Boykin RotaRule 560

The slide rule at right (click the thumbnails for a larger view) was made by the Boykin Products Company. I don’t know much about it other than a brief discussion (featuring the manufacturer’s nephew!) on the ISRG (a news group on Yahoo!). The example that I have is a different, and presumably more modern model (since the model number is higher). It’s quite nicely made, with an ingenious braking mechanism to help keep the cursor in one place. I rather like the scale labels on the cursor; it greatly eases the work of figuring out “where you are” when you’re using the rule. If anyone can help fill in the blanks on this slide rule, I’d sure appreciate it.

But the main reason for this post isn’t to answer questions about the RotaRule 560 — it’s to get opinions and ideas about high-resolution photography of slide rules vs. the more conventional scanning.

What got me started on this was the artificial looking “flatness” of a scan. This is the inevitable result of the fact that a scanner just plain doesn’t work like your eyeball. Scanners by their very nature look at the object being scanned from a perspective that varies as the scanning head moves across the object. This creates an image where every element appears to be directly in front of you — something that could never happen in real life. The human vision system sees such an image as lacking all perspective. A camera, on the other hand, emulates your eye much more closely — especially in terms of recording the perspective in an image. Furthermore, a modern digital camera’s optical system (I’m generalizing here; there are exceptions) is likely to render the object’s colors and contrasts more faithfully than a typical scanner would.

So I built a shadow box (that’s why the background in the photos is completely black) and tried taking photos of the RotaRule. Clearly I have some refinements to make, particularly with respect to glare. I took these photos outdoors, in natural indirect light. The glare visible on the cursors is most likely from the bright white T-shirt I was wearing, which was directly lit by the sun. I used a 160mm (effective) macro lens on a tripod-mounted Canon EOS 10D camera, at 100 ASA, f32, and 1/15 second. I used the pinhole f-stop to maximize the depth of field.

Several things are evident at a glance. First, the overall resolution is somewhat less than what a 300 dpi scan would produce. This makes sense, as the photos work out to about 200 dpi. The photo is less “crisp” than a 300 dpi scan. Second, the “scanner flatness” is definitely gone; this looks like a normal, natural image to my eye. Third, the colors and textures are substantially more sensitively (and accurately) rendered on the photo, as compared with the scan. For a concrete example of this, look at the top of the front photo — the white background of the stator has a distinctly different tone than the background of the rotor. This difference is visible in an identical way on to an eyeball, and completely invisible on any scan I made (I made dozens of them in the course of this experiment).

I can’t say that either photos or scans are so superior to the other that I should drop one approach and take the other. I’m leaning, slightly, toward the photo — mostly on aesthetic grounds. I like the natural-looking images, and I like the fact that I can use the same method to image inherently three-dimensional objects, such as a cylindrical slide rule, where scanning simply will not work.

Thoughts, anyone?