Does binocular objective lens diameter really affect brightness?

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Offline David Cooper

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It's obivous how with a camera lens, increasing the amount of light getting in by opening the aperture will result in more light hitting the film/CCD and thereby generate a brighter image. I'd always assumed the same applied to telescopes and binoculars because it's common knowledge that that the objective lens diameter relates to brightness, but is it really so?

If you draw out the lines of the light paths on paper, it doesn't look as if this can work, because the further out a line comes in from, the further out it will be when it hits the occular/eyepiece and so it will hit further out from the centre of your eye, and indeed far beyond the edge of your iris, thus failing to add to the light reaching the retina. So far as I can see, increasing the size of the objective lens should make the image brighter up to the point where the whole iris is taking in light, and then beyond that size it will make no difference (other than to the field of view angle which will continue to increase).

Unless there's some special optical trick going on which I haven't taken account of, what this suggests is that there should be no brightness advantage in using 7x50 binoculars over 7x30s. I'm hoping that there is a special optical trick to it though, because if there is it might make it possible to make a zero-magnification, optical light amplification device for viewing aurorae.