Why are lens round but photo is rectangular?

Started Aug 8, 2010 | Discussions
gaius Regular Member • Posts: 226
Why are lens round but photo is rectangular?

Why are lens round but photo is rectangular?

trekkeruss Veteran Member • Posts: 3,899
Re: Why are lens round but photo is rectangular?

Because round photos would be a waste of material.

Chris59 Forum Pro • Posts: 15,437
Re: Why are lens round but photo is rectangular?

gaius wrote:

Why are lens round but photo is rectangular?

Lenses are round because that is the easiest shape to grind and polish. Image circles are also round (if they weren't they wouldn't be called circles ).

Photographs are rectangular because that is the way most people want an image to look so the top, bottom and sides are cropped by the rectangluar shape of the sensor.

Rectangular sensors are also smaller than the image circle so are less expensive to produce.

There was a thread not too long ago arguing for circular sensors so that the image could later be cropped any way the photographer wanted and obviated rotating the camera 90 degrees for portrait orientation, but clearly it was not a popular proposition.

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AnthonyL Veteran Member • Posts: 3,269
paper is rectangular

and most viewing devices.

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sherwoodpete
sherwoodpete Veteran Member • Posts: 7,767
Re: Why are lens round but photo is rectangular?

The lens could be made rectangular, but that would needlessly sacrifice either image brightness, or image quality.

From any point on the subject, rays of light are captured by the entire surface of the lens, and are focussed to produce the corresponding point on the image. That is, all of the round lens is used to create any given point on the image.

If the lens was cropped to a rectangular shape, the image would lose brightness. Now it is normal to use a variable aperture to control the image brightness, and doing so tends to improve the image quality. The use of a rectangular lens would give a darker image, but the quality would not be improved to the same extent.

More importantly, it is usually desirable to make the lens capture as much light as possible. A circle is the optimum shape to achieve this. Attempting to capture the same amount of light with a rectangular lens would result in considerable aberrations and loss of image quality, or might even be physically impossible, since the corners of the rectangle would be part of a much larger circle.

Regards,
Peter

igb Senior Member • Posts: 2,637
see this thread

http://forums.dpreview.com/forums/read.asp?forum=1030&message=30588085

There are many others, since it's a question that periodically comes up. short story, -

  • circular lenses cheaper/easier to produce than rectangular ones.

  • Rectangular sensor cheaper¨ user to prduce

  • Many lenses have rectangular internal baffles to reduce flare

  • DSLR systema work much better with rectangular mirrors

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hotdog321
hotdog321 Forum Pro • Posts: 20,754
Re: Why are lens round but photo is rectangular?

The image cast by the lens is circular, but the sensor/film utilizes just part of that circle. Furthermore, the perimeter of the circular image is less sharp than the center. That is why you want to check the corners of the frame first to see if you have a problem with sharpness.

A square film/sensor would be better, but it is not a very popular format beyond the medium format folks.

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sherwoodpete
sherwoodpete Veteran Member • Posts: 7,767
Re: Why are lens round but photo is rectangular?

hotdog321 wrote:

A square film/sensor would be better, but it is not a very popular format beyond the medium format folks.

There are arguments in favour of an oversize sensor, such as used in some Panasonic cameras. That enables a chosen rectangular format to be selected with a diagonal which fits within the image circle of the lens. However, such a sensor need not be square.

Regards,
Peter

WilsonEPhillips Senior Member • Posts: 1,317
Re: Can you imagine what the bokeh would look like

If you think the doughnut bokeh from the mirror lenses is bad, just picture rectangles all in the background of your images.

Deleted1929 Forum Pro • Posts: 13,050
Re: Why are lens round but photo is rectangular?

The lens is round because it's simpler to manufacture and because a lens needs to behave the same way when rotated. If it's did not the distortions would depend on your shooting orientation - very undesirable.

The aperture would ideally be round also, but in practice is a multi-sided polygon with either straight or rounded edges that don't quite make a circle. A perfectly round aperture is desirable for the quality of the out of focus blur.

