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View Full Version : LANCASTER Meniscus f/6 or f/5?



renes
10-Oct-2011, 05:57
It is said that meniscuses are f6 when front plate is removed from the lens barrel.
I found some reports saying that Lancaster is even faster - f5.

Could anyone who use Lancaster achromat confirm it?

Steven Tribe
10-Oct-2011, 12:33
I think there are 3 different versions of the apparently simple Lancaster meniscus lens.
With very poor engraved data - just 1/2 plate etc.

Steven Tribe
10-Oct-2011, 14:46
But the small one I have has a efl of 125mm approx with a diameter of 30mm which gives a figure of F4.16.

This is the type which you identified in your following note!
And I have another anon. meniscus with same focal length - with slightly larger diameter, which brings it down F3.8.

renes
10-Oct-2011, 14:53
I think there are 3 different versions of the apparently simple Lancaster meniscus lens.
With very poor engraved data - just 1/2 plate etc.

I mean most popular version: J.Lancaster & Sons Patent Birmingham with two scales round the front that show stops (starting with f10) and Times 1-8.

I compared lens diameter of a few the same FL achromat meniscuses (TTH, Gundlach, etc) and Lancaster's cell was the biggest. Originally most with plates starts with f/11, Lancaster with f/10...

renes
11-Oct-2011, 04:48
But the small one I have has a efl of 125mm approx with a diameter of 30mm which gives a figure of F4.16.

This is the type which you identified in your following note!
And I have another anon. meniscus with same focal length - with slightly larger diameter, which brings it down F3.8.

Based on your calculation method, my Lancaster 5,5" (140mm) meniscus with diameter of 30mm gives about f/4.7, but WA Lancaster 4" (101mm) which is 27mm in diameter gives f/3.7.

I have also Gundlach 180mm meniscus which seems to have f/7,8 (diameter of 23mm),
and 70mm Darlot meniscus with a diameter of 20mm which gives f/3,5.

I wonder if this method (focal lenght divided by lens diameter) works also for meniscus lenses...

Fotoguy20d
11-Oct-2011, 06:00
I've been using that method and exposure is close but not right, I believe. How are you determining FL? From film plane to the glass? I think (someone correct me if I'm wrong) for a meniscus, with concave side forward, the node point used for focal length is actually somewhere in front of the glass so the FL is longer and the aperture therefore smaller than you estimate.

Dan

Steven Tribe
11-Oct-2011, 06:18
I have been measuring the efl by comparison of image size with lenses of known (engraved) efl. I realise that a single (but compound!) lens doesn't necessarily have it's real optical centre in the plane of the lens.

renes
11-Oct-2011, 07:09
I've been using that method and exposure is close but not right, I believe. How are you determining FL? From film plane to the glass? I think (someone correct me if I'm wrong) for a meniscus, with concave side forward, the node point used for focal length is actually somewhere in front of the glass so the FL is longer and the aperture therefore smaller than you estimate.

Dan

In case of Darlot and Gundlach I determined fL from film plane to the rear glass. If the node point is actually in front glass, the fl change a little and estimated f/stop is f/3,8 for Darlot and f/8 for Gundlach. Both Lancaster fl are based on VM informations (I have checked them and they seems as described althought I will do it again with 140mm becouse it seemed it was approx. 150mm).

I wonder if my Gundlach 7 1/2" meniscus could be the same lens as Gundlach Portrait f/6 but with smaller f/8 aperture?