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Twisted Lip

Hit the DSLR quality buffer with IC434

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F ratio is the photographic speed - so it is crucial to exposures. One minute of F4 is equal to 2 minutes of F5.6 (assuming focal length is constant so the pictures are the same).

OK, but if you go from F4 to F5.6 at constant focal length, you half the area of your aperture. So half the number of photons you can collect.

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Yes, that's the point. F ratio is the ratio between the image scale and the light gathering area.

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A.G, surely it's only aperture that matters here? F-ratio should only affect image scale?

No Russell, it is the Fratio that matters, two scopes of F5 will require the same exposure even if the apertures are different, the one with a larger aperture will have a longer focal length and a narrower FOV, probably better resolution depending on the seeing .

Regards,

A.G

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Sounds like it's both then ;) You can't compare f-ratio, you also need either aperture or focal length. heck, you could ditch f-ratio and just use aperture and FL :)

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