Am I understanding f-stops correctly?

11 years 10 months ago #235498 by PMurphy
I've been trying to get a better understanding of certain photography related terms that seem to be pretty common. Now from what I can see f-stop number is largely changed by what focal length of the lens you are using right? So if someone were to say the aperture opening diameter is f/8 would not be correct, right? So if I had a 50mm lens with aperture setting of f/8 then the diameter of the aperture opening would be something like 50 divided into 8 would equal 6.25mm right? Just as if I was using my 105mm lens with aperture of f/11, the aperture opening would technically be 105/11 = 9.55mm correct? Or do I have this totally screwed up?


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11 years 10 months ago #235505 by KCook
Dividing the focal length by the f-stop is correct, to get the effective diameter of the aperture. However it is still common practice to call the "aperture (opening) f/8", or whatever. Even though this is not the ideal syntax.

Also, note that I said "effective diameter". With modern compound lens designs the actual opening, at the front surface of the lens, is often greater than what the f-stop calculation gave. This is especially true for wide angle and zoom lenses. Thus the filter diameter is usually quite a bit bigger than the max focal length divided by the widest f-stop.

Kelly Cook

Canon 50D, Olympus PL2
kellycook.zenfolio.com/

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11 years 10 months ago #235516 by icepics
The aperture is the opening in the diaphram of the lens.

The f stop is the measurement of that opening. Since it's done in fractions, f8 is 1/8 the focal length of the lens.

That's why the larger aperture (openings) are smaller numbers - being fractions, 1/4 of the length of the lens would of course be larger than 1/16. So f4 is bigger than f16.

Sharon
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11 years 10 months ago #235543 by PMurphy
You guys are so helpful :beerbang: I never thought of it that way, that 1/16 was 1/16 of the focal length of the lens. Good to know. Thanks again!


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11 years 10 months ago #235599 by shelland

Scott

- Twin Cities, MN

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11 years 10 months ago #235833 by Henry Peach
focal length / entrance pupil = f/#

The "entrance pupil" is the aperture as it appears through the front of the lens, as opposed to the actual physical size of the aperture.

But the important thing to remember is that the reason we use f/stops vs just measuring the aperture is that a given f/stop is approximately the same amount of exposure no matter what format, focal length, etc...
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