Circular Lens, Rectangular Image
Date: Spring 2013
All camera lenses are circular but images are rectangular why?
In digital cameras today, the light coming into the camera is usually captured and turned into electrical signals. The camera can then turn those electrical signals into the image. The device that takes in light and makes electrical signals is called a CCD (charge-coupled device). The CCD is in the shape of a rectangle, so the resulting image ends up being a rectangle (and the incoming light outside the rectangle is not captured). It is just the way they are made.
Now, if we think about film-based photography, why would the photo be rectangular? Film cameras work by taking in and focusing light onto film. A "negative" is formed on the film when it is exposed to light and developed. The negative can then be used to create a photograph. The area of the film exposed is rectangular, so the image is rectangular. But why would photographers prefer rectangular images? It is a matter of real estate. If you think about fitting the picture negatives on a roll of film you can fit rectangles with less wasted space than circles, so the amount of film needed to store the negative of a rectangular image is less than the amount that would be needed to store a circular image.
So it does initially seem odd that a circular lens produces a rectangular image, but really there is nothing too complicated going on.
The image is circular. It is the detector (film, or, nowadays, the CCD array) that is rectangular. In other words, the camera detector records only a rectangular portion of the image.
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
The circular lens does project a circular image onto the back of the camera. The reason why photographs are typically rectangular is because the CCD (the chip that records the light) or the film (if you are not using a digital camera) is rectangular, so only the light that falls within that rectangular area gets recorded.
Fisheye lenses are famous for creating circular images. In these photos, the circular image projected onto the CCD is actually smaller than the CCD itself, so you see the entire circle and the pixels outside of the circle (but within the rectangular CCD) are black because no light reaches them. See the Wikipedia article for some examples: http://en.wikipedia.org/wiki/Fisheye_lens
Hope this helps,
Thanks for the email. Camera lens are circular as there is less distortion of the light through the lens as compared to rectangular lenses. Distortion of light can result in blurry pictures and shifted colors. Also, I think circular lenses are less expensive to manufacture than rectangular lenses.
I hope this helps. Please let me know if you have more questions.
The lens may be round, but the image capturing mechanism at the back of the camera is square.
Here is a diagram showing how the (round) lenses form the image on the (square) image capturing mechanism in the back of the camera. This picture is from this URL:
The image focused through a circular camera lens IS circular. The ?apparent? image is rectangular, square, or any other ?shape? occurs because the film, or semi-conductor array, is ?cropped? by a frame that does not let incident light make contact with the light sensitive detector. You can get the same effect with your eye. Look at an object through a rectangular frame. You only ?see? the part of the image that is within the frame. Images whose light hits the frame does not hit your eye, so the image appears to be the size and shape of the frame.
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Update: November 2011