Exposure Time and Exposure Scale Calculator for Alternative Processes

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I’ve been teaching alternative processes for many years, and while some students have an appetite for techy stuff and maths, most are artists who just want to make beautiful prints. To many, calculating correct exposure times and calibrating digital negatives is a painful process to push through in order to get to the good stuff.

Fig. 1: Stouffer T3110 and T2115 Step wedges

Calculating the correct exposure time can be as simple as exposing a strip of light-sensitive paper to UV light for, say, 10 seconds, covering part of it with opaque card and exposing again for 10 seconds, and repeating this until you end up with a strip of paper that shows exposures of 10, 20, 30, 40, 50, and 60 seconds. This is what Christina Z. Anderson calls the ‘low-tech’ method in her book Cyanotype: The Blueprint in Contemporary Practice, and while it works, there are some downsides. It can be fiddly when you’re working with a contact printing frame or vacuum frame that needs to be opened up each time to move the black card (placing the card on top of the glass is not recommended as light will pass under it through the glass). Some exposure units have the light source underneath, pointing up through the glass, so it can be difficult to see what you’re doing if your paper isn’t coated edge-to-edge.

There is an alternative method which requires a single long exposure instead of many short ones. The process of making the actual test strip is much faster and easier, but the trade-off is that you need to do some maths to calculate the correct exposure time. Instead of covering up with an opaque piece of card, this method uses a Stouffer Step Wedge: a small strip of photographic film which is clear at the top, with numbered strips (called steps) of successively darker densities running from top to bottom. When placed over a strip of light-sensitive paper and exposed to light, the step wedge does the same job as we do when we manually cover up successive sections of the strip. The beauty of the Stouffer wedge is that it has been manufactured so that each one of its steps has a known density, therefore blocking a known amount of UV light.

Fig. 2: Cyanotypes exposed under Stouffer step wedge. L: 200 units, R: 600 units. The 200 unit exposure time is too short.

Making the Stouffer test is simple: take a blank piece of the transparency material you’ll be using to print the negatives, and place it on your light-sensitive material. Place the Stouffer on top of the transparency, with the text reading the right way. Place the whole sandwich in your exposure unit, and set it to an exposure time at least twice as long as you think you might need. The paper under the Stouffer will receive the full exposure time through the clear, top part of the Stouffer, which is step 1. Each step below this will receive the equivalent of a successively lower exposure time through the increasing density of each step. If you’ve correctly estimated a total exposure time that is too long, you will end up with a test strip that clearly shows blocked up shadows on one end, too light on the other, and hopefully a sweet spot in one of the steps in between that shows maximum density (Dmax) without overexposure.

So far so easy, but now we need to take the step number where we first see maximum density, and use it to calculate what the correct exposure time is, and this is where the stumbling block lies for many. The traditional way, and one I used for many years, is to have a list of numbers showing what you need to multiply the original exposure time by in order to calculate the correct exposure time. There are other methods which aim to simplify this process: for a long time, I shared a spreadsheet with students that is freely available online (thanks, John Isner), and more recently, I have further simplified the method with a series of charts. Unfortunately, when I open up the calculator app or a spreadsheet while teaching a workshop, there are usually more glazed eyes than there are exclamations of delight.

Fig. 3: Detail of cyanotype with 600 unit exposure time

What I needed was a tool designed for people who are more comfortable with images than numbers, and which would make the process as simple and painless as possible. This is how I came up with the Lux Darkroom Exposure Time & Exposure Scale Calculator. Like a spreadsheet, it will do all the maths for you behind the scenes, but the interface is visual and intuitive. All you need to do is select two options from dropdown menus, and enter the exposure time you used to make your test print with the Stouffer wedge. On a digital image of a step wedge, you can then click on the step where you’re seeing maximum density, and click the ‘calculate’ button, which will give you the correct exposure time you need to produce maximum density.

How to use the Exposure Calculator

We can use the cyanotype shown in Figure 2 as an example. It was exposed under a 31-Step wedge for 600 units. A closer look at the print (Fig. 3) shows us that step 5 is the first step at which maximum density appears: above this step, all the remaining steps to step 1 are the same.

In the calculator tool:

1. Select Negative process because cyanotype is a negative-working process

2. Select 31 Step Stouffer, which corresponds to the step wedge used

3. Type '600' in the seconds/units box, which was the time the test strip was exposed for

Fig. 4: In the Exposure Calculator tool, select Negative process and 31-Step Stouffer from the dropdown menus, and enter '600' into the seconds/units box

In the second box:

4. Click step 5 on the image of the step wedge, which is the step number where maximum density first appears in the printed step wedge,

5. Click 'calculate'. The correct, calculated exposure time is displayed below.

Fig. 5 With the 'Max Black' box selected, click on the first step showing maximum density (5 in the example) and then click 'Calculate'. The correct exposure time will appear below.

The tool works with negative processes like cyanotype and platinum printing, or positive processes like carbon transfer. It also works with dual-exposure photogravure (either polymer or copperplate) to calculate two separate exposure times: the first for the aquatint screen (Dmax) and the second for the image transparency (Dmin).

Tips:

• You can use either a 31 or a 21-step wedge, just be sure to select the one you’re using in the dropdown menu.

• You can enter the exposure time in minutes and seconds, or just use the seconds box if you’re working with a light integrator that uses units.

• You can also click the first step that prints paper white, and the tool will calculate the exposure scale of the process.

About Stouffer step wedges:

Stouffer Industries manufacture many different step wedges and can supply them. They do not have an online shop, but you can place an order via email or phone.

To calculate exposure times, you will need a transmission step wedge on a clear film base.

The step wedges that work with this calculator are:

• T2115 (21 steps)

• T3110 (31 steps)

Both of these are long and thin so use very little sensitised paper for testing.

If you prefer a larger, wider step wedge, the two below are 4x5”

• TP4x5-21

• TP4x5-31

There’s no need to buy the calibrated ones, which are more expensive and not necessary for calculating exposure times.

Suppliers:

The 21-step wedge (T2115) is relatively easy to find and can be purchased from these suppliers:

• Stouffer Industries,USA (ships internationally)

• Intaglio Printmaker, UK

• Fortex, UK

• Photographer’s Formulary, USA

• Bostick & Sullivan, USA

• Freestyle Photographic, USA

• B&H, USA

The 31-step wedge (T3110) is harder to find:

• Stouffer Industries, USA (ships internationally)

• Precision Digital Negatives, USA (ships internationally) this is the step wedge we use and recommend at Lux Darkroom

Note: the 31-step wedge sold by Precision Digital Negatives looks slightly different as it's a custom step wedge designed by Mark Nelson. It's manufactured by Stouffer and performs identically to the T3110 on their website, but Mark has made some improvements to the design which make the test prints clearer and easier to read.