Monogram. Portrait, f-5 Southern Photo Material Co

[marty1107]

Hello

Let me start by saying I'm totally new to large format.

I'm looking for information on this lens, focal length, and do the numbers on the diaphragm adjustment correspond to actual f stops. The lens is mounted on a Century Studio Kodak 10A box. Focusing the camera at infinity, I measured from the diaphragm ring to the focus glass and it measured 15" or approximately 380mm. I also measured the diameter of the front glass and it is approx. 3.75". So if the aperture is the focal length/lens diameter, then the wide open aperture would be f4. However the numbers on the diaphragm run from 1.5 to 64. So how do they then relate to an actual aperture? I've taken three shots and they're all underexposed by at least a stop or 2.

Thank you

Martin

86007 86008 86009

[Mark Sawyer]

Note that the aperture markings skip every other stop in the f/stop sustem. That gives it away that this lens is marked in the old "Uniform System" of stops:

US/16 = f/16
US/8 = f/11
US/4 = f/8
US/2 = f/5.6
US/1= f/4

The pattern is that 16 is the same in both systems, but the US System halves or doubles with a single stop change, while the f/stop halves or doubles every other stop. The f/stop won out because it makes more sense when actually doing the math.

So yours is an f/5 or US/1.5, and a nice Petzval, I'd guess from about 1910-1915, and probably a re-branded Wollensak.

BTW, it's not the diameter of the front glass you measure as the aperture, it's the size of the minimum opening of the lens barrel as measured through the front glass, which has a slight magnifying effect.

[goamules]

Measuring the glass diameter to calculate is wrong. Measure instead the apparant iris diameter, looking through the front. The physical diameter is usually larger than the real exit pupil diameter. When you remeasure you will find your lens speed wide open is slightly slower than F5, instead of f4.

Also, don't forget to calculate for bellows factor. Though if you measure the iris diameter, and the focal length from iris to film plane, when it's focused on your subject, you will have an accurate f-stop reading. If you are shooting at portrait distance, let's say you measure 21" bellows. Dividing 21 / 3 (apparent aperture) gives you F7. That's why your exposures have been underexposed, I'll bet.

[marty1107]

So to measure the diameter, I should remove the front or rear lens elements and measure the iris diameter fully open

Martin


Note that the aperture markings skip every other stop in the f/stop sustem. That gives it away that this lens is marked in the old "Uniform System" of stops:

US/16 = f/16
US/8 = f/11
US/4 = f/8
US/2 = f/5.6
US/1= f/4

The pattern is that 16 is the same in both systems, but the US System halves or doubles with a single stop change, while the f/stop halves or doubles every other stop. The f/stop won out because it makes more sense when actually doing the math.

So yours is an f/5 or US/1.5, and a nice Petzval, I'd guess from about 1910-1915, and probably a re-branded Wollensak.

BTW, it's not the diameter of the front glass you measure as the aperture, it's the size of the minimum opening of the lens barrel as measured through the front glass, which has a slight magnifying effect.

[Chauncey Walden]

No, as Goamules wrote it is the apparent diameter as measured looking through the front elements.

[marty1107]

Hello

Thanks to all for the info and assistance ...

Martin

[marty1107]

Hello

I realized today that I'd taken this photo the day before. I had someone hold a ruler in front of the lens while I stepped back, maybe 4 to 5 feet, and take a head on shot. I think it shows the effective aperture as 3in, 10-3/4 in to 13-3/4 in, as Goamules had said.

Thanks again

Martin

86016


No, as Goamules wrote it is the apparent diameter as measured looking through the front elements.

[CCHarrison]

This lens was rebadged and sold under many labels in the 1902-1912 period. See attached.

Dan

[marty1107]

Many thanks Dan .... is the 3-1/4" they reference the apparent lens diameter?

Martin