Variable maximum aperture.

[john.margetts]

I have an issue that I am trying to get my head around. It is to do with zoom lenses where the maximum aperture changes as you zoom the lens.

First issue. I set the aperture ring to f/3.5 while the lens is zoomed to 28mm. I now zoom the lens to 210mm (my particular lens but applies to any such lens), the aperture is now f/5,6 but the aperture ring still says f/3.5. I am happy that the physical aperture remains the same so the relative aperture must change. Does this affect the relative aperture at the other aperture ring settings?

Second issue. I am using this lens on my 'new' Nikon Nikkormat EL. When fitting the lens to the camera, I have to index the lens by turning the aperture ring to both its limits. I do this with the lens set to 28mm. When I zoom to 210mm, does the camera's light meter take this change of relative aperture into account?

 

[db3348]

Does this affect the relative aperture at the other aperture ring settings?

Yes John all f/ numbers are affected to some extent .
If you weren't already aware, f/number values are traditionally derived from expressing physical diameter of aperture as a fraction of the focal length , e.g., f/4 means the physical aperture diameter is 1/4 of the focal length, f/8 means the physical aperture diameter is an 1/8 of the focal length , etc . So therefore if the focal length changes , while the physical aperture may stay same, but the value of the f/number will change .

does the camera's light meter take this change of relative aperture into account?

On the second issue, yes the light meter should be taking into account, because as the focal length gets longer, a relatively smaller amount of light is reaching the film. The meter is always measuring the amount of light reaching it's measuring sensor .

cheers

db3348

 

[ancient_mariner]

David is quite correct. It's only really a problem if you're using an external meter or flash, otherwise the internal metering effectively just measures the light coming through the lens.