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# Thread: Calculating exp. time for desired density

1. ## Calculating exp. time for desired density

I want to use some ortho film to make general dodging masks. Is there an easy method to calculate the correct exposure times to achieve a 1/2 stop density, 1 stop, etc.? I assume I would start with a step wedge and a densitometer, but once I exposed the step wedge, how would the densitometer readings be used to calculate the proper times to quickly make several densities? I know what specific densities I am looking for (.15, .3, and so on). Is there a way, or is it just trial and error?

2. Originally Posted by Greg Davis
I want to use some ortho film to make general dodging masks. Is there an easy method to calculate the correct exposure times to achieve a 1/2 stop density, 1 stop, etc.? I assume I would start with a step wedge and a densitometer, but once I exposed the step wedge, how would the densitometer readings be used to calculate the proper times to quickly make several densities? I know what specific densities I am looking for (.15, .3, and so on). Is there a way, or is it just trial and error?
Hi Greg, I'm the fellow you sold a densitometer to nearly a year ago. If you've been reading the "Progress on XTOL-Concentrate" thread, you know I've been getting lots of use out of it. Anyhoo...

Let's say you have a 21-step Stouffer wedge, and you've exposed and taken measurements. To get a desired density D, you need to work backwards like this:

1. Locate density D in your list of densitometer values. It corresponds to some wedge W on the wedge (where W is 1 to 21).
2. Compute S = (21-W)/2.
3. Expose your ortho film to whatever you exposed the wedge to, MINUS S stops of light.

This way, you'll expose the film to the same amount of light which the correct wedge saw. I hope this makes sense.

Mark Overton

3. measure the results of a step wedge, graph them, build a curve-fitting function and calculateintermediate data points, but don't forget density is a function of exposure AND developing time

4. Originally Posted by Greg Davis
I want to use some ortho film to make general dodging masks. Is there an easy method to calculate the correct exposure times to achieve a 1/2 stop density, 1 stop, etc.?
To answer that question about calculating: NO

But it is easy to determine the information you need empirically with some test strips. You don't need any calculating or spread sheet or graph, just a pencil and paper to keep notes. Make a test strips by progressively exposing and covering part of the film in the usual manner. Process the film and read the densities with your densitometer. You should be able to easily zero in on the exposure time needed to produce your required densities that way.

5. Hi Greg,

Mark and Ralph are both right. As Mark said, (assuming 21-step .15 increment) two steps on the step wedge means one stop of exposure time. But you want to re-create a fairly accurate replica of a step-wedge yourself?

So as Ralph said, you will need to use interpolation. I use a graph on paper. You will have 21 points on the graph. Where graph crosses the density you want, take the point below and above.

Assume these two points make a right triangle where you know the width and height of the legs. Your desired density will cross the hypotenuse at the point you want to solve. Subtraction is the only math you need to find the height of the leg on the density scale (because it's the density you picked). So you solve the similar triangle for the width of the x-axis.

Run over Rise times new Rise. Tells you the unknown Run.

6. Originally Posted by ic-racer
To answer that question about calculating: NO

But it is easy to determine the information you need empirically with some test strips. You don't need any calculating or spread sheet or graph, just a pencil and paper to keep notes. Make a test strips by progressively exposing and covering part of the film in the usual manner. Process the film and read the densities with your densitometer. You should be able to easily zero in on the exposure time needed to produce your required densities that way.
This is what I thought would need to be done. I just wanted to avoid it if there was a faster method. I'll just have to read each portion and note what time it was. In order to avoid the issue of the bulb turning on and off, thus creating actual time difference between test strip and final exposure, I will try to use my metronome function and move the card during one long exposure.

7. IC Racer and Greg,

You wrote while I was composing so I don't know if you saw how simple it is...

Do you see any difficulty exposing a step wedge, finding the closest steps, then narrowing it down by interpolating with "Run over Rise times new Rise"?

8. Greg, trial and error for masking is really the only way to get the right masks. Different combinations of exposure and development time are used to get different results depending on what you are trying specifically to do. For example, to make dodging/burning-type masks without local contrast and sharpening effects, generally more exposure and less development. Etc.

It can seem daunting at first since there is a fair amount of guesswork and "winging it" but it gets easier with practice.

9. These will be generic masks, much like graduated filters, that I can use on any negative. I want them to be calibrated in density to 1/6, 1/3, 2/3, and 1 full stops. I want to make a bunch of different shapes of each density.

10. Oh ok, I thought you were making masks from negatives.

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