How do I compare optical zoom and cropping when moving from a 10× compact to a 1-inch camera?

Question

I currently use a Canon SX120 IS (36–360mm equivalent, 10MP) and am considering a 1-inch compact for better image quality in lower light. Some options I’m looking at have much shorter optical zoom ranges, such as 28–100mm equivalent lenses on 20MP cameras, but they offer in-camera cropped “digital” zoom modes like Sony Smart Zoom.

How can I fairly compare my current camera’s 360mm equivalent reach with a camera that has a shorter lens but more megapixels for cropping? For example, if I only need about a 5MP final image, how much extra reach does cropping a 20MP image provide, and how should that be compared with a camera’s optical zoom or focal length?

user1943 · Accepted Answer

Cropping and zooming are basically equivalent, so this is easily calculated. Zooming in 2× — like, focal length 36mm to 72mm — is like cutting the frame in half. Of course, that's each dimension, so you need to square it to calculate megapixels. For example, a 24mpix image might be 6000×4000, and to simulate that 2× zoom, you'd cut it in half each way, to 3000×2000 — or, 6mpix.

To zoom in 10×, megapixels get cut by 10², or 100×, so you'd have a 600×400 pixel image — not very much to work with. And, even a 1" sensor isn't very large, so by looking that closely, you're pushing the camera system (lens and sensor) quite a lot, and you probably wouldn't be very happy with the results even for online viewing or small prints where that resolution might otherwise be adequate.

How do I compare optical zoom and cropping when moving from a 10× compact to a 1-inch camera?

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