Is f2.8 the same as f2.8?

[neuroanatomist]

ybe I *should* say "effective aperture is 35 / 2.8 mm" or something

Nope, no 'should' about it. Aperture is the diameter of the iris diaphragm. F-stop is the ratio of focal length to aperture, i.e. = focal length / aperture. With algebraic rearrangement, aperture = focal length / f-stop.


A 70-200mm f/4 has a max aperture of 50mm, while a 70-200 f/2.8 has a max aperture of 71.4mm (43% greater diameter). That's one reason why an f/2.8 lens is so much larger than an f/4 lens of the same focal length - the optical elements need to be able to 'fill' the aperture with light, and more glass means more weight (and cost!).


Front element size is not necessarily the same as or directly proportional to aperture. In camera lenses, it's always the same size or larger, as far as I know. But in some optical systems (certain microscopes, for example), the front element of the objective lens is smaller than the iris diaphragm.

[Jon Ruyle]

ybe I *should* say "effective aperture is 35 / 2.8 mm" or something

Nope, no 'should' about it.

Okay, if "aperture" is diaphram size and is not the same as "front element size" then I don't need to say "effective aperture". Just "aperture". The world makes sense. I can breathe easy.