f-stop range of 12" f/2.5 Aero-Ektar lens?

[Murray@uptowngallery]

Most of the 12" f/2.5 Aero-Ektar lenses in people's possession seen to be be extracted from the original camera for practical reasons.

The one I have has no f-stop scale other than a previous owner's piece of adhesive tape and lines marked with a ballpoint pens.

Has anyone seen an actual f-stop scale from one of these to know what was the stopped down max. f-stop?

I've measured the max & min physical aperture diameter with the cells removed, but this is a lens with a significant shift in effective aperture vs. physical diameter, so I don't want to assume it's close enough. This is true for some lenses but I don't think so for this one (for example, max physical opening is 3.43". 12/3.43 is 3.5. The lens is f/2.5, and I don't know if the iris opened past the f/2.5 marking.)

Minimum diameter is 1.25-1.26" as best I can measure (some error likely).

Thanks

 

[epatsellis]

murray, hold it a arms length and measure the entrance pupil for calculations, will get you close enough.

 

[Murray@uptowngallery]

Someone has sent me instructions for auto-collimation with a mirror several times over the years and it always seemed like a little too much work, but the most recent reiteration finally made it seem easy enough to do, so I will probably try it.

I want some decent measurements because I want to make some accessory stops of smaller diameter and if I'm going to bother, I might as well try and get them as close to 'right' the first time. It will be a useful exercise to be mark a lens barrel anyway, as there are plenty of other unmarked things in my possession.

M