Why are Camera Lenses Round and Film Square?

This is a thinking project. I’ve asked The Straight Dope and not heard back yet… I’m sure I know or can come close to the answer but lets talk it out and see where it goes.

For our purposes we need some definitions:

  • Square = Any object of square, rectangular, boxy or cube likeness. Corners are always 90 degrees.
  • Circle = Any object of circular, oval, spherical or generally round like shape, regardless of deformation on a planar axis (lens shape). Has no corners.
  • Clear = Any material that is optically clear; as in it allows the visible spectrum of light to pass through it.
  • Film = Any image receiving material. This could be old style polyesterPETnitrocellulose or cellulose acetate light sensitive film or new style electronic optical sensors like CCD and CMOS.

So why is a camera lens round and the film square? Why not round film? Why did it get setup like this?

Lenses

In order to focus and direct light the object or medium doing this has to be clear (duh!). This ability isn’t exclusive to round objects though. Windows are square. Windows are round. Some windows are both square and round. Same goes for eye wear. They’re all optically clear, but the shapes vary from square to round.

To focus light, the clear object needs to be distorted (convex or concave if you like). You can see this with old school eye wear.  Remember the coke bottle jokes? A cross section of the lens was more of a shallow bowl shape. Today this is less so, but the lens is definitely distorted.

Taking a closer look at a focusing lens though shows me something interesting- as you get towards the outer edges of the lens, its ability to keep the image in focus fades. Take a look at a magnifying glass. They’re usually convex on the outsides and the only part of the image in focus is what’s in the middle. The more it magnifies the more distorted the image is as you look farther from the center.

If we had film that was round, the outer edges would most likely be out of focus. BUT! If the film was smaller than the image cast by the lens the film would only capture what was in focus.

Makes sense. Right?

Film

Film is square. CCD (charged coupled devices) and CMOS devices are the electronic versions of film. But they’re still square. Why?

Well from an electronic perspective I can tell you that the matrix of dots that make up the CCD and CMOS chips makes sense. A matrix is square or cubic. Think of a spreadsheet. Each pixel in the matrix (cell) has an address. Spreadsheets have letters on the x and numbers on the y axis. CCD matricies are similar. The combination of x and y allow each pixel (cell) to be addresses individually. This makes sense. Old school chemically developed film doesn’t have to deal with this, but it did have to be coated evenly with the photo sensitive material. So why did they start out square?

Here’s my guess. Humans like square objects:

  • they stack perfectly
  • they are easy to handle
  • they store well
  • they organize well
  • straight lines seem to be easier to make and cut as opposed to arcs or circles
  • the written word has typically been on square like tablets
  • humans tend to read in lines- lines are straight (how hard would a book be to read if the text was along a circular path from outside in??)
  • paintings and drawings seem to lend themselves to square canvases
  • they’re not round

From a design and engineering perspective the square (or plate that is square) seems to make sense too. Engineers like to make machines use  round things for moving parts and square parts to hold the round ones. For example look inside a laser printer or any printer for that matter:

Laser printer mechanics

See all the round stuff? Guess what holds it in place? Big squares of metal or plastic. Maybe engineers and inventors have always thought this way. I can’t find an answer to that. I’d love to see another take on this.

So back to film. It’s square. Traditional film was a roll (really big rectangle or long square…) divided into frames. Really really old film was on plates. Square plates. One single square plate per frame. Present day “film” sensors are square. They output square images. Those images are made of square pixels. Square.

Printing or developing film has always been on paper. Paper is square. Printers (the machines, not the people) like square paper. Ever tried to use a round sheet of paper in a printer?

Mechanically it seems to make sense to use squares to print on.

The Conundrum

So we know that light focuses best through distorted round clear things. We know that humans, machines, math and computing devices like squares. I guess light will just have to be the odd one here. (I know math likes circles too, but squares seem to be easier to calculate. They have good symmetry (ok so do circles). Call me lazy.)

Lenses focus the light onto square film. But wait. The incoming image isn’t square. It’s round. Sadly, some of the image is lost. Happily, the lost parts of the image are the ones from the outer edges of the lens that are out of focus. Remember earlier:

Taking a closer look at a focusing lens though shows me something interesting- as you get towards the outer edges of the lens, its ability to keep the image in focus fades. Take a look at a magnifying glass. They’re usually convex on the outsides and the only part of the image in focus is what’s in the middle. The more it magnifies the more distorted the image is as you look farther from the center.

I guess I just answered my conundrum.

My Conclusion

Lenses are round because they focus light better than a square. Round lenses are most likely easier to manufacture than a square lens that is, well, rounded.

Film is square because it is easiest for humans and machines to manipulate. Paper has always been square. Printing has always been done on squares. Old film on plates was square because it was originally done on glass. Which was already square for windows (I assume….). Machines like squares.

Square film also conveniently solves the issue of round lenses distorting the outer parts of the image. The film is only exposed to the most focused center of the incoming light. Call it auto cropping. It seems as though cropping an image is just built into the design. Whether the cropping was intentional or not, it worked and it happened long before we had digital cropping tools.

 

Comments?  I’d love to hear them. Did I screw up something? Let me know!

 

 

 

 

Author: Helicopter Jeff

I'm a vinyl decal cutting and designing, Astrophotography and general photography capturing, RC helicopter/quad copter pilot, Arduino and Raspberry Pi hardware/software developer, network, scripting and troubleshooting ninja living in the metro NY area. I am passionate about my hobbies so much I do them for a living. There's nothing better than getting paid to do what you love.

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