Full frame sensors are sensors in digital cameras the size of which are 36mm x 24mm.
They are based on 35mm film cameras. But how? I hope this diagram makes it clear. The top to bottom height of 35mm film is 35mm. That’s the standard, and it has been the standard for over a century.
But the film has to move along so that with each wind-on of the film advance, a new frame is in front of the lens ready to be exposed.
The manufacturers stamp sprocket holes at regular intervals along the film. The film advance catches in those holes and moves the film along.
If the image that the lens cast onto the film covered all the 35mm depth of the film, then photographs would have holes at regular intervals across them.
So the camera manufacturers put a frame in front of the film to mask off any light that might fall beyond the rectangle. And the size of the frame is a rectangle that’s 36mm x 24mm.
And that’s why ‘full frame’ digital sensors are 36mm x 24mm
As I said in this article How Big Does A Lens Have To Be, not all digital sensors are full frame.
Here are the relative sizes of some of them. I haven’t bothered to name the smallest size because it is intended to be representative of various sensor sizes that camera manufacturers put into compact cameras.
Why Make Different Sensor Sizes?
As I explained in How Big Does A Lens Have To Be, smaller sensors mean smaller cameras and smaller lenses.
So why do camera manufacturers make cameras with bigger sensors?
The answer is because like most things in photography, it’s a trade-off between image quality and convenience. I’ll get into that in the next technical post.