harisA wrote:Is this having a front element with diameter of 184/0.9 =204mm?

Lou Jost wrote:That relationship is not the precise definition of f-number. This lens is analogous to, and has the same proportions as, the Repro-Nikkor 85mm f/1.0.

I have never held either of these lenses in my hands. However, to clarify what I think is going on...

The

*effective* f-number of a lens is well defined by the angle of the cone of light that it delivers to the sensor.

When a lens can be used at infinity focus, then the

*nominal* f-number is also well defined -- it is just equal to the effective f-number at infinity focus. That number will be equal to the focal length divided by the entrance pupil diameter, which is certainly no bigger than the front element diameter and is often smaller.

However, when a lens can

*not* be used at infinity focus, then the nominal f-number is not so well defined and the entrance pupil may be larger than the front lens element. (In some cases it can be

much larger, for example with telecentric lenses where the entrance pupil diameter is theoretically infinite!)

In this case one approach is to measure the effective F-number when the lens is used as designed, then specify a nominal F-number by inverting a classic formula like

F_effective = F_nominal * (magnification+1) , so that

F_nominal = F_effective / (magnification+1) .

I expect that is what's being done here. That is, in use, I expect the lens is delivering effective f/1.8 at 1:1, leading to a calculated value of f/0.9 for nominal f-number.

A good model may be to think of this lens as two f/1.8 lenses placed close together, with their "infinity" sides facing each other.

Perhaps Lou can tell for sure when he gets the lens in hand.

--Rik