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Thanks, that sorts me out nicely. And as a bonus, corrects my thinking in another area, that of the focal plane.
I have always tended to think of the optics as having a focal POINT, which I suppose is perfectly accurate, but it does not tell the whole story, that assumes objects are are a point source, plainly that is not the case for more or less anything but stars (Ignoring atmospheric effects) So to take M42 as an example, it is obviously an extended object and the image formed at the prime focus will not be a point but a similarly extended image. Increase the focal length (Leaving all else the same) increase the image size for the same amount of light captured so the light is distributed over a larger area and any given point will have less light falling on it.
Only leaves me one thing to ponder, if the image size formed at the focal plane by a given diameter objective is dependant on focal length (And therefore ratio) why does the image size not change when you stop a camera down?
I thought I would leave that question in although I reckon I have thought of the answer myself while typing it. Anyone who knows better please correct me.
In a camera lens we stop it down by reducing the apeture not increasing the focal length, so the image size would remain the same due to the focal length of the lens assembly, but the objective is effectively smaller so there is less light to be distributed in that formed image, we change a different parameter compared to doing something like sticking a fastar on my SCT, but the focal ratio is still the same between a 50mm lens at F2 and a 100mm at F2, the images formed will be different sizes but the INTENSITY of those images will be the same so the lesser amount of light captured by the 50mm is distributed in a smaller image and the exposure time will be about the same (Assuming like bojan that it is a perfect world, no light scattered sideways, none absorbed and turned into heat etc)
Nice to start my day by learning something totally irrelevant to me in my line of work!
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