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An Understanding Of Film Masking In The Darkroom Part 1

In this series, Timothy Hall talks us through the art of film masking.

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Words and images by Tim Hall

Film masking, in brief, is the making of a low contrast positive image on film. It is made in contact with a negative and used in conjunction with the original to print from. Sandwiched together, the mask effectively “dodges” certain areas so that those areas reproduce lighter and allows the darkroom technician to use a longer exposure for the print. The mask keeps the detail in the shadow areas and allows for more detail in the high value areas due to the increased exposure for the print.

Paper Response to a Specific Density Range

The best way to approach the subject of film masking is to have a good idea of a negative’s density range and its effect on photographic paper. Eastman Kodak supplies a 21 step tablet for this purpose. Made on black and white film it has increments of steps that progressively get denser. Each step blocks a half stop of exposure; the 21 steps, therefore, covers 10 stops of exposure.

Kodak Tablet

A numeric value can be obtained from the use of a densitometer, a devise that measures the amount of light penetrating a sheet of film. All densitometers are calibrated to display a higher numeric value of .30 whenever ½ of the light is being blocked. That is to say that if you “zeroed” your densitometer to display .00 when you measure an area of film that is clear, then .30 would indicate that ½ the light is now blocked by that part of the film. Consequently .30 is said to equal one stop of light blocking density.


A 21 step tablet has 21 half stops of density, and the steps (on a densitometer) measure .15, .30, .45, .60, .75, .90, 1.05, 1.20, 1.35, 1.50, and so on up to 3.00.

When contact printing a 21-step tablet onto black and white paper you will notice that 5 steps are easily seen. Above 5 steps you will see lighter steps that approach white and below 5 steps you will see darker steps that approach black. The example I am talking of is contacting the step tablet onto a “normal” contrast grade paper (grade 2). When printing onto a lower contrast paper you will gain more steps and when printing onto higher contrast papers you will obtain fewer steps.

Silverstep Tab

The silver step tablet shown above was contact printed onto a multi-grade Ilford paper with various filtrations set in the enlarger head. If you look closely on the label I wrote by hand the filtration used for each exposure. I normally use a color head designed for the printing of color negatives and positives. The enlarger is fitted with filters above the negative stage so they do not interfere with the image forming light such as when filters are placed below the lens.

I also indicate the densitometer measurements below each step on the contact prints. The one labeled “Paper Grade 2” has an indication of print densities from .21 to 1.25 in the most visible steps.

Paper grades

An interesting fact is that when you double your exposure the 5 discernable steps will move 2 steps up along the gray scale, and when your exposure is cut in half the 5 steps will move down the gray scale. This is physical proof that .30 indicates one stop of exposure change.

contact prints with 3 exposures

Note that a density range of .75, or 5 half steps, is what photographic paper will easily respond to. Densities recorded above or below that range will not be easily seen. You will need a greater density range than .75 in your negatives to be sure, but detail can be best displayed on a print when the detail falls within that range. If we look at the maximum black and maximum white on the print of the gray scale you will notice that in fact an overall range of 1.65 or more is needed in order to obtain true black to white.

We conveniently have the work of Ansel Adams to refer to. He has broken down his zone system into 10 zones, Zone I being the maximum black that any paper can produce and Zone X the pure white of unexposed paper. He suggests that the high value detail in a print be easily discernable and that we place it in zone VII. Easily discernable shadow detail is placed in zone III. Quite conveniently .75 happens to be the density range between zones III and VII.

A densitometer has a dual function in that you can use it to measure densities on film as well those reflecting off of a print. Used as a reflection measuring devise, again, .30 equates with one stop of change in the light being reflected off of a print. That is, if you “zero” the densitometer to display .00 on the white of the paper base, then .30 would indicate ½ of the amount of light is being reflected back. In fact .00 means that if 100 units of light is falling on the paper then all 100 units is reflecting off of the print. The white of a print never reflects an equal amount of light that is falling on it and so normally .07 is used to indicate the white of any paper base.


