Beseler 18" (460mm) f4,8 Series III opaque projector lens - 11x14"?

[darkroom_rookie]

Anyone used this lens? It is most likely a Cooke triplet. I'm curious about the sharpness and coverage.

 

[Maris]

I use a Wollensak (= Beseler?) 18" f5 (approx) opaque projector lens on my 8x10 camera. The lens is a Cooke triplet and it "illuminates" 11x14 with plenty to spare. At full aperture the challenge is field curvature which means soft corners on 8x10 and seriously soft corners on 11x14. Central image quality is brilliant and contrasty but film sharpness is hampered a bit by a hint of secondary chromatic abberation. This would not have been a problem on an opaque projector.

 

[darkroom_rookie]

What is the diameter and the length of that Wollensak 18" f5 lens? Beseler 18" lens dates to around 1950s or 1960s. Maybe they're the same.

 

[Maris]

By steel rule measurement of the chrome plated barrel I get 103mm diameter, 107mm long.

 

[darkroom_rookie]

The diameter of the Beseler lens is almost exactly the same, but it's longer in size, at least by 45mm. Which type of Cooke triplet is preferable, from your experience, for the same focal length? A longer or shorter one in length (barrel)? Where does the nodal point lie for a longer-sized lens? Could one simply insert an aperture, cut from a black piece of cardboard, inside, right in front of the middle element?

 

[Maris]

An informal test of my Wollensak 18" f5 (approx) reveals the first principal plane is about coincident with the front surface of the middle concave element. The lens swung about a vertical axis in this plane shows negligible image shift for small angles of swing. Cooke triplet lenses, in my experience, tend not to be very fussy about whether the aperture goes in front of or behind the middle element. Even more convenient is to use a series of "lenscaps" with different size holes according to the apertures required. I can't find any image quality compromises with this approach and as expected image quality improves at small apertures.

But the reasons for me to shoot a 18" f5 aren't about small apertures so I always use it wide open.

In broad optical engineering terms the physically longer the lens and the more shallow the curves on the glass the better the image quality. The compromise then becomes reduced field coverage due to mechanical vignetting. Just look at all those long barreled Petzvals that only cover small plates. Incidentally when I measure just the glass, not the barrel, of my 18" Wolly front vertex to rear vertex I get about 103mm; a lens as long as it is wide!