inconsistent aperture zoom lens

[thomson_chan]

Dear all,

 

I have some questions regarding inconsistent aperture zoom lens.

 

For example, my 24-85 F3.5-4.5 G AF-S lens, when its at 24, its

aperture is 3.5; when zoom to 85, the aperture changes to 4.5.

 

My question is: When it changes to F4.5, does it only mean the light

intensity received by the lens is equivalent to F4.5? Does the depth

of field change?

 

Cheers,

Thomson

[jitterbugboy]

depth of field naturally compresses as the lens zooms. Even if it was a constant 3.5, depth will be a lot greater at 24 than at 85. Otherwise it just has to do with metering.

[vedearduff]

The depth of field changes. This is due to two factors; the aperture and the focal length.

 

The depth of field changes even on my 35-70mm f/2.8 constant aperture zoom when I zoom from one extreme to the other.

 

I hope this helps.

 

Vernon

[steve_phillipps]

When it goes to f4.5 it is actually f4.5, as the aperture is related to focal length/diameter of front element, it's not just an effective aperture value but a true one.

 

Steve