April 20, 2011

Hacking St. Ansel: A Homemade Densitometer

If, like me, you are still holding fast and shooting some film amidst the digital deluge, chances are you have delusional visions about Saint Ansel going forth and spreading the the light of the pure craft of photography (the light, of course, is divided into zones neatly marked I-X). But pure or not, the craft of analogue photography requires quite a bit of exactness and repeatability to give optimum results and for this, testing film and developer combinations often becomes necessary. But the one thing that hindered me most from really testing my film and having sleepless nights over geeky things like N-1 development was the lack of - or rather the cost of - a densitometer. I know those things cost a lot less than they used to - a few hundred instead of a few thousand several years back - but they are still expensive and bulky beasts. But not to be discouraged from my vision of photographic nirvana, I managed to put together a kit that cost me all of $30 - and can conceivably be done for less. NASA wouldn't trust their pictures of space aliens to its vagaries, nor perhaps would Adams be satisfied with the texture of the moon over Hernandez had he used this contraption, but at the moment it works for me.

For this densitometer I use the cheap photodiodes easily available at Radioshack or any other electronics store. It cost me about $2 for a handful of them. Next, I cut a small square piece of cardboard about 3x3 inches to form the base of my densitometer. Take a diode and bend its legs outward at right angles near the middle so as to form a small inverted 'T' with the light sensor at the top and the legs sticking out on each side. Two pieces of tape should be enough the stick the legs of the diode to the cardboard. Black electricians tape works well for me. Once you have done this you should have the inverted T standing on the board with the sensor sticking out. Now, we need something resembling an empty cylinder with a small hole on top to cover this so that there is only one inlet for light reaching the diode. This is easily made by cutting a small circle in the base of an opaque film canister - I made mine using a plastic reloadable film cassette with the center spool removed. This has the advantage of having a ready made hole in it. After you are done, you should have a structure that allows light to reach the photodiode only from a hole on the top and the two arms of the diode should be protruding from the sides.

Now onto the more interesting bit. As the intensity of light falling on the diode changes, so does the resistance that can be measured by sticking the sensors of a cheap multimeter (I prefer the digital ones) to the ends of that diode. How does this translate into film densities? Well, if you think of the formula for film densities its the difference of logarithms and not tied to any units. So put your 'densitometer' under a steady and relatively bright source of light. An enlarger works fine but I just use a table lamp for this. With full light falling on the sensor take a resistance reading - say the reading is 6. Now hold a piece of unexposed but fully developed film over the hole. The intensity of the light reaching the sensor decreases and the resistance correspondingly increases to 9, say. So now you have your filmbase + fog density which is log 9 - log 6. It's that simple, really. Now, hold a negative that you have exposed at zone 1 over the hole and say the reading changes to 12 - you can now easily calculate the density as log 12 - log 6 and then subtract your Fb+F density from it to get your zone 1 density.

But that is not the end of the story. The chief drawback of this setup is that even though it's quite accurate for the lower densities around zone one, the resistance curve of the cheap photodiode is not linear. That is to say, as the light reaching it decreases the increase in resistance is not quite proportional. As a consequence you will get lower readings than expected for higher densities like zone 8. A simple way around this problem is to 'calibrate' your densitometer. You will need either one of the calibrated step wedges that Stouffer, Kodak and perhaps others make. Or you can get by if someone with a densitometer just agrees to read a strip of film exposed by you at various densities. If you have a set of known densities, it is then easy to read those in our meter and find out by how much the reading is 'off'. So if you know that a particular strip has a density of 1.1 above Fb+F and your meter is coming up with 1 then you know that at that density the reading needs to be 'corrected' by 10 percent. I do all of this in a quick spreadsheet so it's easy to keep track of. Once you have read a set of densities and noted their correction factor, your densitometer is good to go on it's own. It's definitely accurate enough for the purposes of film-developer tests and produces repeatable results. Finally, I might mention that this setup works great if you are exposing a test roll of 35mm film - be sure to hold the negative over the light hole for a few seconds for the multi-meter reading to stabilize.


Anonymous said...

Do you people have a facebook fan page? I looked for one on twitter but could not discover one, I would really like to become a fan!

AB said...

It's just one person :), not a group - so this blog is pretty much self contained. The easiest way to follow it is to click on the RSS feed link. Or you can follow the "Blog RSS Feed Reader" app on facebook. Best, -A.

Anonymous said...

WOW!! That's almost exactly the approach I used before I splurged and bought a factory made densitometer off ebay. But now the lamp's blown so I'm casting about for other ideas, once again. $150 for a replace lamp just seems a bit perverse, IMHO.