Looking at a small computer through rose-colored glasses
and a tangent about focussing.

 

November 29-30, 2006. As of last night, the little Vaio now has a custom-made red filter for its screen. I found 12x12 sheets of red acrylic on eBay for real cheap and bought a handful for such projects. A few scraps of left-over aluminum channel, some Permatex glue, and few minutes with a straightedge and utility knife produced a slide-on and -off filter. I gilded the lily with black electrical tape to conceal the acrylic's raw edges and then spent as much time ungilding it (couldn't see the top and bottom of the screen until I trimmed the tape back) as on the rest of the project combined. Ain't it ever thus?

I went to sleep last night thinking about how to make a fine focus adapter to drive the focus of the 200mm F2 by the microscopic amounts required to find excellent infinity focus. I settled on a tangent-arm design and awoke with a scheme in my head involving leftover black cherry, aluminum channel, bolts, wingnuts, Forstner bits, the Dremel and its router attachment, split rings, and metal dowels. It's a wonder I slept at all with that rattling around in there.

I ended up using two very different designs (thank goodness) both built around a hose clamp and a Mitutoyo micrometer head. The device has gone through three versions since this morning. Versions zero and one were really clumsy. Key parts of v2.0 are still curing, but it's a nice, smooth, compact, easily adjusted thing. In this scheme, the micrometer is mounted on and travels with the focussing ring.

The micrometer head is mounted in an aluminum frame which is JB Welded to a 14-inch hose clamp which wraps around the focus ring of the 200mm F2.0 EDIF. Nevermind... just look at the picture to get a sense of how it's put together.

The micrometer head pushes along a tangent to the focusing ring against some fixed portion of the mount. Four revolutions of the micrometer drum move the probe 0.1 inch. The focus ring turns through a circumferance of about 12 inches, so if I've got the geometry down right, a full revolution of the micrometer drum will turn the focus ring through 1/480 of a turn. Allowing for some mis-measurement, this is a fine-focus knob with a ratio of about 500:1. That should do.

I coated the bracketry in JB Weld, set it in a jig, and did my best not to mess with it for 16 hours. Works great! With any luck, this mighty lens -- a 4-inch F2 ED-glass refractor -- is ready for critical manual focus for the first time ever. I don't think it'll be particularly easy to dial it in (surprise me, please), but at least is should now be possible. The few outings this lens has had under the stars have been spectacular . . . and frustrating and tedious. I've always backed it off to F2.8 to improve focus, which is a shame all things considered. Usually, I've wrapped the lens and camera in a cat's cradle of control and power cords plus a timing belt to a Robofocus stepper to tweak it. Stay tuned! This looks good enough to call forth at least a week of clouds.

 

 

New red light district: Vaio SRX87. The case
is my latest attempt at cable management: plug in
the case, then run cords from it to the toys.

 

JB-Welded, micrometer-fitted, and dry-fired on
the kitchen counter, this may be the first 200mm EDIF F2.0
lens that can be focussed critically by hand.
It takes two point five turns of the mike's drum to move
half the infinity symbol past the index.... smoooooooth.

 

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Text & Photos by David Cortner
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