densitometer (with tip for homebrewed densitometer)

[jeremy_rardin]

Does anyone out there have an accurate densitometer? I'm running some

personal film speed tests and I need one. Suprisingly, there is not

one at the local college science lab (so they say). If you have one

and don't mind me sending you some negatives to find the one in the

.08 to .10 range (as suggested by ol' Fred Picker) just send me an

e-mail with your mailing address if you dont mind I would really

appreciate it! :)

 

thanks in advance!

 

- jeremy

jeremyrardin@gmail.com

[arraga]

<p> If you have an enlarger, you only need a photo cell (about US$ 1) and a digital electronic tester (US$ 10-20).

 

<ol>

<li> Mount the cell at the bottom of a opaque black 35mm canister (not the cassette; the plastic that the cassete comes in); make two smallish holes at opposite sides of the canister walls for the wires to the cell, and a bit larger hole in the top of the canister to let light in.

 

<li> Connect the wires leading out of the canister to the tester, and put it into resistance measuring mode.

 

<li> Turn on the enlarger with a lens, but no negative in the carrier , note the value displayed in the tester. Let's call it 'nothing'.

 

<li> Put a blank negative in the carrier, and note the value displayed now. It'll be bigger. Call it 'fb-f'

 

<li> Put the negative which you want to measure its density in the carrier, and note the resistance value. Call it 'neg_density'.

 

<li> Now, the fun part. Turn on a calculator (or a logarithm table, if you are so inclined - when I was in highschool I had to learn it the hard way :), and take the logarithm of the values collected.

 

<p>To know fb-f density, do log (fb-f) - log(nothing).

<p>To know your selected negative frame density, do log (neg_density) - log (nothing).

 

</ol>

 

<p> I've checked the method against a Stouffer calibrated transmission tablet. It works.

[arraga]

In fact, it'll probably be better than a densitometer, as the enlarging conditions where you'll make your print will be automatically considered. Commercial densitometers take a middle road between specular and diffused density.

[ryuji_suzuki]

What kind of photoresistor are you talking about? If CdS, usual ones' intensity-resistance relations are not very linear to use as you described...

[edward_zimmermann]

"<cite>In fact, it'll probably be better than a densitometer, as the enlarging conditions where you'll make your print will be automatically considered.</cite>"

<P>

Better for what? Without a well defined illumination and spectral response, highly linear circuits (which are not trivial) <U>AND</U> calibration against a known and well-defined standard whatever values one gets are not more than personal. Its a bit like a homebrew thermometer without calibrations and scales. Even if one hand calibrates the measures against transmission references you still don't know the response to other negatives and other conditions. For this one needs to have a well-defined illumination and filtering. For doing some basic film testing and B&W quality control it might be sufficient but there is more to sensiometry.

<P>

"<cite> Commercial densitometers take a middle road between specular and diffused density.</cite>"

<P>

Its ISO (and national ANSI, DIN etc.) Standards.

<P>

Even among "<cite>commercial densitometers</cite>" there are many different types and responses (Status types). The most common transmission types for our uses is Status-A (for transparencies), Status-M (for colour masked negatives), Visual, Ortho and UV. For B&W one probably wants to use "Visual" (not Ortho!) but for contact printing using UV sensitive papers such as platinotypes you, by logical contrast, want "UV".

[arraga]

I use a CDs cell. Regarding linearity, I assume you are talking about the log(value) graph, and not the actual resistance values, which are exponential.

 

To check it, I took readings of the light coming out of the enlarger lens at f-stops 2.8, 4, 5.6, 8, 11,16 and 22. Each f-stop is, of course, a reduction in half of the light reaching the cell. I've attached a gif of the resulting graph. It's sufficiently linear for me.

[arraga]

With regards to the differences to commercial densitometers, I'm afraid I wasn't clear enough.

 

This in no way substitutes a commercial one, but I think that for this narrowly defined application domain, is more than enough.

 

I do use it for checking negative densities, which will be printed by projection in the same enlarger as the check is conducted. No contact printing, no AZO, no transparencies, no color negatives, just plain old enlarged 135 and 120 B&W.

 

As you are aware, projection densities are different from contact densities. A commercial densitometer does not take in account that, or the light source characteristics of the enlarger.

 

With regards to ISO standards, remember that a film speed test is all about differences between densities (eg. 0.10 above fb-f, 1.30 above fb-f, etc) and not absolute values. I'm not submitting film speeds to ISO, but defining a personal EI. I really don't care about the exact density value of a negative, but I care very much about its difference against a clear base negative.

 

And yes, I'm aware of the complexities of linear circuits. I come, (as if it's not evident) from an engineering background.

[eric rose]

If you go to feeBay you can find lots of great color analyzers for around $25. Use there photocells and meters for the described precedure. Beats having to micky mouse something together.

[lowell_huff1]

Introduce yourself to a local printer. A good printer prepress department will always have a densitometer in service.

