Slide Rule Hacks
In the spirit of O'Reilly Publishing's "Hacks" series, here are useful things you can do with any garden-variety slide rule. (If you don't have a slide rule, you can buy a brand new one from ThinkGeek or an antique at your favorite four-letter auction website.)
Each hack identifies the scales you will need to have available on your slide rule (C, D, etc.). Not all slide rules have all possible scales!
Converting megapixels to image resolution
Scales used: A, B, C, D
Before you can estimate image resolution from a quantity of megapixels, you need to have an idea of the aspect ratio of the device you're looking at. 4:3 is typical for cameraphones, while 5:4 is common for digital cameras.
The hack:
- Set the slide rule to the aspect ratio, lining up the higher number on B with the lower number on A.
- Move the cursor to the number of megapixels on B.
- Leaving the cursor in place, slide the rule to set the aspect ratio on the C and D scales now (higher number on C, lower on D).
- Read the results at the cursor on the C/D scales, in thousands.
Example: Estimate the image resolution of a 12-megapixel camera, assuming a 5:4 aspect ratio. Line up 5 on the B scale with 4 on the A scale. Move the cursor to 12 on the B scale (remember, this is a 1-100 scale, so move it to the 1.2 mark to the right of center). Leaving the cursor in place, slide the rule to the left until the 5 on C lines up with the 4 on D. Take the reading: The cursor is just a hair left of 3.88 on the C scale and at 3.1 on the D scale. So the image is roughly 3880 by 3100 pixels.
