Homemade lens - determining aperture

[Michael Slade]

Hey all,

I've built a view camera lens out of an old magnifying glass and want to determine the aperture.

Where is the formula for determining aperture?

Thanks!

[bdial]

It's the focal length divided by the effective lens diameter.
Jason Brunner has a good video to illustrate; http://www.jasonbrunner.com/videos.html

[Michael Slade]

Thanks!!! I knew APUG would come to the rescue!

[Chazzy]

You're going to need some tortellini.

[Joe VanCleave]

One thing I did recently to my homebrew nested box camera (5" x 7" format) was to use "waterhouse stops" (homemade insertable discs of premeasured openings) for the aperture stop, inserted just behind the lens inside the box. I rate each aperture with its diameter in millimeters, rather than an "f" number. The reason for doing this is that I can then measure the bellows extension of the box, in millimeters, after focussing on the subject and then simply divide this by the diameter of the aperture stop in use to get a direct calculation of the "true" focal ratio, already taking into account the bellows extension factor. Using apertures rated by their diameter thus makes life much easier if you're focussing close-up on objects.

I carry an el-cheap plastic millimeter scale for use in measuring focal distance on the box.

BTW, my camera's main lens is a plastic, credit-card sized fresnel magnifier. It's not a Holga, but it has large format Holgaesqueness. Or Holgiosity.

~Joe

[bobwysiwyg]

Joe,

Interesting. Can you post a pic or two taken with this setup?

[Joe VanCleave]

Joe,

Interesting. Can you post a pic or two taken with this setup?

Here's a still life from my F295 posting, shot in my living room with afternoon sun streaming in through the window blinds. Preflashed grade 2 paper negative, f/18 waterhouse stop, 2 second exposure.

~Joe

[Joe VanCleave]

And this shot of the Sandia Mountains, east of Albuquerque, a 7 second exposure with the fresnel lens stopped down to around f/55 with a waterhouse. Preflashed grade 2 paper negative.

The negatives are held in a homemade black foamcore film holder, using corner mounts, giving the images a pseudo-19th century plate camera appearance.

~Joe

[Joe VanCleave]

And this still-life, in my living room with indirect afternoon light. The fresnel lens is operating "wide open" at around f/3.5, so you can see the lovely off-axis artifacts. Preflashed grade 2 paper negative, 25 second exposure.

~Joe

[bobwysiwyg]

Very nice, thank you.