How To Use A Step Wedge To Find a "Standard" Print Time.

The Quick And Dirty Method:

Expose your step wedge so that you can't distinguish between step one (the clear base material) and step two. Record the time it took to do this. There you're done. You have your "Standard Exposure". Remember if you ever change a variable (emulsion, paper, coating technique etc.) you'll have to do this test again to ensure your Standard Exposure hasn't changed.

Why This Works: In short because the Stouffer wedge's polyester base material (see table below) is known to measure around 0.10 logD in UV density and Pictorico OHP a little higher at 0.18 logD UV. If you blend step one and two (remember the wedge goes up 0.10 per step) it means your density is at approximately 0.20. On the Stouffer wedge that's 0.02 over (close enough without owning a UV densitometer) what we need to negate the base material of the OHP. Remember the days of calculating Base + Fog? Well we don't have the fog on a digital negative but we still have a base. 

It seems there has been very little published on the Internet as to how to correctly use a Stouffer step wedge. Even Stouffer's own site is of very little help. Seems everyone assumes that we're just born with the knowledge of what it does. In short a Stouffer tablet tells you what density range your negative should be to match your paper/emulsion combination. If you contact print a 31-step wedge and step 20  is the last toned step before white and step 2 and 3 are equal on the dark end of your paper your negative's density needs to be about 1.8 logD. There are two terms you need to know here. "Density Range" refers to the negative and "Exposure Scale" refers to the paper.

Digital Negative Objective: we want to find an exposure time that brings out the best qualities of our emulsion, in particular we want a dark shadow, as dark as the emulsion is willing to go, within reason. This is called dMax short for maximum density.  If you don't give your emulsion enough exposure not only will shadows will look like mid-tones but you'll also get no highlights and faded and weak mid-tones. dMax is achieved when we cannot tell our darkest steps on the step wedge apart.

Which Step Wedge Would You Calibrate To?


 1. Exposure too short. Could have a little more dMax.


 2. Pretty close. I can still see a slight border between step 2 and the base material.


3. This is good. Step 2 and Step 1 (base) are blended.


4. Too much exposure. Steps 4 & 3 are blended. You can use this but your inkjet printers and inks will be required to produce more density.

Fig 1. Stouffer T3110 step transmission guides shot at 6, 12, 16 and 24 minutes. Black has line indicates the border of step one and two. I decided that 16 minutes gave me the best Standard exposure.

Count the first black step and subtract this number from the step adjacent to the first white step. For example, if we were to look at the bottom step wedge and we said that "5" is the first black step and 14 was the step adjacent to the first white step we come up with 10 steps (ten). Multiply this by the step factor (T3110 is steps of .10 density) and you get your density for the emulsion/paper combination you're using. In the case above it's classic cyanotype so the range is 1.0 logD which is within my usual range for this process.

 

Known UV Density Values of Base Materials

Stouffer 21-Step .10 logD

Stouffer 31-Step .10 logD

Pictorico OHP .18 logD

Photo Warehouse Crystal Clear OHP .06 logD

 

Other Reasons To Own a Stouffer T3110 or T2115

While not as accurate as a UV densitometer the step wedge is also one of the most valuable "debugging" tools in alternative photography. For instance, it will tell you when your emulsion has gone bad (for me that would be often). I personally prefer using the T3110 because it uses increments that work in 1/3rd stop equivalents, which make it a little more accurate for printing purposes. It's also easier to calculate the density, if the wedge prints to step 17 at the standard print time you know your density is about 1.7 logD.

What's the "Range" of My Process

Every emulsion and paper combination have combined properties which determine the range of tones that they can print. Some high-end art papers print beautifully smooth full tones. While some other art papers print like toilet paper for no apparent reason. Emulsions can be quite the characters too. At one end of the chemical spectrum you have short-ranged process like gum and cyanotype. In the middle you have processes like silver, iron-silver processes like VDB, Pt/Pd and at the far end of the spectrum you have processes like Salt prints and Albumen prints.

The trick in contact printing (as with enlarger photography) has always been producing a negative which exploits the properties of the emulsion/paper combination. No more, no less. Alt photographer printers who use silver negatives for example would have to "extend" development on their contacts negatives to 30 to 70 per cent or greater to make a negative best suited for Pt/Pd rather than for Silver.

 

Notes

Density. How thick something is. In this case photographic emulsion holding light back on a negative.

How is Density measured? For practical purposes it's a numerical scale that goes from 0.0 to 3.1 (it actually can go higher). In photographic terms every 0.30 increase on the scale is one "Stop" (just like your camera). The T3110 scale from Stouffer increases in steps of 0.10 which is one-third of a stop. This means that for every three steps on the scale the exposure will have doubled. In contact printing where you are using a static light source this means the time of exposure doubles every three steps.