Modern f/stops

[Christopher Walrath]

Whitey, that was some great referential info there. I'm curious, why would US1 equivocate f/4? Was that an old aperture extent or something?

[Whiteymorange]

Guessing here. You keep cutting the US in half as you go down, US16 = f/16; US8 = f/11; US4 = f/8; US2 = f/5.6 and US1 would equal f/4. The system wasn't very flexible and seems, for the most part, to have kept to whole numbers.

[Ole]

At the time the US was introduced, the fastest lenses were f:3.5 Petzvals. So there was simply no need to be able to mark anything faster than f:4 - or US 1.

[GeorgesGiralt]

Hi !
I was told that in order to measure the exact opening of the diaphragm of a lens, ons should focus the camera at infinity, and then, without moving the focus, place a piece of B&W paper in the front cap and expose it from the rear (with a spotlight placed at the ground glass). the diameter of the black spot on the paper is an exact measure of the lens aperture as it should be optically.
I wonder if this method is good but I think it is making sense as it use a spotlight at one focal lenght from the lens, and measure the opening as projected...
Maybe one of you can comment and explain this to us ?

[phfitz]

GeorgesGiralt,

That should work well BUT some lenses will give truely strange measurements that way. Some lenses are measure in the reverse direction or they do not match the aperture scale. Optical illusion of the lens design. Try it front and rear to see which way matches wide open.

Have fun with the hunt.