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mylek
8-Jun-2014, 07:35
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
8-Jun-2014, 07:53
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
8-Jun-2014, 08:09
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
27-Jan-2016, 18:05
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
27-Jan-2016, 19:59
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
28-Jan-2016, 17:52
Just like automobile fuel mileage!

Dan Fromm
28-Jan-2016, 18:40
Diameter of the entrance pupil/focal length, not diameter of aperture/focal length

Mark Sawyer
29-Jan-2016, 11:16
"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/hbase/geoopt/stop.html

Emmanuel BIGLER
30-Jan-2016, 09:38
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.info/forum/showthread.php?114769-Entrance-Pupil-and-how-to-properly-measure-Lens-Speed&highlight=entrance+pupil

Dan Fromm
30-Jan-2016, 10:32
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 ...

Dan Fromm
30-Jan-2016, 10:33
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.info/forum/showthread.php?114769-Entrance-Pupil-and-how-to-properly-measure-Lens-Speed&highlight=entrance+pupil

Thanks for reminding us of this old discussion. I've added the link to it and to y'r explanation of how to measure focal length to my list of useful links.

Emmanuel BIGLER
30-Jan-2016, 15:29
Thanks for reminding us of this old discussion.

HI Dan!
Actually it is amazing to see the number of outstanding photographic pictures recorded since 1826 by photographers who had absolutely no idea of what an entrance pupil is.

Reminds me the old joke about the bumblebee: according to the most recent advances in aerodynamics, scientists say that the bumblebee cannot fly, this has been published in a serious peered review scientific journal.
Fortunately, bumblebees never read scientific journals, and continue to fly as they have always done, at least since the last glacial era (I doubt that any bumblebee existed in the US or in France when both countries were covered by a massive ice cap).

Dan Fromm
30-Jan-2016, 16:27
Emmanuel, there's a really strong tendency here to think too hard about theory and not simply try things out. For a recent example, see http://www.largeformatphotography.info/forum/showthread.php?128348-Optics-Question-What-remains-constant-as-we-focus-closer

The OP's question's is a practical one that comes up every time someone acquires a lens without an aperture scale or in a shutter with the wrong scale and wants to use it correctly. I'm not sure that the ancients had to answer the question. But they, too, could and did ask the question by shooting and seeing what happened.

Cheers,

Dan

Nodda Duma
30-Jan-2016, 18:53
It's not wrong to want to understand the underlying physics, but in the end the best part about optical design is when I first get to try the lens out ;)