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Why Does A Circular Lens Capture Rectangular Pictures? Explained

This might seem like an obvious thing to some people, but for others it can be mind boggling! 

The short answer is:

💡 Photographs and images from cameras are square or rectangular in shape because the image sensors or photographic film that we use is always square or rectangular in shape. The shape of the lens is mostly irrelevant.

Why Are Pictures Rectangular?

The reasons pictures are rectangular or square is because of the shape of the film or image sensors.

Since the only light-rays that are recorded and stored are the ones that reach the image sensor, the shape of the sensor determines the shape of the image. 

Traditionally, image sensors and film frames are rectangular.

Take the example of 35mm film frames – they measure 36mm x 24mm which ends up being the aspect ratio of the final image of a 3:2 rectangle. 

The round lenses actually focus light-rays in a circular image onto the sensor however most of the time the sensor or film will be smaller than the image of light-rays that the lens projects.

Since the circle of rays is larger than the sensor, the sensor simply takes a portion of the circular image produced by the lens. This leaves some light-rays ‘cut off’ and they are not used. 

If you were to use a lens which produced an image of light-rays which was smaller than the sensor, then you would find that the image produced would have a circular dark border. 

Similarly if you had a round sensor which was exactly the same dimensions as the round image projected by the lens into the the sensor then you would end up with a round image…

But of course we don’t use round image sensors.

Why Are Camera Lenses Round?

Without getting into the details of it, round lenses are simply easier to produce compared to rectangular ones. 

Camera lenses are produced using lathes and the grinding process lends itself more towards round lenses.

Since the process requires grinding, polishing and spinning, a round lens is easier to shape than a rectangular one.

Why are Image Sensors Rectangular?

Similar to the reason that lenses are round, image sensors tend to be rectangular for a manufacturing reason.

Image sensors are printed onto a silicon wafer and using rectangular sensors would be more cost effective.

This is because when the sensors are ‘cut out’ of this wafer, if you were to use circular sensors you would have a lot of empty space and material which goes to waste.

Using a rectangular shaped sensor minimizes waste of materials.

In addition to this, rectangular or square shapes lend themselves well to how we tend to portray images in our world.

Think of paintings in frames, projector screens or computer screens as examples. They’re all rectangular.

What Is The Role Of A Lens? 

In the camera system the lens itself doesn’t produce the final image.

The lens’ role in a camera is a pathway for light and it focuses light rays onto a photosensitive surface.

The photosensitive surface in digital cameras is the image sensor and on a film camera it’s the film. 

The lens does actually focus light-rays in a circular image so there will be a circular image being beamed into the camera body and the sensor.

To understand why this doesn’t result in a final image which is circular, you need to understand how a photograph is created and remember it is the lens’ job simply to focus the light-rays on a photosensitive surface.

How Is A Photograph Created? 

Once the light-rays have passed through the lens and been focused on to the film or image sensor the the data is recorded.

In film cameras, you have to develop the negatives, but in digital cameras, an image processor will take the data from the image sensor and then produce a digital rendition of the image. 

You should now know why we get rectangular or square images even though lenses are round.

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