How to Determine Your ISO Rating for Ilford Harman Direct Positive Black & White Paper

April 05, 2016  •  Leave a Comment

Ilford Harman Black and White Paper is a refreshing and fun way to create positives on fiber paper directly in your large format view camera.  I think the limited tonal range and unique characteristics of this specific medium help me to unleash new ways of thinking about expressing my creative vision.  

The first step when starting with this paper is selecting a developer and the proper development time to achieve DMAX.  You may want to read my article on this topic if you need to do that.  

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For my style of photography and my creative vision, I rate my paper at ISO 6 most of the time.  I work in natural light, typically first thing in the morning or the evening as the sun is going down.  If I alter from this approach, I adjust my exposure based on experience.  However, you should not take my rating as your own.  You should test and figure out what rating produces the type of print that you want to create.

The first thing you have to keep in mind when working with direct positive paper is that when you give the paper more light (bigger aperture opening), the tones on the print will be lighter, which is just the opposite when working with negatives.  I use this principle to influence the mood of my print.  I use the dark slide on my film holder to simulate different ISO ratings when I test for the base ISO rating that I want to use.  

ISO Testing Procedure

You can perform a simple ISO test of your paper by pulling out your dark slide out half way across the paper for the first expsoure and then pulling it out completely for the second exposure.  Make sure to keep your aperture the same for both exposures.  

By doing this, you are doubling your exposure time on the first expousre and therefore testing for 2 different ISO ratings on the same sheet of paper.  The first exposure will represent the lower ISO rating because it received twice the light as the second exposure when you pulled the dark slide out.  

For example, if you were testing for ISO 3 and 6 at your meter said to expose for 1 second, the first exposure would be for 1 second as metered, and then when you pulled the dark slide all the way out for the second half of the paper, the right half of the paper only receives 1 second of exposure, but the first half receives a total of 2 seconds.  The left side of the paper would represent ISO 3 and the right half ISO 6.  You could use this method to test a range of ISO ratings until you find what works best for you.

Comment below and share your ISO rating of your paper and be sure to include details about your style of photography, developer, and other pertinent details that impact the tonal scale of the print.

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Tim Layton
Darkroom & Large Format Photography
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