Your lens projects its circular image onto your camera’s sensor. The image circle could be smaller or larger than your sensor–the lens doesn’t care. So what would happen if your sensor is significantly smaller than the image circle projected by the lens? You’d see only the center of the image. And vice versa, what happens if the sensor is larger than the image circle? In this case, you see the entirety of the lens’s image, and the black area around that image circle where the sensor is not exposed to light. This darkening around the edges usually isn’t this extreme in real life, but it’s what we call vignetting. Lens manufacturers generally try to find the sweet spot where their lens is small and light enough to just cover the entirety of the sensor without too much noticeable vignetting. It is quite normal for a lens to fall off both in brightness and sharpness towards the edges of the field. So it makes sense that a lens would be designed for a specific sensor size.
Sensors come in many different sizes. Many people assume the bigger the better since large sensor cameras are generally more expensive. Larger sensors mean larger photosites (the little photo receptors that actually gather light on the sensor) and that means improved light sensitivity and dynamic range, but the difference is not so large as it once was. Larger sensors make shallow depth of field more easily achievable. We’ll talk more about what that means later, but it’s essentially how narrow your area of focus is which relates to how blurry your background can be. Low-budget productions often benefit from the ability to blur the background when sets are less-than-high-end, and it’s an easy way to direct focus to a subject. That being said, in some situations these “advantages” can be undesirable. Larger sensors require larger lenses making them comparatively less portable, they provide a wider angle of view meaning “less zoom”, and they can make deep depth of field (think macro photography) more challenging. The tricky part about crop factor is that lenses are classified according to their “focal length”: a value (usually in millimeters) that refers to the distance between the back of the lens (where light rays converge) and the sensor. The focal length is what you’ve probably known as the “zoom” of the camera. The problem is this: what if your lens is projecting this displayed image onto your sensor but the sensor is only wide enough to capture the blue box in the center? Your true focal length hasn’t changed, but your “effective focal length” is now much greater because the sensor is only picking up the center of the image making the image appear more “zoomed.” The recorded image you see will be very zoomed in or “telephoto” looking because you’re only seeing part of it. For this reason, we need a baseline to determine this “effective focal length” or “field of view” and we’ll compare all others against it. That baseline is 35mm “full frame” film. Anything smaller is a “crop” of that and will have an associated “crop factor”. Multiply the focal length of the lens (in mm) by the crop factor and you have your effective focal length. For now, just know that it’s important to know the size of your sensor because it determines how much of the scene you see, which we’ll call your “field of view”.
35mm “Full Frame” is about 36mm wide. Again, that’s the gold standard. In photo cameras, this film was run horizontally through the camera, meaning the perforations run along the top and bottom. This leads to an image area of approximately 36mmx24mm or “full frame”. Many of today’s digital cameras still use 36mmx24mm sensors based on this initial 35mm sizing. Examples include Canon’s 5D line, and Sony’s popular A7 series cameras. It’s also become quite popular for motion picture video cameras to use this same full frame sized sensor. High end cameras like the Sony Venice, Arri Alexa LF and Red Monster all cover something similar to this full frame size. However, this historically has not been the case, and a lot of people shooting on their Canon 5D Mark II cameras at the start of the “DSLR revolution” didn’t realize that they were shooting to a larger full frame ’sensor’ than most of their favorite Hollywood movies had used. In motion pictures, that same 35mm film was run vertically, rather than horizontally. In this case the area between the sprockets becomes the width, instead of the height of the image like it was in a stills camera. This makes the full frame image approximately one and a half times larger than the image from the 35mm film run horizontally, and it’s very close to the format still photographers know as APS or “cropped sensor” because it’s a ‘cropped’ version relative to the full frame size. Popular cameras include the Canon Rebel and SL series (e.g. T6i, SL2, 80D, etc.) , Nikon’s DX line (D3500, D5600, D7500 etc.) and the Sony alpha 6X00 line (e.g. a6500). “APS” is what most people refer to if they talk about “cropped” cameras vs “full frame” cameras. Again, the crop factor of most APS sensors is around 1.5X. APS is about 25mmx19mm. MFT (micro four thirds), another popular sensor size is about 22mm wide. iPhone XS is about 5.6mm wide (but classified as a 1/2″ sensor-see the bonus section for more information there.)
So what’s the takeaway? Know your camera’s sensor size and what that means to you. Know how it relates to your lens options, especially concerning field of view and depth of field. It’s nice to know how it compares to other cameras, but don’t obsess over it. You likely don’t need “full frame” any more than your favorite directors of photography didn’t need it when shooting your favorite films.