Crop factor and f/stop

[HiFiGuy1]

Thanks George. I felt like it must, but my concept of f-stop and irisup until now has been simply "lower f-stop means larger iris". I guess it does hold true, but obviously focal length must be considered when comparing f-numbers. I never really thought about it in the context of zooms, but now this thread is making me!!

[Colin]

In terms of taking the picture and setting exposure within a single camera, the f/X number is really what we want to know. If the F/X number is the same, the same amount of light hits the sensor. the actual iris diameter will be larger at a longer focal length, but the angle of view is less, so it collects less light from an angular sense, so makes it up with a larger iris diameter.