What crop factor actually measures
Crop factor compares the diagonal of a sensor with the diagonal of a reference format, usually 35mm full frame at 36 × 24mm, which measures 43.27mm corner to corner. A sensor with a crop factor of 1.5× records an image whose diagonal is two thirds as long, so any lens on it frames like a lens 1.5 times longer would on full frame.
The lens has not changed. A 35mm is a 35mm on any camera. What changes is how much of the image it projects the sensor actually records.
There is no single Super 35 crop factor
“Super 35” is a family of sensor areas, not one size. On the same ALEXA 35, open gate works out at 1.27× against full frame, 4.6K 16:9 at 1.35×, and the 3.8K 16:9 mode at 1.62×. Quote a single number for “Super 35” and you can be a full focal length out by the time it reaches the lens list.
Before converting anything, find out the recording mode. The camera body tells you very little; the mode tells you the sensor area.
Why the diagonal can mislead
The diagonal comes from stills photography, where sensors mostly share a 3:2 shape and the convention works well. Cinema modes do not share a shape. Take that same ALEXA 35 in open gate and again in 4.6K 16:9. The diagonal crop factors differ, 1.27× against 1.35×, yet a 35mm lens gives exactly the same 43.6° horizontal angle of view in both, because the sensor is just as wide. Only the top and bottom of the frame have gone.
That is why the comparator shows horizontal angle of view alongside the equivalent focal length, and why its matching column works from the width of the sensor. When you are cutting two cameras together, the width of frame is usually what has to match. On those numbers, a 35mm on an ALEXA 35 in 4.6K 16:9 frames the width like roughly a 45mm on an ALEXA Mini LF in 4.3K LF 16:9.
What changes, and what does not
Field of view changes. That is the whole of what a crop factor tells you.
Perspective does not. The relationship between foreground and background is set by where the camera stands. Swap to a larger sensor and a longer lens from the same position, and the perspective is identical.
Depth of field does, indirectly. To hold the same framing on a larger sensor you use a longer lens, and a longer lens at the same stop gives shallower focus. As a rough rule, multiply the stop by the crop factor between the two formats to find the stop that gives a similar depth of field. It is a starting point for a conversation with the focus puller, not a substitute for one.
Coverage: the image circle has to reach the corners
Every lens projects a circle of usable image. If the sensor mode is larger than that circle, the corners fall away into vignette. A Cooke S4/i has an image circle of about 31mm. An ALEXA Mini LF in open gate has a diagonal of about 44.7mm. Put one on the other and the lens coverage tool shows exactly what you would expect.
The delivery crop is where it gets useful. A scope extraction removes the top and bottom of the frame, and the corners that remain may sit much closer to the image circle. The tool marks the delivery frame and tells you whether the picture you actually deliver is covered, which is often the real question on set.
Anamorphic lenses are measured in the squeezed plane, the image the lens lays on the sensor before it is desqueezed. The tool draws them desqueezed so the frame reads the way it will be seen.
Image circles are manufacturer figures, often quoted at a particular stop, and they change with focus distance and stop. Treat a near miss as a reason to test on the camera, not as a guarantee either way.
Camera and lens figures come from manufacturer specifications. If you spot one that is wrong or missing, tell us and we will fix it.