

Round lenses are easier to mount in many types of mount because the rim thickness is the same all the way around. They're probably easier to make, at least with traditional grinding techniques. But there's no purely optical reason for round lenses that I'm aware of - see the huge range of spectacle shapes that are available, for example.

 

[Baluncore]

In a magnifying glass, the lenses are always circular. I don't understand why because no matter what shape the lense is, it doesn't deny its physical properties and refraction would still occur, so why are they so consistent on making circular lenses.

Glass lenses have spherical surfaces and are ground and polished in machines that rotate the lens and the lap, so all parts of the lens, meet all parts of the lap, at all angles. The result of the polishing process is symmetrical, the lens and the lap are best made circular. If part of the circular face contact was missing, the depth of grind would change due to change in the contact pressure, so the result would not be a spherical surface lens.

A convex lens with a spherical surface will be circular after grinding and polishing. The edge is defined by the intersection of spherical surfaces. If the sharp edge is then ground back to a specified thickness, the lens will remain circular.

The support for a lens can be made in a centre lathe by cutting an internal groove and/or a machine thread in a circular tube. A circular lens does not need to be aligned when it is clamped into the circular tube or frame.

Nowadays, cheap magnifying lenses are accurately cast from clear plastic, then clipped into plastic frames. There is no requirement that those lenses or frames be circular.

 

[Robert Jansen]

This is not to say that there is no utility to circular design. Manhole (personhole?) covers are circular for a good reason. Considering how heavy they are, it's much easier to drag them back on compared with, say, square covers that have to be oriented correctly before placing them on.

Manhole covers are round for a simple reason: circular covers cannot fall into the hole.

 

[Mister T]

Also, they don't drop down the hole if you put them in with the wrong orientation.

That is the reason manhole covers are round. Now, why did the Siamese twins move from the US to the UK?

 

[phinds]

SPOILER:
So the other one could learn to drive

 

[Robert Jansen]

As long as the topic is humor (in bad taste or otherwise), let me suggest The Best of the Journal of Irreproducible Results.

 

[Cutter Ketch]

Glass lenses have spherical surfaces and are ground and polished in machines that rotate the lens and the lap, so all parts of the lens, meet all parts of the lap, at all angles. The result of the polishing process is symmetrical, the lens and the lap are best made circular. If part of the circular face contact was missing, the depth of grind would change due to change in the contact pressure, so the result would not be a spherical surface lens.

A convex lens with a spherical surface will be circular after grinding and polishing. The edge is defined by the intersection of spherical surfaces. If the sharp edge is then ground back to a specified thickness, the lens will remain circular.

The support for a lens can be made in a centre lathe by cutting an internal groove and/or a machine thread in a circular tube. A circular lens does not need to be aligned when it is clamped into the circular tube or frame.

Nowadays, cheap magnifying lenses are accurately cast from clear plastic, then clipped into plastic frames. There is no requirement that those lenses or frames be circular.

I would add that for the reasons stated eye glass lenses actually are circular initially. The after grinding, polishing, and coating, the edges of the circular lens is then ground down to match the shape of the frame. This is the only part of the process that LensCrafters or similar retail outlets do “in about an hour”. It is also why they don’t have to stock a zillion lenses in every frame shape.

Another reason that hasn’t been mentioned is thermal stability of the lens centration. In multi element lens systems like a camera lens or objective the performance of the lens can be very sensitive to centration, I.e how well the centers of the lenses are aligned. The symmetry of circular lens tubes make them very stable for centration over temperature. It’s not that you can’t make other forms stable. There are many examples. However, with a circular lens tube it is very simple to manufacture something cheaply and quickly that is assured of working well. Anything else requires some more careful engineering

 

[Cutter Ketch]

oh, one more thing. The image at the focal plane is basically a faithful copy of the scene convolved with the blur spot. Circular apertures make circularly symmetric blur spots. (Airy pattern) Non-circular apertures don’t. Because it is blurred the same in all directions an image looks better with a circular aperture. A noncircular aperture blurs a little more in the short dimensions and a little less in the long dimensions. The non symmetric blur can be noticeable in some edges looking sharper than others and this is often perceived as less natural looking.

 

[sophiecentaur]

, the edges of the circular lens is then ground down to match the shape of the frame.

There's more to it than that. Once you have chosen your frames, the 'fitter' establishes where your pupils sit with respect to the frame (lining you and the frame up from a distance) and a spot is placed on each dummy lens (plus XY axes). The lens is ground with its axis in the centre of the disc and any cylindrical component is orientated in the right direction. Then it's cut out to fit the frame with the centre in the right place.

Varyfocals are more critical and have to be matched precisely as the prescription varies between looking up and looking down. I bought a pair many years ago and , as soon as I tried them on, I could tell they had been mis-registered. I had to deal with "try them for a while and see how you get on" and persuaded them to check the registration. Sure enough, the 'little dots' didn't match my pupil spacing.

But the OP certainly has a point about circular lenses. Nearly all high spec optics has circular symmetry. Diffraction spikes are even visible due to the shutter or iris leaves in cameras. Telescopes, apart from the really huge ones, have circular symmetry. Large optics need multiple elements because they are too heavy to support all in one piece.

 

[sophiecentaur]

. . . . .not to mention Fresnel lenses.

(I thought I asked you not to mention them.)