Calculating the focal length and speed of a Darlot Hemispherique 2 lens

[Jersey Vic]

When calculating the focal length, do I measure from the back of the lens glass to the ground glass? And in measuring the diameter, do I measure from the edge of the actual glass or the the rim?

Also-does anyone know anything about this lens? Speed? Focal Length? Coverage? Year made?

Thanks in advance for all of your help,
JV

 

[bobfowler]

I have a Darlot Hemispherique #1. These are Rapid Rectilinear lenses of about f/8. Measure from the ground glass to the waterhouse stop slot with the lens focused at infinity and you'll get a ballpark focal length. To measure the speed, you need to measure the diameter of the exit pupil, NOT the size of the glass. A really good way of finding out the actual speed (T-stop) is to use an incident light meter and a lens of known speed and make direct comparisons at the film plane.

 

[Ole]

To measure the focal length, see my article about just that.

For f-stops, you measure the diameter of the entrance pupil. Or you measyre the t-stop with a light meter.

 

[phfitz]

Hi there,

From an 1890 Benjamin French & Co. catalog

Darlot's Rapid Hemispherical
#2
dia. = 1 1/4"
back focus = 9"
portrait = 5x7"
view = 6X8"

Have fun with it however it turns out.

 

[Jersey Vic]

Hi there,
From an 1890 Benjamin French & Co. catalog
Darlot's Rapid Hemispherical
#2
dia. = 1 1/4"
back focus = 9"
portrait = 5x7"
view = 6X8"
Have fun with it however it turns out.

Excellent-Thanks so much!
I'm going to shoot looooong exposure paper neg portraits...sounds like fun to me!
Cheers

 

[Jersey Vic]

To measure the focal length, see my article about just that.

This is genius, fun and very useful. Thanks