Apature

[Zeva]

So heres another nubbish question from me :P ... So i was wondering if lets say i have a 70-200mm f/2.8 lens what exactally does the appature on the camera do? isnt it fixed at f/2.8? thanks

[TheWengler]

You're right, that is newbish! f/2.8 is the max aperture which is available throughout the focal range. If you see something like f/3.5-5.6 then that means the max aperture is 3.5 at the wide angle end with a max of 5.6 at the telephoto end of the focal range.

[fionndruinne]

No, it isn't fixed at f/2.8, it's just the maximum aperture remains f/2.8. You can still "stop down" to smaller apertures (larger f/numbers).

Edit: Wengler beat me to it. And I was quick, I thought!

[SpecialK]

First off, the word is "aperture".

The aperture is the hole formed by the blades of the diaphragm of the lens. The actual size of the aperture is calculated by dividing the focal length by the f-number. The smaller the f-number, the larger the aperture and the more light it can let in at any given shutter speed. Depending on the lens, the maximum aperture can stay constant, or change to a larger f-number as the focal length increases. Large aperture/constant aperture lenses are the more expensiv versions.

[Zeva]

So... You mean like for the 17-70 sigma... its F/2.8-4.5 which means at 17ish its maxed at 2.8 and 70 its maxed at 4.5? so it cant go below 4.5 at 70 but like the 70-200 F/4 COULD go to f/4 at 70 AND 200? so its just the max? and its not fixed and can be shrunk? thanks BTW those were fast replies!

[TheWengler]

So... You mean like for the 17-70 sigma... its F/2.8-4.5 which means at 17ish its maxed at 2.8 and 70 its maxed at 4.5? so it cant go below 4.5 at 70 but like the 70-200 F/4 COULD go to f/4 at 70 AND 200? so its just the max? and its not fixed and can be shrunk? thanks BTW those were fast replies!

Yep, you got it.

[Zeva]

haha ty for help w/ nubbish question

[Razr]

So... You mean like for the 17-70 sigma... its F/2.8-4.5 which means at 17ish its maxed at 2.8 and 70 its maxed at 4.5?

Only if you are in "P" (PROGRAM) or "A" (AUTOMATIC) modes.
The aperture stays the same in "Av" and "M" (MANUAL) mode, no matter what focal length you choose, making any f/2.8-f4.5 etc. lens a "fast"-constant apeture lens throughout the zoom range.

[fionndruinne]

Uh... only at f/4.5, Razr. Not at f/2.8.

[zmikers]

Only if you are in "P" (PROGRAM) or "A" (AUTOMATIC) modes.
The aperture stays the same in "Av" and "M" (MANUAL) mode, no matter what focal length you choose, making any f/2.8-f4.5 etc. lens a "fast"-constant apeture lens throughout the zoom range.

Maybe I misread your post here but I think you are wrong. If you are in Av or M mode with a f/2.8-f4.5 lens on your camera and your Aperture is set at f/2.8, as you zoom the aperture will not stay at f/2.8, it cannot. It will change to match the max aperture to that specific focal length. So the sigma 17-70 f/2.8-4.5 has a max aperture of f/2.8 only at 17mm and a max aperture of f/4.5 at 70mm.