Is f2.8 the same as f2.8?

[LJ2b2c]

Thanks for the education. I'll have to read this a few more times, but I think you hit the nail on the head as to what I was looking for.

[Jon Ruyle]

The sensor size and the crop factor are more relevant. For example, f2.8 on a full frame lens, like the EF 16-35, has a larger absolute opening than f2.8 on the EF-S 17-55.

In theory this is false. f number is defined as focal length divided by aperture, period. Sensor size does not matter. So the EF-S 17-55 must have an aperture of 55 / 2.8 mm, while the EF 16-35 has a smaller aperture of 35 / 2.8.


However, it is true that for wide lenses, the front element is sometimes larger than the theoretical aperture because the lens has to do something weird to illuminate the large sensor. In this case, an EF-S lens may indeed have a smaller front element than
an EF lens of the same focal length and f number. I would still say the 16-35mm f/2.8 has an aperture of 35 / 2.8 mm (because that is what those words mean to me) even if the front element is larger, but maybe I *should* say "effective aperture is 35 / 2.8 mm" or something.

[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.

[ShutterbugJohan]

I was wondering when you'd jump in Daniel....now where's George Slusher?
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I've been wondering that too... I don't think that he's been online much since his camera and a bunch of his lenses were stolen (see his profile for a list). I hope he's back soon.

[Colin500]

Although I didn't ask the original question: Thanks for this excellent explanation, with so many example calculations that it's impossible not to "get" it!


It clears up some things that I had been wondering about while considering whether to upgrade to a 7DmkII or a 5DmkIII (from my current 500D/T1i) ... and since it confirms my suspicions about crop factors, apertures, and iso-numbers, I'm starting to lean much more towards the 5DmkIII.


Regards, Colin