It's not going to cover 20x24. 19" (48cm) Dagor is what you want for that.
Nobody says the 14" Series III Dagor will cover 20"x24" because it won't. The linked catalog page suggests 18"x22" stopped all the way down which is 90 degrees, which is in fact, the limit of these lenses (90 degrees at f/64 or f/90).
A 16.5" Dagor (420mm) also f/7.7 will just cover 20"x24" stopped down and will likely render darkened corners. As Karl French tells you, a 19" Dagor is a better bet for a good usable wide angle on 20"x24".
I share this as a 20"x24" user who owns and uses many focal length Dagors, including 14"(won't cover 20x24), 16.5" (86.9 deg)and 19"(78.8 deg)(both do, read above).
I downloaded and zoomed in on the photo showing your aperture scale. It appears to be marked in "the Goerz System" rather than International stops(the F/stop scale we all know today).
You can consider your marked apertures to be:
6 = f/7.7 or f/8
12 = f/11
24 = f/16
48 = f/22
96 = f/32
192 = f/45
Being that it has a 5 digit serial number, and marked in "Goerz System" it's on the early side. I have a black painted one that looks very similar, but with 6 digit ser, and marked in "International Stops". It should perform very well, even compared to a much more modern lens, though being uncoated it'll be a little less contrasty.
Last edited by Tracy Storer; 15-Sep-2020 at 18:35. Reason: tweak and typo correction
Sometimes people say that those Catalogues doesnīt tell the real world Coverage. They say that the lenses can go higher in coverage. Thatīs why I asked.
I opened a new Thread with a question about an other lens I owned, because the seller said it can go higher than the catalogue numbers. Itīs a Voigtländer Euryscop No. 6 Series IV. He said I could use it for 16x20 inch portraits fully opened aperture.
Coverage increases with magnification. Portraits are shot at higher magnifications than shots of distant landscapes.
That said, there are two coverage concepts. Some say coverage when they mean illumination, i.e., does the lens put enough light in the format's corners. Others say coverage when they mean useful image quality, i.e., does the lens put sharp enough image in the corners.
You have to ask yourself whether your proposed application -- portraits -- needs good image quality in the corners.
You also have to remember that sometimes sellers (choose one) don't know what they're talking about or lie.
The VM says:
In the USA list in 1915-1916 Series IVa was listed for 50° as:
No2 10in focus 1.75in dia 5x7in covered
No3 11.5in 2.33in 6.5x8.5in
No4 14in 2.5in 8x10in
No5 17in 3.125in 11x14in
No6 20in 3.5in 14x17in
So a 17 inch Euryscop Ser. V. will cover around 16". This is considerably shorter than 16x20's diagonal.
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