Why are lenses circular?

[dschuleman]

I am sorry if there

is a blatantly

obvious answer to

this question, but

if the film and

sensors in cameras

are rectangular,

wouldn't it make

more sense for

lenses to cast a

rectangular image

instead of a

circular one?

[Charles_Webster]

Probably, but it's lots easier to grind round lenses than square ones. A few plastic cameras over the years have had injection molded rectangular plastic lenses, but they were "toy" cameras, not capable of or expected to take sharp pictures.

 

<Chas>

[tommyinca]

Because our eyeballs are circular. Our brain doesn't like non symmetry lens. Some of us may get sea sick with them :-)

[phule]

[[but if the film and sensors in cameras are rectangular, wouldn't it make more sense for lenses to cast a rectangular image instead of a circular one?]]

 

Wouldn't a rectangular lens make aperture design more (perhaps needlessly) complex?

[eric friedemann]

In every lens I own, elements rotate into position to acheive focus and, in the case of zooms, to change focal length. This fact would seem to encourage- if not dictate- the use of circular elements.

[subbarayan_prasanna]

In my digital camera the lens is circular but the opening in front of the lens is rectangular, in proportion to the sensor, I guess.

[colin carron]

Charles has the answer. Lenses are made with a circular grinding action so they automatically tend to have a circular shape. The can then be mounted in a circular screw action mount for focusing. You could of course cut off sectors to make the lens square or rectangular but that would complicate things without gaining anything.

[ty_mixon]

Perhaps the question should be reversed - why are sensors rectangular? Film was rectangular, I believe, because of the way it had to be fed.

 

:)

[lex_jenkins]

Good point, Ty. Sensors don't need to be rectangular. Many moons ago one of the earliest compact cameras for handheld photography used circular glass plates. The photos were surprisingly good, especially considering the plates were probably sensitized to what would now be the equivalent to ISO 10 or slower.

 

At some point, at least during the 1960s, some lens designers must have wondered the same thing. Oh, the lenses were still barrel shaped with circular elements. But they fitted the rear mount with rectangular masks. Presumably the idea was to reduce internal flare.

[steve_dawson1]

Until aspherical elements were developed - when lenses could be modelled in computers - lens surfaces were a section of a sphere.

[steve_dawson1]

Until aspherical elements were developed - when lenses could be modelled in computers - lens surfaces were a section of a sphere.

[hugh_stevens1]

Hello Folks

 

Great answers - I agree with them all. Yes, a circular lense would then need to be made rectangular increasing

the amount of work needed to accomomplish the same objective, placing a low distortion image on palte, film,

etc. Rectangular lenses, like circular lenses, if not properly designed add distortion to an image.

 

As far as a circular sensor versus a square or rectangular sensor has to do with the design of the CCD/CMOS

sensor and the way the information is retrieved from the sensor. Yes, Nikon and other cameras allowed use of a

smaller sensor area in exchange for a faster frame rate, just think 'digital zoom'. The CCD/CMOS really is a light

sensitive fast reacting grid that transfers it's electric charge value per pixel into a memory structure. Given that

memory addressing schemes mostly follow an 'X-Y' or 'horizontal by vertical' layout, a circular sensor would pose

some interesting memoery addressing schemes to properly integrate the info. I am not saying that it isn't done,

I'm just saying that I don't want to be the electrical engineer who has to come up with the memory 'polling' driver

circuitry. As with everything with the briallant people who write here, there will be those who say that it can, or is

currently done, i.e. circular sensors. This is true, I concede that point. I also concede it can become a debate

and PN isn't the place for that debate. Hope this helps - Hugh

[markci]

A physically square lens would not cast a square image. That's very naive.

[lex_jenkins]

Mark summarizes the issue succinctly.

 

I have a rectangular shaped magnifying glass. It still focuses light into a circle. That's what a lens does. If it doesn't, it's not a lens. It's just a bit of see-through stuff.

 

However, in the Borg Collective, alternatives may be possible. Not probable. But possible.

[derek_hofmann]

But doesn't being in focus mean that the image is the same shape as the subject?

[ken_jeanette1]

Everyone is wrong! The big bang exploded material out from the epicenter in all directions at once, hence a circle. In homage, only the circle is perfect. Early lens designers wanted their product to be perfect, hence the spherical lens. What I'm really getting at is does it really matter?

[eric friedemann]

"(D)oes it really matter?"

 

 

It might. Glass is heavy- imagine how much weight you could leave out of a lens by making the deminsions of the elements conform to the actual image area they need to project.

 

 

This is analogous to DX lenses, which are smaller and lighter than their full frame counterparts. DX lenses use less glass and are lighter, because they don't need to project as large an image circle.

[steven_p]

A rectangular shaped lens for "film" would be the same as a round one only with the edges cut to square. Basically, lens elements have one optical center that align with one another to correct focus and focal distance.

[michael_dimarzio]

Some lenses are not, some are not even glass.

 

An IR Line Scan the "lens" maybe a spinning 4 sided stainless steel mirror. There also have been lenswds made of Germanium or Selenium, I don't know a thing of those though.

[igor_jarm]

If you were thinking about reducing the existing lenses to rectangular shape:

1) aperture is idealy circular, with fast lenses for example all of the area of the front element is used, hence almond shaped bokeh off center - the front element is too small as it is, now if it was rectangular...

2) weird bokeh with rectangular openings

3) you'd have even more light falloff

3) mounting the lens elements with circular retaining rings as oposed to what?

4) unnecessary difficulty in grinding the rectangle

 

It would make a little sense to appropriately enlarge the existing lens to rectangular shape, but that would mean a lot more weight, size, cost, difficulty in manufacturing etc.

 

PS cameras that seem to have rectangular lenses I believe only have rectangular shades over circular glass.

[yann1]

Why are our pictures rectangular, and not circular...

[mohir_ali]

'The big bang exploded material out from the epicenter in all directions at once, hence a circle.'

 

Sphere - the universe is like a giant ballon; everything is receeding from everything else.

 

'Why are our pictures rectangular, and not circular...'

 

Get a fisheye lens if you want a circular image.

[Brian]

It's a good question because it's made us (well some of us) think outside the box. But it's about basically the simplest way to do things - the radially symmetrical element is easy to grind and easy to mount in a focussing mechanism. The change to rectangular at the image stage is again about the simplest way to do things. Film transport is simplest with straight edges, likewise sensor construction. And cutting rectangular paper is a whole lot easier than circles!

[mrstubbs]

For photography? The corners of a rectangular/square lens would not be useful in maintaining focus or transmission of light equivalent to the sides of a rectangular/square lens. The closer to the edge of a lens the more distortion occurs. Cropping image collected by a lens is much more economical than wasting more lens not used to collect a useful image.

[charlesheckel]

The thread so far is useful and edifying. Let us proceed to consider the next question: why are balls round?