The sensor is the heart of a camera system. The camera lens forms an image and the sensor captures this image as a grid of pixels.
It’s the sensor that sets the number of megapixels that a camera has. But far more important than that is the sensor size.
When you choose a camera system you want the largest sensor that you can comfortably carry around with you.
People carry their camera phones every day because the sensor is tiny. It doesn’t need a very big lens and it takes up very little space in the phone body.
The downside of a small sensor is that it gathers less light. Indoors and at night this becomes a problem. Phones have lots of tricks to work around this including capturing multiple images and merging them together.
Another problem with small sensors is that the lenses must be made more precisely in order to form an image that is sharp enough.
For years the standard film camera was the 35mm SLR. The size of each frame was 36x24mm.
A digital camera that has a sensor this size is called “Full-Frame”. But, most cameras have sensors that are smaller than this, mainly because it’s cheaper. A large phone sensor might only be 8x6mm. A common size for proper cameras is called APS-C and this t is around 24x16.
From the point of view of light-gathering, the area of the sensor is what matters. A full-frame camera has over twice the area of an APS-C one and over 20 times a phone.
The size of the sensor also affects the field of view of any attached lens. As the sensor gets smaller the area of the scene it sees reduces. So the lens has a smaller field of view than it would on a full-frame camera. It acts as if it’s focal length is longer than it actually is.
This effective focal length can be calculated. As an example an APS-C sensor is 24mm wide, whereas a full-frame is 36mm, 1.5 times wider. This ratio is called the crop factor of the sensor and used to calculate the effective, or what we call the equivalent focal length. You just multiply the lens’s actual focal length by the crop factor. So a 50mm lens on APS-C is equivalent to a 75mm lens on full-frame.
35mm SLRs were in use for decades. Because of this when people talk about a 50mm lens it’s often assumed that it is on a full-frame camera.
It’s important to realise that the focal length of the lens does not change. But it behaves as if it does. An APS-C lens with a focal length of 35mm has 35mm written on it, but it has the same field of view as a 50mm lens would have on a full frame camera.
This might seem convoluted but equivalent focal length gives you a common language for comparing lenses between systems.
Compact cameras have small non-standard sensor sizes. They normally have zoom lenses and display the current focal length in the viewfinder. But they display the equivalent focal rather than the actual one. Because it’s more familiar.
To summarise. Sensor size is the most important parameter of a camera system.
A lens attached to a camera with a sensor smaller than full frame has its focal length multiplied by the crop factor to calculate an effective focal length.
For homework, find a compact camera with a zoom. Look at the end of the lens and see if you can find the focal length as a range of values in millimetres. Compare that with what the camera displays in the viewfinder as you zoom.