info on triple convertible lens apertures

[Darryl Baird]

Hi,

I'm looking for the equivalent f-stop for front and rear 'only' configurations... I have a Velostigmat Series 1, 12.5 inch that converts to a 20" (508mm-rear) and 28" (711mm-front). Is it the same as other convertible lenses, double the aperture number for the effective f-hole? (example 6.3=12.6) ?

... or any experienced users of triple convertibles, do you know something about this lens?

thanks

[Gene McCluney]

Each focal length will have a slightly different aperture scale. That is why when these lenses are shipped from the factory in-shutter, the shutter dial has three scales. If a lens ships with identical back and front cells, then there will only be 2 scales engraved.

[Darryl Baird]

thanks Gene,

I search auctions and found a shorter version of the lens...

with both lenses -- f/6.3
with rear only -- f/11
with front only -- f/16

that's what I'll start with and do a test, bummer it get so slow at the longest focal length

[Darren Kruger]

I'm looking for the equivalent f-stop for front and rear 'only' configurations... I have a Velostigmat Series 1, 12.5 inch that converts to a 20" (508mm-rear) and 28" (711mm-front). Is it the same as other convertible lenses, double the aperture number for the effective f-hole? (example 6.3=12.6) ?

I have the later version of this lens, the Wollensak Series 1a Raptar 13". The maximum apertures start around the same spot but the next stops do not line up to each other on the scale.

According to the page on Velostigmat series 1 lenses in 1916 Wollensak catalog on http://www.cameraeccentric.com,

The approximate working speed of the single elements when the diaphragm indicator is set at F:6.3 is F:16 for the front lens and F:12.5 for the rear.

This roughly corresponds to the scale on my raptar 1a.

-Darren

[Darryl Baird]

excellent, thanks

[Lynn Jones]

Hi Darryl,

First you need an 8 to 10 dollar calculator.

Usually on these triple convertible lenses, the apertures are simply marked in millimeters. For maximum aperture, divide the focal length (F) by the max in mm, that will give you the f number. After that, for everything smaller, divide the F by the desired aperture and that will tell you the correct number of mm. Now make a chart of each of the focal lengths.

Lynn

[Philippe Grunchec]

Darren, would you sell yours?

[Vick Vickery]

Go to www.cameraeccentric.com/info.html and look in the 1916 Wollensak catalog on page 4...you'll find a complete description of the lens which was recommended for use with 8x10 format and had maximum open aperatures of f/6.3 for both lenses, f/12.5 for the rear alone, and f/16 for the front alone.