Greetings,

I just purchased a B&L Tessar IIb "6-1/2 x 8-1/2" size barrel lens. (The lens has not yet arrived.)

Its appearance is that shown in the 1906 B&L lens catalog (all-brass finish except for the "shade", notations engraved on the side of the barrel), as distinguished from the appearance shown for that lens series in the 1920 B&L catalog (black finish, notations engraved on the front of the lens).

In the 1906 catalog, the No. 8 size of the Tessar IIb lens is shown as suitable for 6-1/2" x 8-1/2" plates at full aperture (f/6.3).

In the 1920 catalog, the No. 8 size of the Tessar IIb lens is shown as suitable for 8" x 10" plates at full aperture (f/6.3) and for 10" x 12" plates at "smaller stops".

The diameter specification (which turns out to the the overall actual glass diameter of the front element ), and the focal length (12") is the same in both catalogs for the No. 8 size, suggesting that the "Number" designations had not changed.

Note that in these lenses the "size number" is not usually marked on the lens, just the "coverage rating" (full aperture).

Do we know what (if anything) changed in the design of this lens between the 1906 catalog version and the 1920 catalog version, or is the difference in "coverage" specification just an advance in marketing engineering?

For reference, here are links to the catalog pages on which I have relied:

B&L 1906 lens catalog - Tessar IIb (f/6.3):

http://www.cameraeccentric.com/html/...hcata/p11.html
http://www.cameraeccentric.com/html/...hcata/p13.html

B&L 1906 lens catalog - Tessar IIb (f/6.3):

http://www.cameraeccentric.com/html/...t/lensesg.html
http://www.cameraeccentric.com/html/...coveraged.html