Aperture Question

[Peter De Smidt]

I have a 420mm Fuji L in a Copal #3s shutter, and I also have a Kowa Graphic 305mm lens in a barrel. The cells from the Kowa screw right into the 3s shutter. Is there a formula for figuring out the aperture compensation? When I try to match aperture sizes by measurement, I get that I need to close down the 300mm 2/3 of a stop compared to the 420. When I use a Broncolor film plane probe to measure the actual light coming through each lens, I need to close the 300mm down a full stop, with both lenses at their focused on infinity extension.

[Jim Galli]

I have a 420mm Fuji L in a Copal #3s shutter, and I also have a Kowa Graphic 305mm lens in a barrel. The cells from the Kowa screw right into the 3s shutter. Is there a formula for figuring out the aperture compensation? When I try to match aperture sizes by measurement, I get that I need to close down the 300mm 2/3 of a stop compared to the 420. When I use a Broncolor film plane probe to measure the actual light coming through each lens, I need to close the 300mm down a full stop, with both lenses at their focused on infinity extension.

A full stop. I cheat. Here's my method, weird, but it works. 305mm like f30 420mm like f42. f30 ~ f42 bumps to f32~f45 which is one stop.

[Peter De Smidt]

Hi Jim,

Thanks!