Lesson 2 of 18 · 8 minute read
Sensor size and what crop factor really means
A smaller sensor keeps a smaller slice of the image the lens projects. The lens itself never changes.
A lens projects a circle of light. The sensor sits behind it and keeps a rectangle out of that circle. A bigger sensor keeps a bigger rectangle.
This is the entire truth behind crop factor, and it is worth being precise, because the shorthand confuses people for years. A 50mm lens is 50mm regardless of what it is bolted to. Put it on an APS-C body and you keep a smaller portion of what it projects, so the resulting frame looks like what a 75mm would have given you on full frame. The lens did not become a 75mm. You simply threw away the edges.
Two consequences follow. First, for the same framing, the crop body needs you to stand further back, and standing further back changes depth of field. That is why full frame has a reputation for shallower backgrounds: it is a side effect of distance, not magic. Second, a smaller sensor collects less total light for the same exposure settings, which is why larger sensors generally hold up better as ISO climbs.
None of this makes small sensors bad. Micro Four Thirds gets you smaller, lighter lenses for the same reach, which is why wildlife and video shooters often choose it deliberately.
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