There are a variety of reasons why you would want to determine your base exposure time for your contact printing processes, so I will discuss a few of the most common in this article. The main difference between using the proper proof time as a “proofing” tool to make adjustments in your exposure and development processes, the base exposure time gives you DMAX in your printing time and removes the guesswork out of making a perfect contact print. If you want to make contact prints in the darkroom (silver gelatin, AZO, etc.) or any number of historic/alternative prints such as salt, van dyke, cyanotype, platinum, palladium, and others, then you will absolutely want to nail down your base exposure time.
Determining base exposure must be done for each paper and printing process based off of your environment. This means if you were using two different papers for salt printing, then you would need to do this exercise twice. If you change work location, it is very likely that you will need to determine your base exposure again to ensure your results are still consistent. Also, if you are using the sun as your UV source for one of the alternative processes then you will not likely be able to determine a consistent base exposure. In this case I would suggest using a consistent UV light source. I produced a video showing you how to make your own UV printer that you may want to review. Your goal is to determine the shortest amount of time that it takes to create DMAX (maximum black) for your paper, chemical and environment variables. By doing this you will be able to create consistently beautiful prints.
For example I do a lot of Palladium printing from my large format sheet film. I had to determine base exposures for all of my film and paper combinations because the film is literally different for each process. If you used Tri-X and T-Max then you would also need to determine base exposure for each film. I think you get the idea.
Base exposure testing applies to all printing methods to include silver gelatin darkroom printing as well as hand coated alternative printing.
I have developed a 7 step process that outlines the required actions. It is actually much easier to do then it is to read and type out the steps.
Base Exposure Steps
1 – For alternative prints tear a piece of watercolor paper approximately 5×7. If you are making a silver gelatin darkroom print then just use an 8×10 piece of the paper you will be making your final prints on. You will tear watercolor papers and cut darkroom papers.
Now tear or cut this piece into three equal strips for testing. Make sure you tear and are not using scissors for watercolor paper, which can leave metal filings on your paper and produce undesirable effects on your alternative print.
Using a ruler and pencil measure and mark a three lines at the 25%, 50%, and 75% of length on your paper. You are creating four blocks or testing zones for your exposure test. If you are doing a darkroom base exposure test then you will not need to be tearing the paper of course.
2 – Based on your printing process you should have a general idea of the recommend exposure time. For example, if you are making a Palladium print most textbooks state a 6 to 8 minute exposure is normal. This may or may not be true for you based on your working conditions, but it at least gives you a general idea.
Based on this example I would expect to make 4 test exposures in 2 ½ minute increments (2 ½, 5, 7 ½, 10). If you are making a silver gelatin print on fiber paper then your base exposure time will probably be significantly shorter. A typical range for darkroom fiber papers like Ilford MGIV FB is somewhere between 10 and 20 seconds. Simply do a little research for normal base exposure time for your specific printing process as a means to create the testing times. For my darkroom papers I usually conduct 2 second test strips as a general rule.
3 – Either get a piece of the blank OHP Premium film that you will be using for your digital negatives or get a piece of traditional silver film for your darkroom prints. If you use film at times and OHP for digital negatives as well then these are two different tests.
Your blank film creates a base (film base + fog) of your film material so that you can factor this into your exposure because the film acts as a diffuser. If you are using regular film (e.g., Tri-X, T-Max, etc) then you will need to develop a blank sheet or roll via your normal development process first so that you can effectively get your film base + fog for this medium. If you are doing digital negatives then just get a new piece of your Pictorico OHP film.
Now laying the film lengthwise over your test strip, cover up the top half of your paper. You should have your print paper that is open on the top half and has a piece of clear film laying over the bottom half.
You will be able to see the paper DMAX as compared to the DMAX you will get with your film laying over your paper (factoring in your film base + fog). As discussed in the final step, you will want to identify the test strip section (1, 2, 3 or 4) that matches your paper DMAX revealing your base exposure time and the purpose of this exercise.
4 – You will need a piece of 2 ply mat board or something similar to cover up and block light from your test strip. You can use your contact printing frame, possibly a clip frame or even a piece of glass laying on your film and paper during the exposure tests.
Place your mat board over ¼ of your test strip and make the first exposure. In effect you are covering ¾ of your test paper at this time. Using my example in step 2, make a 2 ½ minute exposure in your UV printer. Now slide your mat board down to expose ½ of your paper and make another 2 ½ minute exposure. Next, slide the mat board down to expose ¾ of your paper and make another 2 ½ minute exposure. Finally, remove the mat board and expose your paper for another 2 ½ minute interval. You have created a test strip with 4 exposures in 2 ½ minute intervals. If you are doing silver gelatin darkroom prints your light source (i.e., enlarger, low-watt incandescent, etc) will be different and much shorter exposure times as previously mentioned. The times and light source may change, but the process remains the same.
5 – Remove the paper and process it as if it were one of your fine prints. This will vary based on your printing process. Make sure you process this test strip in the exact same way you do your fine prints. Once that is completed, dry the print as you normally would and then evaluate the test print.
6 – Examine your test strip and look for the block where it is the same color as your uncovered area on the top of the test strip (DMAX). This is your base exposure time!
If none of your test sections match the strip above then you need more exposure time. Calculate a new test and follow steps 1 through 5 again. If all of your squares are black then you need to shorten your test times and follow steps 1 through 5 again based on the shortened exposure times.
7 – Document the base exposure time for each printing process, paper, environment, and light source. If any of these variables change then you need to do the base exposure test again. Use this time when making your prints in the future and this time is also used for the creation of custom correction curves in the next section.
When making contact prints I use my base exposure time as the foundation of my printing process. I know what kind of negative I need for each printing method so I make sure I use a lot of control in this area to ensure my contact printing time remains constant. In my alternative printing processes, once I get my base exposure time I literally never have to worry about exposure again because it is right on the money for every single print.
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