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Focal ratio is focal length divided by aperture: 750 mm over 150 mm is f/5. Because image brightness per unit area scales with the inverse square of the ratio, an f/10 system needs four times the exposure of an f/5 for the same extended-object signal. Aperture alone determines total light gathered and resolution; the ratio determines field and image scale.
Focal ratio
f/ratio = focal length / aperture
Exposure scaling
time is proportional to (focal ratio)^2 for extended objects
For point sources, no - aperture decides that. Faster ratios help extended objects like nebulae, where signal is spread over area.
Because a long focal length gives the image scale needed to sample fine detail across the sensor pixels.