David Krooshof

A Contact Printer and an Exponential Darkroom Clock

Contact Printers

A contact printer is a device that can hold a negative and a sheet op photographic paper, that you can shine a light onto, so the negative can leave an image on the paper. Using a contact printer, you can make prints of negatives without the use of an enlarger. If the negative is large, that is well exposed, these types of prints can look very good.

I was looking for a classic contact printer, that looks like a picture frame with a clamp. It takes a light source to make the image.

What I also found, was a box version, with two incandescent lamps in it, and very scary wires, with crumbling cotton around the cores in stead of rubber or plastic. I replaced the inside with a LED light panel. And I treated the wood so it looks beautiful again. And I made it safe to use.

Me, musing about my printer and exponential darkroom clock

An Exponential Darkroom Clock

When I got the light panel, I wanted a way to time exposures reliably, repeatable. I looked into darkroom clocks, and noticed they are all linear. You can set time in seconds. In photography, this is an unnecessary braindrain.

Everything on cameras works with expontential curves.
We see light and photograph in stops, clicks, EV.
Each click on the shutter doubles the mount of light: 1/500, 1/250, 1/125, 1/60...
Each click on the aperture doubles the amount light: 16, 11, 8, 5.6, 4, ...
ISO is also in doublings per EV value. Per stop.

Once you are used to it, you do everything in stops and avoid the number crunching.

And for those who are more at home in acoustics:
DIN is exactly like decibells: 3˚DIN = double the film sensitivity like 3 dB is double the energy.

So then why would a darkroom clock be in seconds?

I guess no clock maker ever gave it a thought. A second is a second, right? But not in photography. The difference between 2 and 3 seconds is half a stop, but between 12 and 13 it is a small correction.

Then why would you need to start calculating time based on a feeling about your print? Why can't you look at the image, see it is 2/3 stop overexposed and dial down the time 2/3 of a stop accordingly, regardless of the current setting in seconds? Why not make test strips in 1/3 stops increments, rather than seconds?

I noticed Stephen DiRado explaining darkroom exposure times to his students and half his class blanking out when the numbers went up more than they anticipated. And those students are correct. Having to calculate fractions of a power of two, is not nice hurdle to take while making art from the heart.

So this is what I built.

One coarse knob that sets the time in full stops: 2, 4, 8, 16, or 32 seconds.
One fine knob that sets the time in third stops: plus or minus 1/3, 2/3, 1, 4/3, 5/3, 2 stops.
All in al, two knobs set the time from 1/2 second to 128 seconds in a very compact interface, that responds logically and intuitively across this range, without having to calculate ever.