Graflex Optar, 90mm f/6.8; Making and Testing an SSG Cam

[Bart B]

Made a P4 cam for my 4x5 Super Speed Graphic from a nail plate. Cutting and griding it for a 90mm Graflex Optar was a learning experience. Hardest part was getting the high/distant end fit all the way to the end. Filing and smoothing the high end tended to round it off too much. I finally put a scrap piece of nail plate next to and aligned with the high point to make the slope at the top fairly well correct. Next time, I may make the 7 degree angle at the high end only half way up then straight up so the curve goes past the infinity and .703" max height. Then make the curved cam surface easier to finish better to the peak. That part of the cam could stay further to the left side until the cam curve's finished to match rangefinder coincidence and lens focus. But it finished up darned close to perfect and final smoothing the cam slope made it as smooth as the Graflex P34 cam.

Two problems came up.

First, the nail plate was .035" to .036" thick in some areas and .040" or so in most others. I used the thinnest part of it for the cam. After dressing all edges smooth as well as the front and back sides, it often sticks in the cam tube when focussing. The Graflex P34 cam for the 135 Optar is about .031" thick and it slides easily and never sticks in position. I don't know what's causing the home made P4 cam stick unless it really needs to be thinner. Checking the edges where it rides in the SSG's cam tube and surfaces don't show any sharp or ragged edges that could catch on something.

Second, with the 90mm Optar lens wide open and sharply focused at f/6.8, the rangefinder's not coincident. Best focus shifts when stopped down to f/16 and smaller. Subjects have best sharpness at all distances with good rangefinder coincidence but only when the aperture's stopped down to at least f/16. Both lens groups are tight in the shutter. The distance the lens board moves forward for all distances is different than what lens formulas calculate it should move.

There's no evidence a clumsy mechanic took the lens groups apart and reassembled them wrong. But for some reason, this lens has a definite focus shift from wide open to f/16 and smaller. Stopped down, it's pretty sharp all the way to the corners of a 4x5 image.

Any ideas why this happens?

I can live with this issue as I don't plan on using any apertures larger than f/16 96% of the time. Focus at rangefinder coincidence with smaller apertures is very good. If I run across a genuine Graflex P4 cam to borrow, I'll put it in then make a comparison.

[Jim Jones]

First; a disclaimer. I've never tried to make a SG cam. The specs for the original cam may well have called for 1/32" material. Hardware stores should have small sheets of flat smooth 1/32" brass. It may not be as durable as steel, but is more pleasant to work with. The original depth of field table that came with my 90mm Optar says, "Like most Wide Angle Lenses it is recommended the lens be stopped down to f11 or smaller for critical definition." This may be optimistic for this old and simple design. Experienced users often recommend stopping down much more.