Question about variable aperture

[edbayani]

practically speaking, aperture number is the distance from the sensor to the lens opening. so if the diameter of that opening is 2.8 x from the sensor, the number is f2.8. since the design of the lens can focus nearer at micro the distance is extended that when you measure the distance with the diameter it is 4.8 times so you get the usable f4.8 aperture even without changing the lens opening.

[OsmosisStudios]

Originally Posted by edbayani

practically speaking, aperture number is the distance from the sensor to the lens opening. .

No, it's not. The aperture is the diameter of the opening of the lens at the focal point. More specifically, the number is the factor by which that diameter differs from the focal length which is, in turn, the distance from the sensor to the focal point of the lens.

Kirbinster: Remember that all the stats provided on lenses are at infinity focus: that counts for focal length and aperture. Its the same reason you get focus breathing on the new 70-200 f/2.8 lens: they play tricks with the design to get better performance corner-to-corner, but as a result they effectively shorten the focal length at shorter focusing distance (IIRC the focus group is within a magnification group or vice versa)

[Digidave]

Originally Posted by kirbinster

Oh so this is a "feature" of the famous Nikon 105mm micro then?

Yes it does it on all three bodies.

My Nikon 60mm Macro does the same thing. I read an article about it once, or more precisely, I read a portion of an article. My head started hurting & I stopped reading before it exploded with information overload.