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Old 10-01-2016, 05:15 PM
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Ryderscope (Rodney)
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Originally Posted by RickS View Post
The f-ratio is focal length divided by aperture, so it's pretty easy to calculate. For f/4 at 70mm focal length you'd want a 17.5mm aperture. At 200mm focal length the aperture would be 50mm to give you f/4.

I've puzzled over this question myself at times.

With my Canon 200mm F2.8 lens, the filter diameter at the front of the lens is 72mm. At F2.8 the lens will be wide open. Calculating this = 200/F2.8=71.43 so that's pretty darn close to 72mm. This being the case, calculating the size of the aperture mask to place on the front of the lens to achieve a particular f stop is straight forward.

However, for some lenses it doesn't seem to be as straight forward as that. With my Canon EOS 100mm Macro F2.8 lens the filter size on the front of the lens is 58mm diameter.Calculating the required aperture withe lens wide open gives 100/2.8 = 35.7mm which is less than the diameter of the front of the lens.

I have the same issue with my 50mm f1.2 lens which at 50/1.2 gives me 41.6mm which again is less than the diameter of the front of the lens (58mm again).

On this basis it seems that the diameter of the aperture at the front of the lens must be less that I would have normally calculated.

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