Brass lens unknown focal lens

**captainscot** · 6-Mar-2014, 17:58

I found an old brass 5x7 rapid rectilinear that i forgot i had, so i put it on my recently new purchased 5x7. The image on the right is a Dagor 8.25, the image on the left is the brass lens, any thoughts on the focal length? I taped up the waterhouse stop slot with black tape and took a guess and exposed as an F8, i think i underexposed by a stop.I think more tests will need to be done! Will there be any difference in the image if i can get a waterhouse stop for it versus no stop in it all? I also have the 4x5 back for the wisner that i may try it on.

**8x10 user** · 6-Mar-2014, 18:24

~10"? You could always measure the aperture and multiply by 8?

A water house stop of a smaller aperture will give more depth of field but many prefer the look of F/8 for portraiture. I'd say use it wide open for selective focus work and stop down with the dagor when you want a lot of depth and sharpness.

**Tim Meisburger** · 6-Mar-2014, 18:30

To get focal length focus on something at infinity (cloud, mountain, etc) then measure the distance from the center of the lens to the ground glass and that is focal length. Measure the smallest internal diameter of the lens and divide the focal length by that to get aperture (for example, and 8 inch lens with a two inch diameter will be an f4). Waterhouse stops will restrict the smallest diameter of the lens, changing the aperture the same way a diaphragm does. Smaller apertures need longer exposures, but in the image will increase depth of field, so that objects in the background will be in focus..