Make a Calibrated Step Tablet without a Densitometer?

[james_ollinger]

I'm a weekend warrior and now occasional darkroom worker so I can't justify the expense of a calibrated step tablet,

but I've wanted one. I've got a couple Macbeth densitometers I picked up a surplus store but they need to be

calibrated. I've got an uncalibrated tablet, but can't use the densitometers to calibrate them without a calibrated

tablet. Catch 22.

 

Anyway, I was thinking about it and it seems to me you might be able to make one without using a densitometer. I

don't have access to my darkroom now and won't for quite awhile, but I'm just thinking out loud and thought I'd share

and see what people think.

 

You need:

 

A roll of general purpose b&w film (your choice)

A camera/meter you trust

An 18% gray card (or some other suitable item)

An enlarger with a color head you trust

A darkroom meter

 

1. Set up for photos of a gray card on film. Meter card and shoot exposures. Begin at -5 stops through +5 in 1/3rd

stop increments.

 

2. Process normally.

 

3. Find the first exposure where there's discernable density, not just clear filmbase + fog. This exposure gets

marked as 0.1 density.

 

4. Set your filter pack to 0M 0Y 0C

 

5. Load this exposure (0.1) in the enlarger and measure with the meter. Null the meter or write down its reading

(depending on your meter type).

 

6. Go to the next densest exposure on the negative strip. Note the difference on the meter but don't change

anything.

 

7. Go back to negative marked 0.1. Add color filtration in equal amounts until you get the meter reading for your

unknown density negative. When you get that on the meter, you've added that density. E.g. if your filter pack is

10M 10Y 10C, then your test negative density is 0.2 (10 of MYC is 0.1 density, so add that to the 0.1 on our known

negative, and you get a density of 0.2. If the filter pack were 50M 50Y 50C, you'd end up with a density of 0.6).

 

8. Record the density for that negative.

 

9. Repeat until you've gone through the strip.

 

10. You now have a calibrated step wedge.

[waldo_lee]

Step wedges from Stouffer Industries are very reasonable, even for the calibrated ones (http://www.stouffer.net/Productlist.htm). I strongly advise going with a manufactured step wedge. Also, in my experience uncalibrated step wedges are usually close enough to calibrated ones for the purposes of B&W. Sure, it's ideal to set up your densitometer with a calibrated step wedge, but I wouldn't hesititate to use an uncalibrated one if that's what I had, and I wanted to get on with getting some work done.

[james_ollinger]

Wow. The price must have come down. It seems like the last time I looked (years ago) they were >$100 and there was just no way I could justify it for what I do.

[conrad_hoffman]

If you have a calibrated densitometer you can, with great difficulty, make a step tablet. Without it, it's near hopeless. The problem is of course, the film curve. If you're making a reflective target, it's even harder, because the paper curve is much more severe. Now, there are some ways to check your densitometer at a few points. If you expose a sheet/strip of film to a very low density, you can cut out little squares and stack them up. The densities will add, so you can at least check the linearity of the densitometer. If you can find a lab to measure it, just make a step tablet that looks more or less even and have at it. Trivia- one way they used to make density wedges was to cast them using a wedged mold, and a neutral gelatin- filled with carbon rather than dye. Like you, my darkroom is down at the moment, but I could measure for you if you send me a PM.

[rodeo_joe1]

Lee filters do ND acetates in a variety of densities. If you ask them nicely, they'll probably send you a couple of small

samples for nothing. The 0.15 and 1.2 densities are probably all you need, since if you set your densitometer up at

0, 0.15 and 1.2D it shouldn't be too far out anywhere else, and the critical density range for B&W negs sits below

1.2D anyway.

 

I have a swatch of filters from Lee, from the days when they gave a whole book of acetate samples out on request. It

includes 0.15, 0.3, 0.6, 0.9 and 1.2D filters, so I've been able to check all these. The densities of their acetates are

quite accurate (< 5% out) - plenty good enough for photographic use.

 

In any case, you just don't need the accuracy of an NPL or Stanford Labs-traceable calibrated wedge. We can only

set our lenses and shutters to the nearest 1/3rd of a stop, at best, and this is equivalent to a density step of about

0.06D at mid densities (assuming a negative C.I of 0.6), so I don't see much need for greater accuracy than this in

calibrating our measuring equipment.

[patrick_gainer]

If you buy or borrow a copy of Gordon Hutchings' "Book of Pyro" you will find instructions for building a Zone board. This will give you the equivalent of a density wedge whose accuracy of separation of the Zones does not depend on measurement by a densitometer, but by a tape measure.

[james_ollinger]

Thank you everyone, for the advice and info.

 

Conrad: please don't go to the trouble. That's a neat idea but I don't see myself actually doing it. Not for the relatively little darkroom work I do anymore.

[james_ollinger]

Also thanks, I'll check out the BOOK OF PYRO. I don't have a copy, but I'll borrow one from the library and see what it says (I enjoy reading this kind of stuff regardless).

[patrick_gainer]

It is a good read for a number of reasons, whether you ever use any pyro developer or not.

[profhlynnjones]

No James, the $100 or more were from Kodak. I've used Stouffer for many years, I used to know the owner founder going back to the 70's, trust them absolutely.

 

Lynn