Film was rectangular because it's convenient to make and because paper has always been delivered as a rectangular shape - mainly because it's both efficient to make that shape and efficient to use it. Round paper and round film would be nightmarish to work with and very inefficient to make.

Sensors are round because they must mimic existing film shapes to work with existing technologies in a predictable manner. However the same manufacturing efficiencies apply to sensors as to film, and, if anything, more significant with sensors.

In principle there is no reason why you could not make round sensors and use them, however the cost of such sensors would be double the cost of existing sensors, but offer very few advantages in practical terms.

It's also worth remembering that processing image data from a rectangular sensor is a relatively simple task but with a round sensor you add complexity.

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Piecart Forum Member • Posts: 91
Re: Original Kodaks produced cirdular images/prints.

In the days of the original Kodak cameras, which came loaded with "film" and which you sent back for reloading and processing, the image on the negatives and the actual prints was circular.

If you play around with a rectangular print (using a circular magnifying glass), you will notice how easy it is to compose pleasing circular "crops".

Piecart Forum Member • Posts: 91
Re: Why are lens round but photo is rectangular?

trekkeruss wrote:

Because round photos would be a waste of material.

But rectangular photos are a "waste" of the lense's image circle: square would be "better".

TechOutsider Senior Member • Posts: 2,979
Re: Can you imagine what the bokeh would look like

Ha, but the aperture need not be round.

WilsonEPhillips wrote:

If you think the doughnut bokeh from the mirror lenses is bad, just picture rectangles all in the background of your images.

Piecart Forum Member • Posts: 91
Re: Can you imagine what the bokeh would look like

TechOutsider wrote:

Ha, but the aperture need not be round.>

Exactly. The Olympus Trip 35 (10 million sold) has a two-bladed aperture which produces a diamond-shaped opening but still gives excelent bokeh.

sherwoodpete
sherwoodpete Veteran Member • Posts: 7,767
Re: Why are lens round but photo is rectangular?

Piecart wrote:

trekkeruss wrote:

Because round photos would be a waste of material.

But rectangular photos are a "waste" of the lense's image circle: square would be "better".

Only if the final result is required to be square.

For many purposes, both aesthetic and practical, a rectangular image is required. Cropping a rectangle from a square gives a smaller and more "wasteful" image than fitting a rectangle of those proportions into the original image circle.

In fact the only real justification for the square format was the practical difficulty of turning the old roll-film TLR using a waist-level viewfinder.

Regards,
Peter

sherwoodpete
sherwoodpete Veteran Member • Posts: 7,767
Original images fitted the shape of the material

Early art-works fitted the shape and contours of the cave-wall or animal-skin, or other material such as bone or antler.

However, there is also long tradition for producing images which are rectangular.

Certainly artists or painters generally opt for the rectangular format, unless the target is something like a round pot or plate, in which case the shape of the object dictates the image format.

Regards,
Peter

p51d007
p51d007 Senior Member • Posts: 1,284
Re: paper is rectangular

LOL...yeah, but I go back far enough to remember working on old Zenith
TV's with the 70 degree "round" picture tubes

They were a MAJOR pain in the you know what to get the convergence
to align properly.

AnthonyL wrote:

and most viewing devices.

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chuxter Forum Pro • Posts: 21,714
Re: Why are lens round but photo is rectangular?

gaius wrote:

Why are lens round but photo is rectangular?

The same reason pie is round and cake is square...convention.

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Roger99 Forum Pro • Posts: 11,572
Because round photo's would roll away. (nt)
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It is the mark of an educated mind to be able to entertain an idea without necessarily accepting it. -Aristotle

...oh, and I see by the lack of responses that I am right yet again.

Roger99 Forum Pro • Posts: 11,572
It's really because...

Photo's are rectangular because paintings were and paintings were because walls and stretchers were. Some early photography was round or ovoid but even then when up against a fight for artistic credibility as a medium, following fine art conventions was the norm.

Even when those early ovoid or round images were printed it had little to do with the nature of the lens but more to do with referencing contemporary fashionable painting of the time.

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It is the mark of an educated mind to be able to entertain an idea without necessarily accepting it. -Aristotle

...oh, and I see by the lack of responses that I am right yet again.

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