[arthur_mcculloch]

Santiago

 

A brilliant homebrew idea. Just wanted to check the formula for negative density. I thought it would involve subtracting the film base fog reading (not log nothing)from the negative density reasding.

 

But I was never any good at maths. Let me know.

 

Thanks

Arthur

[arraga]

Hi Arthur,

 

yep, absolute negative density would be log(neg_density) - log(nothing). For fim testing you want relative densities agains fb-f, instead of absolutes.

 

For example, if you want 0.10 above fb-f density, you'll have [log(neg_density) - log(nothing)] - [log(fb-f) - log(nothing)] = 0.10.

 

This simplifies to log(neg_density) - log(fb-f) = 0.10, which is what you tought.

 

I just let a spreadsheet take care of the drudgeries for me. A column has read values, the other log of those values, and the next one has its differences.

 

I like to have a density value for fb-f, in order to check for possible fog creeping in during longish development times, but it's not needed; you can take your zero as a clear negative (log fb-f), instead of pure air (log nothing).

 

Some additional tips.

 

a) The CdS cell takes some time to stabilize readings; it's the same problem that older exposure meters had, before silicon blue cells. Give 4-5 minutes at low light levels, or until numbers are not changing anymore.

 

b) Of course, this must be done in a darkroom where the only light source is the enlarger.

 

c) Reducing the size of the opening at the top of the canister reduces the posibility of flare, but you are limited to taking readings of the negative center, as moving to the sides will block light entering the hole. In real life testing, flare was inconsequential, even without a canister top, but if you have problems, reduce the opening diameter, and/or matte paint the canister walls.

[patrick_gainer]

I built one using a silicon phototransistor in current mode as input to an IC op amp with a silicon diode feedback. The current through the phototransistor is a very linear function of illumination. The voltage fed back through the diode is a quite accurate exponential function of the current through it, which is accurately equal to the photocurrent. The net result is that the voltage output of the op amp is accurately proportional to the log of the photocurrent over a wide range. I used a dual op amp, the second half being set up as a log amp with constant, but variable, current. The output amp goes to a potentiometer that allows calibration for reading density, and also for reading zones. The "constant" level adjusts for exposure time, base density, and other factors.

 

The phototransistor has a very small sensitive area and responds very rapidly. Silicon is sensitive from the violet to the near infrared. If need be, you can use filters to control its response. Remember that with a log amp, a filter factor is a constant. You do not set the sensitivity, but the constant level, to adjust for filter factor.

 

Have fun.

[doremus_scudder1]

Jeremy,

 

You can do the neg calibration you need with no densitometer at all. Call me old fashioned if you like, but I have been calibrating Zone I negative densities (ie. finding film speed) for years now by simply finding where the first significantly visible departure from maximum paper black is.

 

I simply expose one lf negative in stripes using the darkslide to differnt densities, making sure to leave one stripe completely unexposed (one could use several negs as well). Use the unexposed stripe or neg to find where film-base-plus-fog prints maximum black by making a test strip. It is a good idea to position the negative in the carrier so that a stripe of "nothing" (ie. pure enlarger light that does not pass through the neg) gets to the paper as a control. Process and dry the print and examine it. When you can no longer tell the difference in shades of black under reasonably bright lighting, you have reached an effective Dmax and determined the time/aperture/lens/head position to use for the rest of your tests. Now take your other negs (or the one with the stripes) and simply make a test exposure of them (it) as well, using the same exposure as previously arrived at and also allowing a stripe of pure enlarger light to hit the paper. Process and dry the paper and examine it under the same light source (a light source that approximates ideal viewing illunination is best). Look for the first stripe/neg that emerges as a different tonality from maximum black. This is your Zone I density (and your EI). The rest of the Zone System calibrations go similarly, using the equipment, chemicals and paper you plan to use and, therefore, taking all the variables into account. Yes, you may end up not working with exactly .10 over fb+f, but you will know exactly what exposure gets you Zone I on the paper. Better in my opinion.

 

This film speed test and the rest of similar Zone System calibrations are explained in the Minor White/Richard Zakia book (The Zone System Manual, I think... Maybe search on the authors' names in Amazon).

 

I do all my Zone System calibration this way and teach this way as well. I really believe it helps with visualizing the tonalities for beginners too.

 

Hope this helps,

[arthur_mcculloch]

Thanks Santiago

 

I've got the cell and will try this out in the next week or two.

 

And thanks for the supplementary information. I've been trying to standardise my b&w processing around fp4 and hp5, and this will help.

 

Regards

Arthur McCulloch

[mike_buehler]

Where did you buy the cell? any links?

[pa.patriot]

Cells available here (For USA)

http://www.radioshack.com/product/index.jsp?productId=2062590&cp

[robert himmelright]

a good printer may or may not have a densitometer. Some labs use what is called a fuji oasis, which is a densitometer but from my brief interaction with it i saw no actual readings...feed controll strip in...it calibrates itself, strip feeds out...fuji says "run","caution" or"dont run". The thing is connected to a phone line so fuji sees all your numbers and you dont. Scarry eh?