When measuring a print, the densitometer can easily distinguish differences in the dark steps. Reflection densities of 1.90 and up become difficult for the human eye to distinguish, but the densitometer will see distinct steps from 1.90 to 2.10 to 2.2 to 2.4. Now, if we find the corresponding steps on the 21 step tablet that produced this range of density we find that if we want to produce a true white to maximum black on the print we would need an overall density range of 1.65. That is, we would need to expose and process a full 11 half steps. These are how many steps we can measure from the print made with the 21 step-tablet. But we need to keep the important detail within the middle ranges that cover a density range of .75. That’s a tall order.

I am not proposing that all black and white prints need to cover a reflectance range of .07 to 2.40. Many successful prints do not. I am just saying that what you’ll need in order to get a true black to white on a print (paper grade 2) is a density range of at least 1.65. I will add that 2.40 on a print is only achievable when you take a print with a maximum density of 2.20 and selenium tone it to 2.40. Black and white prints won’t achieve that dense of a black otherwise. On the other hand a print that has a 1.90 black can be selenium toned to achieve a 2.10 black. And I have found that a toned print with a 2.40 black when placed in a window facing western light for a period of one month will still retain a 2.10 black. Un-toned prints with a 2.20 black will fade to 1.90 in the same period. So having a true maximum black that is toned will yield you a more archival print.


Brief Review of The Zone System

At this point I will apologize to those who find the following overview unnecessary. The Zone System is, by now, a standard procedure that many have studied and mastered and rightly so. It is the best method to learn in order to have a better understanding of the science of photography, and it is most helpful to review it before we venture into film masking itself.

As mentioned, Ansel Adams’ Zone System is a method whereby you measure the camera’s subject matter in terms of “zones” and through exposure and development you place the important detail within zones III and VII. The system helps to teach photographers how to visualize their images prior to the making of an exposure. Camera meters measure the amount of reflectance coming off of a subject, and it calculates an exposure to reproduce a middle gray (zone V). So, for example, if you fill your camera’s frame with a white towel, the camera’s meter will recommend an exposure to produce middle gray. Therefore it is recommended in the zone system to use a hand held meter or a spot meter for measuring subject matter. In this manner you can measure different areas of the photographic point of interest and “place” those areas into the zones of your choice.

It should be clear that when a numeric value is given by any metering devise that that value is only meaningful when used as a reference against other values. That is, if a towel reflects a value of “8” on a light meter it can be placed in zone VII relative to another area in the photograph that has a reflectance value of “6.” Let’s say that second item is a kitchen stove. One would expose according to the meter’s suggestion for the stove placing it in the middle gray step of the zone system. The towel, which reflects 2 stops more light, will then photograph as a lighter shade of gray (zone VII). Likewise a third item, let’s say the area under a chair, indicates a reflection value of “4” on the meter. That area will be recorded as zone III since it reflects 2 stops less light than the stove. Given “normal” development all the detail from under the chair to the towel will be recorded and readily seen when a print is made from that negative.

Camera films come with a numeric value (ISO) indicating the speed, or sensitivity of the emulsion. Great importance is given to the ISO in the Zone System and testing procedures are outlined in determining the actual ISO of any given type of film. The manner is which the ISO is determined in the Zone System is quite different than the ones film manufacturers use. The Zone System, through testing, finds the minimum exposure required to produce a minimum recording of density after processing. Film manufacturers establish their ISO based on how much exposure is required in order to achieve a certain density range given specific amounts of exposure. To top it off chemical manufacturing companies establish their own the time and temperature specifications for the processing of films. It all becomes a bit confusing and it can take quite a bit of time and effort to fine tune the ISO of any given film.

What I will propose here is to perform a single test with your camera and choice of film and developer. First, set up a scene on an overcast day where the lighting will remain stable for a good period of time. Using objects of different shades set them before the camera. Use black velvet for the blackest area in the scene. Take a black piece of thick cardboard and cut a rectangle in the center. Place a bright light bulb behind it; if possible use a daylight-balanced bulb. The light should shine directly at the cameras. Then use towels, garbage cans, etc. to create a large range of tones. When you set up your camera make sure that these areas will be large enough to be measured on a densitometer once the film is processed. A densitometer’s aperture is normally 3 millimeters in diameter so make sure the gray area on the processed film will be at least that large. In case you are photographing with a 35 mm camera shoot sections of the scene to ensure that the areas remain large enough on the film. This may require that you place the camera on a tripod and pan it to photograph the whole area. In this case you may even need 2 or 3 exposures in order to cover the scene.

