How do I calculate exposure?

[chad phillips]

I just purchased a Marvel 5x8 dry plate camera. The Scovill barrel lens has rotating plate with holes marked 15,20,25 etc. Looking at the holes, they sure dont look the same size as lets say f22. Can anyone gimme light as to how they are to be used? Are they roughly the same as a "normal" fstop? I am going to try film with this camera and really would like to understand how the old barrel lenses worked.
Any info would be appreciated before I start wasting film.

Thanks,
Chad

[Scott Davis]

Those aperture numbers are from some defunct system. The best way to figure out what they compare to is to calculate the focal length of the lens, then measure the aperture diameter, and then you can calculate what the corresponding f-number is. If for example, the lens is 100mm focal length, and the aperture is 25mm across, you have an f4 aperture. 12.5mm would then be f8, and 6 mm would be f16, if I've got my math right.

[Jim Jones]

It's best to do as Scott says. Measure the diameter while looking through the front of the lens. I would expect the focal length to be around 8 or 10 inches. The basic cemented doublet achromat on my 5x8 Scovill Marvel measures about 8" in focal length. The five apertures on the waterhouse stops aren't numbered, but are about f/14, f/18, f/21, f/32, and f/41. The image is woefully unsharp in the corners, even when stopped down. Some photographers treasure those unsharp corners, though. Unless you already have film to cut down to that size, you can use photographic paper for test shots. It's cheaper and more convenient.

[chad phillips]

Thanks Scott & Jim. That information is great. I'm gonna measure it tonight. The paper idea is cool. I will have to look into how to time it for exposure.