Topic – Exposure Slide Rule

Just a quickie today.

Yesterday I finished sewing darkroom cloths for covering the widows. The material is synthetic polyester which is almost impenetrable to light. I ran around in the shop with a flashlight and tried out all black cloth they had and with some help found this final one. It is shiny black and would do wonders as a back drop in a studio (as long as the cat’s hairs are removed… It is amazing how much these attract attention).

Self-adhesive Velcro tape was attached around the window and its counterpart sewn onto the border of the cloth. It appears to stick quite well to the ceiling and can be taken down in a matter of minutes.

Here are some photos of the window without and with the cover:


Unfortunately the sewing machine went automatic when I was away and sewed happily on its own! Then a puff of smoke and the acrid smell of burned electric machinery filled the kitchen… Luckily I could borrow the neighbour’s machine for completing the cover.

That was yesterdays photographic achievement. Today I finished a first version of my mechanical exposure calculator. It is basically a slide rule with logarithmic scales set in seconds (or minutes) one one scale and adjusted exposure on the other:


(The above file is a png, here is a pdf: exposure)

I made this for testing if it would be useful in a darkroom. The idea is to get a decent exposure for the highlights with yellow light and then add contrast and number of f/stops with magenta according to the exposure scale. The calculator would be useful for calculating the magenta exposure in seconds.

Before use print it on a large sheet of paper (perhaps sticking it to some cardboard is a good idea too?). Then make a cut separating the two vertical scales, so they can slide close to each other, and it is ready for use.

For calculations do this:

  1. Set the right scale’s zero at the current exposure time. As it is (if not cutting it), where the zero is close to 3.2, the current exposure is 3.2 seconds (or 3.2 minutes).
  2. The time for underexposure by 1 stop is now read off the seconds scale opposing the ‘-1’. Here about 1.6 seconds (or 1.6 minutes).

Similarily an overexposure of 1 and 1/3rd stop is read off at ‘+1 1/3’ as being 8 seconds.

Note: It might be unusual to use decimal minutes but a simple ‘multiply by 6’ of the decimal takes care of that: 3.2 minutes is 3 minutes and 2*6=12 seconds = 3m 12s, 1.6 minutes is 1 minute and 6*6=36 seconds = 1m 36s and so on.

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