2x zoom lens with same absolute aperture instead of extender ?

mermaidkiller wrote:

MitchAlsup wrote:

But you are not measuring the size of the entrance pupil! You are measuring how much of the lens can be seen through the viewfinder--which needs to be the whole lens (or the manufacture would save money by making the lens smaller.

You can observe the size of the entrance pupil by opening up the aperture all the way and see how much of the view finder you can see from the front of the lens. Hint: its a lot smaller than the size of the objective. The objective is large because it has to fit multiple entrance pupils across its face in order to fully illuminate the off axis portions on the sensor.

That is exactly the objective / entrance pupil diameter. This IS the test to measure the objective diameter. This website tells how to measure the diameter of objective of binoculars. This is valid for telescopes as well and ... cameras. A DSLR woth OVF is just a.... telescope with objective and eyepiece, regardless of how many mirrors, lens elements or baffles are in the light path. Any obstruction will be shown in the projection image on the objective site.

...

I believe you are measuring the "effective" aperture.  In other words what diameter aperture you would need if the lens was composed of a single element, and the aperture was inside that element.

Most photographic lens are composed of multiple elements, the physical aperture is not co-located with the front element.   A 100mm lens at f/2 has an effective aperture diameter of 50mm, but the physical opening may be a different size.

This is one of the factors that make constant aperture zoom lenses practical.  The physical aperture is located such that the effective aperture does not vary wildly with focal length.