Portrait Euryscop Series III N°4A

[Laurent Laval]

Hello group,
I have a Portrait Euryscop Series III N°4A witch is upon the Voigtlander catalog a 10"5/6 : 275mm F/4.5
I want to make some Waterhouse stop.
When I mesure the full aperture at the Waterhouse stop place I get 55,5mm
so I found that the full aperture of the lens is more F/4.95 than F/4.5 (275/55,5=4,95)
does it sound right to you ?

Laurent

[Oslolens]

For a pinhole, it would be right. Once you get lenses involved, things change.

If you place a SLR in back of your camera, you can find effective aperture pretty close when comparing with a modern lens you trust the aperture of.

Sent fra min SM-G975F via Tapatalk

[Steven Tribe]

Yes, it does Sound right!

It is quite normal that the longer focal lengths do not quite met the formal speed give to a Series.
For instance, the size 8A - focal length 25”, front lens diameter 5” - is even further away from F.4.5.

Or even more dramatic, size 9 series IV has a focal length of 46” and a diameter of 6.5” and doesn’t come close to the “promised” F6!

[Laurent Laval]

Yes, it does Sound right!
focal length 25”, front lens diameter 5”

yes you may be right, but I understand than you don't have to mesure the front lens diameter but the "entrance pupil" that it is placed at the diaphragm

[Steven Tribe]

yes you may be right, but I understand than you don't have to mesure the front lens diameter but the "entrance pupil" that it is placed at the diaphragm

………..as seen from in front of the front lens!