Hopefully you will have plenty of objects creating a good range of tones. I would suggest crumpled aluminum to render the high value detail of zone VIII and IX. When you meter the scene use a rough drawing of the set up and write down the reflecting densities given by the meter. It will be a good idea to place a neutral gray card in the center. Most likely this will provide you with the zone V reading and basic exposure. Note the object with 2 stops more reflectance. It will provide the zone VII area. Also note which object will provide the detail in zone III. These 3 areas will be your main concern and so make sure you will be able to measure them on a densitometer. The light bulb should be placed far enough away so that no flare from the bulb will go into other areas and affect your measurements from the densitometer.

The black velvet and light bulb will provide the maximum and minimum densities on your processed films. These are usually not easy to measure, especially the maximum densities on your film. These are usually highlights (the reflectance off of shiny objects or the sun coming through clouds). They are usually too small to measure accurately on a densitometer. Most likely you may not have a densitometer but your local photo lab or school may have one for you to use.

After you have set up your test area make at least 8 identical exposures. If exposing 4”x5” films then you can simply expose 8 sheets. Expose a whole roll with the same exposure if you have a medium format camera. 35mm cameras pose a different challenge since the film is so small you may have to shoot 2 or 3 frames to cover the area. Again expose the whole roll with the same exposure, but you’ll have to pan the camera after each section is exposed.

When you process the film be prepared with a pair of scissors so that you can snip off sections of the roll of film after a given period of processing time. I would suggest that you use a temperature you can easily maintain, say 68 degrees. Then after 4 minutes snip off a length of film that you know will include the whole picture area. Give that piece of film a single slit in the corner with the scissor and place it into the stop bath. Snip off another piece after another minute of processing and slit that piece twice. After another minute and a half you can snip off another piece and give it 3 slits. Do this until all the film is processed. Try to cover from 4 minutes to 20 minutes. At the end of the processing you will have varied the processing time to produce a number of different films with various densities on them. I would suggest the following times: 4 mins, 5 mins, 6.5 mins, 8 mins, 10 mins, 12 mins, 15 mins, and 20 mins.

After the film is dry you can measure the zone VII area as well as the zone III area to find its density range. The one that is closest to .75 will be your normal processing time. Looking at the scene determine which area represented zone VIII. Find the negative that has a density range of .75 when zone eight subtracting zone III produces .75. That processing time will be your normal minus one standard. Likewise find the area that the meter determined to be zone VI and find the negative where zone VI is dense enough to measure .75 above zone three. This processing time will be your normal plus one standard. You can interpolate the data to find accurate estimated times if your data falls in between the desired .75.

Now if you print these negatives you will find that the “normal” processed negative will produce the most pleasing print. The normal plus negatives will produce prints that will have too much contrast. The zones above VI will look too light, but VI will contain a good amount of detail. Likewise the normal minus processed negatives will produce prints that will look flat or low in contrast. The exercise is not necessarily to make prints. It is to allow you to determine how to control the values in the negative that you process. It also helps you to learn to predetermine how a photograph will look. The negatives will serve a good purpose for film masking though. That is another matter.

The black velvet will serve as the minimum density and the lighted bulb will serve as the maximum density on your negative. See if you achieve an overall density range of at least 1.65 between those two areas. If not, then you may not achieve a true black to white on the print. Any larger number will only insure that you achieve true black to white. The math is straightforward.

I realize that this is a short-cut method to processing films in the manner suggested by the zone system, but it simulates a real world situation. If you find yourself photographing on a foggy day you just might wind up measuring the lightest area of fog and placing it in zone V but processing it up to a density equal to zone VIII (and it taking a full 20 minutes or more in the developer). The resulting print will appear unlike the actual scene on the day of the photographing, but it might come out as you want it to.


This article will be continued next week in part 2.


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