Focal Length & Crop Factor — A brief example
Focal length is the distance of the focus.
Imagine a pinhole camera. It’s just a box with a pinhole in one side. Light rays converge at the pinhole & create a mirrored image on the rear surface of the box.
In this example, the pinhole is the lens and the rear of the box is the “image plane” — where you would put the film or the sensor.
Let’s say, for the sake of argument, this box is 50mm cubed — 50mm per side. We would call this a “50mm lens” — because the distance between where the light rays converge and the image plane is the focal distance.
As you move the pinhole further away, the image gets larger.
What would happen if we doubled the size of the box to 100mm cubed? What do you think would happen to the size of the image projected on the rear of the box?
It would also double in height.
But — if the film itself didn’t double in size, you end up with a much smaller portion of the image projected onto the film. When you put a telephoto lens on a camera, the whole camera doesn’t increase in size, only the focal length increases.
As you move the “pinhole” further away, the image gets bigger — but if the film stays the same size, it sees less of the scene.
We call this “angle of view” — we can see less of the world, so we get a smaller angle of view.
The effect of increasing the focal length while keeping the film size the same is to change the “angle of view” — you can see much less of the world when the focal length is farther away from you. Makes intuitive sense right?
If you make a frame with your fingers — the further away from your eye the you hold out the frame — the smaller the view of the world that you see. This is because your eye doesn’t get any bigger as the box through which you see the world gets further away.
We call lenses with longer focal lengths “telephoto” or “long” lenses and we call shorter focal lengths “wide” or “short” lenses.
What does this have to do with “crop factor” — everything.
The angle of view (how much of the world you see) is a combination of the focal length & the size of the sensor/film. To get a smaller angle of view — you can either move the focal length further away, or make the sensor smaller.
