Aperature Question.

[twistedphrame]

So I've been wondering for a while about aperature settings. My
understanding is that the aperature is basically a ratio between the
actual diamater of light being let through and the diameter of the
barrel correct? What I've been wondering is if say you have two
lenses one is 80mm barrel diameter. and the other is 50. at f/2.8 is
the aperature of the 80mm larger than the 50 mm because of the size
ration? and thus allow more light through? So would the 80mm be a
faster lens than the other since it has more light or am I just making
things up here?

[Daniel Browning]

My
understanding is that the aperature is basically a ratio between the
actual diamater of light being let through and the diameter of the
barrel correct?

You are correct if you replace "diameter of the barrel" with "focal length". Aperture is focal length (e.g. 80mm) divided by f-number (e.g. f/2.8), which in the case of 80/2.8 is 28.6mm.


(Photographers often talk as if f-number and "aperture" are the same thing but they are not. F-number is the "relative" aperture, but "aperture", by itself, is as described above.


I'll replace "diameter of the barrel" with "focal length" in your quote below:

What I've been wondering is if say you have two
lenses one is 80mm focal length and the other is 50. at f/2.8 is
the aperture of the 80mm larger than the 50 mm because of the size
ration?

Yes.

and thus allow more light through?

No, the wider aperture allows for the *same* light, but higher magnification (smaller field of view).


An 80mm f/4.5 has the same aperture as a 50mm f/2.8, but it focuses light at a lower intensity (less light through per second). In order to increase magnification (or "zoom in") without losing light gathering power, the aperture must be increased.


Incidentally, there are such things as "focal reducers" that take a long, slow lens (e.g. 400mm f/8, aperture of 50mm) and turn them into short fast lenses (200mm f/4, aperture 50mm), like the opposite of a teleconverter. They work great on telescopes, but focal reducers for camera lenses are much harder due to flange focal distance at patent restrictions.

So would the 80mm be a
faster lens than the other since it has more light or am I just making
things up here?
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It's correct to say the 80mm has the same f/number, but a wider aperture. It is not faster.


If there was a focal reducer for the 80mm f/2.8 that converted it to
50mm, then it would be a 50mm f/1.8 (and, indeed, faster than the 50mm
f/2.8).