In my digital sensor size post, I talked about the different image perspectives you gather when using a particular sensor or film cell size. What the camera “sees” is determined by the space occupied by the sensor or film cell within the lens’ projected image circle. In a nutshell, this describes crop factor.
Think of it this way: picture a magnifying glass projecting a beam of sunlight on a tree leaf. The beam of light is the image. The circle of sunlight projected on the leaf is what we call the “image circle”. So the lens is responsible for the image the camera “sees”. Repeat after me: the lens is responsible for the image.
Crop factor vs image size
But what if the leaf were smaller than this image circle? The leaf would only experience a certain portion of the projected image at any given time. Never the whole image. Since camera sensors are square-ish, most every digital camera behaves this way.
The term “crop factor” describes how much of the image the sensor experiences. The thing is, crop factor is really a misnomer. The camera doesn’t crop anything; the entire image is there, the camera sensor just doesn’t see it (and therefore, doesn’t record it). The lens still transmits the full image circle. You can put a lens on one camera with a particular sensor size and capture an image. Then, put the same lens on a different camera pointing at the same subject and you’ll get a different image. The image appears zoomed-in or out. It’s not; the sensor sizes are different.
A phrase that’s often thrown around goes something like this: “the Super 35 camera turns a 35mm lens into a 50mm lens”. No, it doesn’t. A 50mm is a 50mm is a 50mm. What this misguided thought process actually means is that a 35mm lens behaves like a 50mm lens when used on a Super 35 sensor (even this isn’t totally correct). To confuse things even more, the “this lens turns into that kind of lens” wisdom only describes a specific relationship – full frame to everything else (as if full frame is some sort of standard we care about).
Should you give a damn?
No. Any project worth doing is going to afford the cinematographer, director, and all the important people the time to at the very least turn the camera on and mount a lens before you actually film the thing. If the project consists of you and you alone, then do it yourself. Turn the camera on, point it at something, and observe how it sees the world. You should know how your camera performs and how shots look with various lens types.
Even if using multiple cameras with different sensor sizes on the same shoot, the likelihood of running into true problems is small: a shoot that can afford multiple lenses and cameras is likely one that is already using the Super 35 industry standard in every camera, but even if not, different sensor sizes are often found with different mounts, making crop factor a moot point anyways (the image is based on the lens, remember?).
Instead of obsessing over minutia on the Internet, how about film something instead? I promise that eager young filmmakers with bright eyes and bushy tails are much more apt to make an enjoyable film than somebody concerned about crop factor.