I clearly understand, for example, a single element 300mm f/2 lens has a diameter of 150mm, by definition.
I do not understand how this calculation for front element diameter applies to a multi-element lens group. Can someone explain it to me? I have a gut feeling the focal length not being measured from the front element to the image plane factors into this.
Focal length is never measured from the front element to the image plane, but rather it's the distance from the nodal point (optical center of the lens) to the image plane, with the lens focused at infinity. As
@GMCPhotographics pointed out in another thread, the defining feature of a telephoto lens design is that the lens is physically shorter than the focal length, i.e., the nodal point of the lens is in front of the front element, beyond the lens itself.
Entrance pupil diameter is focal length / f-number. The entrance pupil is the optical representation of the physical aperture (iris diaphragm). The front element must be
at least as large as the entrance pupil to fill the wide open aperture with light. With telephoto lens designs, the entrance pupil is located at (or just behind) the front element. That's why we typically approximate the front element diameter of telephoto and supertelephoto lenses as focal length / f-number.
For other lens designs, that approximation doesn't hold. For example (because it's the lens on the camera sitting next to me), the RF 20/1.4 has an entrance pupil diameter of 14.3 mm, but the front element of the lens a bit over 44 mm in diameter, 3 times larger than it 'needs' to be.
On a related note, I do not understand what is happening here: On my EF 100mm f/2.8 macro lens (both L and non-L versions), as I focus from infinity to 1:1 magnification, the light meter indicates exactly 1/4 the transmission at 1:1 relative to infinity. Does this mean 200mm f/5.6 effective at 1:1?
The effective focal length is actually getting shorter due to focus breathing, the EF 100/2.8L Macro at 1:1 frames like a lens of ~68mm. Focal length doesn't really factor into exposure, that's the point of using f-numbers (for example, a 400/2.8 is letting in a lot more light than a 100/2.8 because it has a much larger entrance pupil, but with both lenses set to f/2.8 the exposure will be the same).
At high magnifications, the effective f-number is f-number x (magnification + 1), so at 1:1 your wide open f/2.8 lens is effectively f/5.6 [2.8 x (1 + 1)].
My MP-E 65 at 5x and f/11 (a reasonable compromise between DoF and diffraction) is effectively f/66 [11 x (5 + 1)] and that is
dark. That's why I typically use the MT-24EX Twin Lite with it, even in bright daylight.
