CIRCLES OF CONFUSION
When we talk about a lens, we often describe it by the film format or sensor size that it’s designed to work with. Super 35mm lenses are designed to work with Super 35mm film or sensors that are around the same size; Super 16mm lenses are designed to work with Super 16mm film or similar sensors, etc. This can be confusing in itself, as we talk about lens focal length in millimeters, as well. You can have a 35mm (focal length) Super 16mm lens, for instance, and a 16mm (focal length) Super 35mm lens.When we describe a lens as being designed for Super35 or 16mm film, we’re actually talking about the image circle of a lens. Lenses are designed to project a circular image onto a sensor or film plane. The size of the projected image circle is based on the design of a lens. An average "Super 35mm" lens has an image circle diameter of around 32mm to 35mm, while an average "Super 16mm" lens has an image circle diameter of around 15mm to 18mm. Lenses designed for smaller formats, usually in video cameras, are often referred to in inches. A 2/3-inch-type lens has an image circle of around 11mm, and a 1/2-inch lens image circle is around 8mm.
In the graphic above, you see Super35, Super16, 2/3-inch and 1/2-inch image circles on top of each other. This photo was taken on a 50mm (focal length) lens. If it were a Super 35-type lens, we would get the outside image; if it were Super16, we would get the image outlined for that and so on. The focal length of the lens is exactly the same; we just get more of the image with the larger image circle. Focal length is unrelated to image circle, which is a big point of confusion for many of us. A 100mm lens has the same focal length, regardless of its lens type; the difference is just how big of an image circle it projects.
So hopefully you understand the image circle concept. Now let’s discuss how it plays together with sensor (or film) size. Super 35mm film is standardized at 24.9×18.7mm (31.1 diagonal), but our modern digital sensors come in a variety of shapes and sizes. From the large full-frame 35mm sensors found in many DSLRs to the small 1/2.5-inch sensors found in many POV cameras, sensor sizes vary a lot. Below is a graphic with a variety of camera sensors overlaid in relative size. As you can see, each sensor is a bit different.