70-200mm f/2.8

[canonguy]

hi all, this may sound like a silly question, if you are using a 70-200mm f/2.8 lens, but you set an apperture of say f/8 for example then you are not shooting at a constant f/2.8 are you? can someone explain how using this lens you shoot at a constant f/2.8, thanks..

 

[Danny133]

leave it at aperture 2.8 lol ... shooting wide open means shooting at the lowest possible aperture .. as the lens is fixed at 2.8 .. it stays at 2.8 all the way through the focal range

if you shoot at f8 your letting less light in but "usually" get a sharper picture

 

[specialman]

Modern lens only close their aperture blades down to the inputted value (say f/8 ) as the shutter is pressed. Otherwise, the lens is as wide open as possible to aid focussing by allowing as much light in as possible. So if it's a f/2.8 lens but you are shooting at 1/100th @ f/8, the lens only closes the aperture when you take the shot

 

[MindofMel]

Modern lens only close their aperture blades down to the inputted value (say f/8 ) as the shutter is pressed. Otherwise, the lens is as wide open as possible to aid focussing by allowing as much light in as possible. So if it's a f/2.8 lens but you are shooting at 1/100th @ f/8, the lens only closes the aperture when you take the shot

+1

 

[dinners]

Constant f2.8 just means that f2.8 is available as your max aperture all the way the max zoom range not just at it's shortest length.

In terms of focus - a f2.8 lens at f4 has an advatage over an f4 lens at f4 as it will use it's max aperture to focus regardless of the fact you're using f4.

 

[canonguy]

thanks for your replies, now i get it.

 

[pm79]

That was quite helpful. I didn't know that if you take picture at f/4 or f/8 it only closes down to that aperture when you press the shutter and otherwise open at f2.8 all time and focuses at 2.8. Very interesting...

 

[Craikeybaby]

If you want to see the aperture in action there is the DoF preview button (it's below the lens changing button on my 20D).