F4

[Digital Doug]

Hi Guys & Gals, Was wondering if the f4 aperture on a sigma 17-70 mm lens with a 72mm diameter lens would let the same amount of light in as the f4 aperture on the Canon 70-200mm f4 with a 67mm diameter lens ?

 

[CT]

F4 is f4 Doug regardless of the physical size of the aperture. It's a measure of the lens ability to pass light in relation to it's focal length. F4 with either lens coupled with any given shutter speed would result in exactly the same exposure value.

Hope that makes sense?

 

[Digital Doug]

Great thanks CT

 

[apodo]

The reason the thread size on the Sigma 17-70mm is 72mm is so that you do not get vignetting at wide-angles, nothing to do with the aperture.

Michael.