Calculating f stop

[mylek]

Hello,

i have a Seroco (Conley/Vista) portrait lens series II 10in f/5 with an iris but there is no marking or scale on it. I''ll like to marked it.

If i'm using the focal length/diameter formula, the front lens have the proper diameter (254/5=51mm) but the iris wide open has a diameter of 42mm, which bring it to f/6. Something seems to be wrong using this formula with such a lens.

I encontered the same problem as i was trying to make Waterhouse stops for a petzval lens. Some of them have a narrower inning ring to receive the stop. What is the proper way to calculate it?

[goamules]

Your formula is correct, as long as you measure the apparent iris opening, with the front glass on. Do not simply measure the glass, or the iris inside, the magnification of the glass has to be taken into consideration for the formula. Also, many lenses are not really as fast as advertised, especially once the shutter or iris is in place.

So you can make a waterhouse template out of cardboard, with a crosshair reticle like a submarine periscope. Put it in, look in front, measure with a ruler what your calculations say the next fstop should be. remember which "ring" or "dot" on your recticle that corresponds to, remove and cut a hole that size.

[Bob Salomon]

Just meter through your gg and mark the stops on the lens. Just first take a redaing of a grey card through the gg and directly from the card. The difference is the absorbtion of the gg. Enter that as a filter factor and meter trhough the lens.

[Jim Andrada]

I think this would be a calculation of T-stop rather than F-stop since it would include the effects of the (less than 100%) transmittance of the lens elements themselves.

[Mark Sawyer]

Also, many lenses are not really as fast as advertised, especially once the shutter or iris is in place...

I've noticed this a number of times.

[Jim Andrada]

Just like automobile fuel mileage!

[Dan Fromm]

Diameter of the entrance pupil/focal length, not diameter of aperture/focal length

[Mark Sawyer]

"The entrance pupil is defined as the image of the aperture stop as seen from an axial point on the object through those elements of the lens which precede the stop. "

http://hyperphysics.phy-astr.gsu.edu...oopt/stop.html

[Emmanuel BIGLER]

There is a good discussion in this forum's archives on how to find the diameter of the entrance pupil in a lens, without need of any expensive instrumentation.
"Entrance Pupil and how to properly measure Lens Speed "
http://www.largeformatphotography.in...entrance+pupil

[Dan Fromm]

Diameter of the entrance pupil/focal length, not diameter of aperture/focal length

Oops! I wrote them upside down. This is an LF forum, but still ...