Copal No.3 - with lens?

[Ole]

That one is very very simple to determine the f-stops on:

The lens focal length is 300m, that is clearly marked on it.

The f-stop is the diameter of the effective aperture divided by the focal length - or to get the f-number, take 300mm/diameter. Since there is no glass in front of the aperture, the effective diameter of the entrance pupil is identical to the diameter of the aperture opening - as a circle, but the aperture is close enough to a circle to make the difference negligible.

Simply measure the size of the opening, and divide 300 by that number in mm. I have a cheap plastic measuring thingy which is perfect for the purpose - I think I bought it in Clas Olson.

[Two23]

I have the 250mm Imagon. As I recall, the lens is f4.5 without any disks in it. The lens was made in several lengths. The 200mm was used on medium format, the 250mm on 4x5, the 300mm covers 5x7, and the 360mm covers 8x10. This seems to be a challenging lens to learn, but then again I don't shoot portraits and have been using it for b&w landscapes.


Kent in SD

[masimix]

So Ole, in norwegian:

Jeg måler diameteren på åpningen og deler 300 på tallet? F.eks, full åpning er ca 40mm - 300/40=7,5 - f7,5. Minimum åpning ca 2mm - f150..?

And what if the disc is still on, should be extremely little light passing through?

[Ole]

det stemmer, det! Med disken på, blir det mer komplisert: Husk at åpningen der er konstant, så du skal IKKE addere. Hvis blenderåpningen er mindre enn hullet i skiven er det blenderen som dominerer, hvis blenderen er mer åpen er det skiven som dominerer. Ekstra ille blir det hvis blenderåpningen dekker deler as de ytre hullene! Det ekleste er nok å bruke den ene eller den andre, ikke begge samtidig...

[masimix]

Okei, takk, da sier vi det, enten den ene eller andre.. Det holder med discen regner jeg med.