View Full Version : combining wollensak convertable cells

Darren Kruger

3-May-2008, 14:05

I recently purchased a Wollensak Raptar 1a 31" element (half of a convertable lens.) It fits in a Alphax #4 shutter, same as a 330mm 1a I have. The 330mm has 20" and 25.5" elements. If I were to use the 31" element with one of the ones from the 330mm, how would I calculate the new focal length and aperture? I found a table with combinations on cameraeccentric but I'm interested in the math behind the combinations.

-Darren

Ole Tjugen

3-May-2008, 14:25

1/F = 1/f1 + 1/f2 + 1/df1f2

The last term is a correction factor for optical spacing, which is not the same as the distance between the cells.

For rough estimation of LF-relevant focal lengths, it can mostly be ignored.

Ole Tjugen

3-May-2008, 14:28

Which, of course, means that your 330mm has the wrong cells - it should have 25.5" and 26.5" cells to be 330mm. As it is, it's a 280mm. ;)

Darren Kruger

3-May-2008, 14:43

Which, of course, means that your 330mm has the wrong cells - it should have 25.5" and 26.5" cells to be 330mm. As it is, it's a 280mm. ;)

The 20" and 25.5" cells are what's listed in the Wollensak catalog for the 330mm focal length at cameraeccentric (http://www.cameraeccentric.com/html/info/wollensaki/p4.html). I just measured and the lens focuses around 330mm so Wollensak did something where that spacing factor becomes important (at least for that lens. What would be the formula for determining the aperture for each combination?

-Darren

Ole Tjugen

3-May-2008, 15:10

You're right, and the "d"-factor is important. Oh bugger...

Anyway the f-stop is given by F/d (a different "d"), where "F" is focal length and "d" is the diameter of the entry pupil. Now the entry pupil is "enlarged" by the glass in front of the aperture, so simply measuring the opening isn't going to work - unless there's no glass in front of the aperture, as when using a single cell.

But in this case the "loupe effect" can be ignored, and very often was. I've checked all my old triple convertibles and casket sets for which I have aperture tables, and every single one ignores it!

If you mean by 'each combination' using one cell at a time, the formula for aperture would just be f.l./aperture diameter. It can get hard to measure accurately at smaller diameters, especially for polygonal apertures.

When the aperture is between two cells, it's apparent diameter is affected by the glass curvature (magnification/reduction) and it's get more complex. It is often close enough to estimate as previously mentioned, but combine that with an 'iffy' shutter speed and you may be off further than expected on your exposure.

There is a method I haven't tried yet with a mirror and flashlight to measure exact f.l., then measuring the exit pupil projected by the lens onto a piece of paper at the lens front. Then you have exact rather than nominal f.l. and can measure the projected iris more easily than the aperture itself.

I think the mirror f.l. method does something like put a mirror at one lens end and a flashlight beam is projected through a very small hole in a piece of paper. Somehow you get 2x the actual f.l. I understand the path traveled by the light is 2x the lens f.l. and this supposedly happens if you take a photo of in a mirror...you and your reflection are 2x as far apart as you are from the mirror.

I know others have posted it here and there, but Richard Knoppow has posted a step-by-step procedure on rec.photo.large-format.equipment (and I must have it buried in my inbox somewhere).

Murray

oops, slow typer

Darren Kruger

3-May-2008, 16:51

Thanks all for your help. I'll probably make a table up and use that if decide to start using the various combinations.

-Darren

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