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Module 4 — Exposure

[Note from the editor: since the millennium, films cameras are rapidly moving towards obsolesence, what with digital cameras having (at least) matched them in quality, and gone way beyond them in speed and convenience. With digital cameras (we recommend one with exchangable lenses) not only is the light meter usually right, you can also check the result on the screen as you are shooting.]

Module Objectives

By the end of this module, you will be able to:

  1. Explain the relationship between film speed, f-stop and shutter speed in creating a properly exposed photograph
  2. Determine the right tool to use to meter a scene
  3. Distinguish between the correct exposure for print vs. slide film
  4. Calculate the correct camera settings based on the meter reading



Although today’s cameras come with sophisticated built in metering systems, you must understand how a meter "sees" light in order to compensate for anything other than average conditions. This module will explain what a meter is telling you when it indicates what settings you should be using. Understanding this concept will also help you determine why your perfect shot comes back from the lab looking less than perfect, since the lab will also assume your shot was made under average conditions.

First, you need to understand what happens to light when you take a photograph.

  1. Light strikes your subject
  2. Depending on the reflectivity of your subject, a percentage of the light that strikes it is reflected towards the camera
  3. The light goes through the adjustable lens opening
  4. The light strikes the film through the shutter, which opens for a period of time determined by the photographer.

Once again, understanding one step in this process sets the professionals apart from the amateurs. In this case, step 2 is the secret. Being able to interpret your meter reading based on your analysis of the reflectivity of your subject will make the difference between a poorly exposed picture and a perfectly exposed one. We’ll describe how to develop that skill later. But first, we need to define some of the terms used in this process.

Film Speed, F-stops, and Shutter Speeds

As light reflects off your subject it passes through the lens of your camera, through the open shutter, and strikes your film. Adjusting the settings on your camera involves understanding the relationship between all four of these components: light, lens opening, shutter speed, and film speed.

Film Speed

All film has a precise, built in sensitivity to light, or speed, expressed as an ASA or ISO number. Very slow film might have an ISO 24 rating, while extremely fast film might have an ISO 1600 rating. Why wouldn’t you choose the fastest film available? Well, the faster the film, the grainier it is, because the more sensitive a film is, the bigger the grain must be. Today’s films have much finer grain than those of even 10 years ago, so an ISO 400 film gives very acceptable results even in fairly large prints. For DOMAI type photos, I’d suggest an ISO 100 or 200 for outdoor work, and ISO 400 for your indoor work. If your photos will primarily be exhibited on a computer, and you are working with existing light, you may even go to an ISO 800 film.


As the light passes through your lens, a diaphragm allows a certain percentage of that light to reach the shutter, depending on how wide the diaphragm is opened. The size of the opening is expressed as an f-stop, and depending on the "speed" of the lens, can be as big as f/2 or as small as f/22. This can be a little confusing, since the bigger number denotes a smaller lens opening. As with film speed, you might ask why you wouldn’t always use a wide opening. And as with film speed, there is a tradeoff. The larger the lens opening, the narrower the depth of field, or the narrower the range of distance that the photo is in focus. When you’re doing portrait work, this frequently isn’t a problem. Since the subject is the face and figure of a person, you probably don’t want a lot of distracting background detail, so throwing it out of focus is a good way to minimize its effect. But sometimes you do want background detail, so a smaller lens opening might be required.

Shutter Speed

Before the light can expose your film, the shutter must open to allow it to expose the negative for a precise amount of time. In the early days of photography, this time would be expressed in seconds or minutes. Having your portrait taken was almost like having a picture painted; you might have to hold the pose for a long time! Today’s film, however, can be properly exposed in a fraction of a second. In fact if the subject is in bright sunlight, an exposure time of 1/1000 of a second or less is not uncommon.

Tying it All Together

When you set your camera to take a picture, your meter makes a mathematical calculation which ties all four components together. The amount of light reflecting off the subject can be measured, the speed of the film is known. So the last variables that you usually select will be the lens opening and shutter speed. We’ll show you how to figure that all out later.

The last concept you need to understand is how the ratios work between all these variables. Without getting into a lot of technical detail, photographers talk about these ratios as stops. An exposure setting which allows twice the light to strike the film is referred to as having an extra stop. ISO 200 film is one stop more sensitive than ISO 100 film. A shutter speed of 1/250 of a second is one stop shorter than a speed of 1/125 of a second. Lens openings are a little different, since you’re dealing with the area of a circle, but a lens opening of f/2 means that the lens opening is half of the focal lenght. Typical settings are f/2.0, f/2.8, f/4.0, f/5.6, f/8, f/11, f/16 and f/22, with each of these setting being one stop smaller, meaning half as much light.

Determining the Right Tool to Use

If you are taking a picture of a scene that has an average amount of light being reflected from it, your through-the-lens metering system will do a great job. Steps 1, 3, and 4 in our process are purely mathematical, can be performed perfectly by any automatic camera, and will, under perfectly average conditions, yield a perfectly exposed negative. The number of poorly exposed pictures in the world is proof that there are lots of situations that don’t fall into the category of "perfectly average conditions."

What is "Average?"

Meters assume that the light falling on them has been reflected by a scene that is18% gray, i.e. 18% of the light falling on the scene has been reflected. Why 18%? Simple. 18% gray happens to be half way between pure white and pure black. (By the way, it doesn’t matter whether it’s really gray, or a combination of colors, it still works out the same.) Interestingly enough, this works really well outdoors on a sunny summer day when you’re shooting a large landscape. Nature happens to be nicely balanced that way. However, it doesn’t work when a dark-skinned beauty fills 80% of your frame, or your model has decided to pose outdoors in the snow.

What do you do when your scene isn’t average?

Fortunately, there are three solutions to a scene that doesn’t reflect 18% of the light that falls on it.

  1. Take an educated guess.
  2. Use a gray card
  3. Use an incident light meter

The Educated Guess

While the first option may seem rather unprofessional, it’s actually the basis of a very sophisticated, extremely successful system developed by a famous photographer, Ansel Adams. What Mr. Adams did was split reflectivity into 10 zones, and assigned a zone to the amount of light reflected by some common objects. Without going into a lot of detail, Mr. Adams was able to carefully meter a scene, from the darkest patch to the lightest, and expose and print his photographs so that the detail in all those areas was captured perfectly. You will learn about a simplified version of his technique in the section Calculating the Correct Camera Settings later in this module.

Using a Gray Card

A gray card is a rectangular card that may be purchased at most photo stores. It usually has one side that is pure white, and the other side is 18% gray. The way you use it is to take a meter reading of the gray side of the card in the same light that falls on your subject. If your subject is a person, simply have him or her hold the card facing the camera, stand close enough to the card so that it fills your frame (without casting a shadow on the card!), and meter the light reflected by the card. The reason this works is that the card is reflecting the same amount of light that your meter is expecting, providing an "average" reflectance. If you’re metering a very fair-skinned girl, your meter assumes that she is reflecting 18% of the light hitting her, so everything will come out very dark. Similarly with a dark-skinned subject, the meter will assume she is reflecting 18% of the light, so everything will be overexposed. With the gray card, the light being reflected is the same as the meter’s expectation, so everything comes out right.

Using an Incident Light Meter

An incident light meter works virtually the same way your camera’s light meter works, with one key difference. It measures the amount of light falling on the meter, rather than the amount of light reflected by the subject. The reason this works better in some situations is that it doesn’t matter whether the subject reflects 18% of the light falling onto it or not, it will give a correct reading regardless of the subjects reflectivity.


Print vs. Slide Film, Black and White vs. Colour

As we’ve seen before, the camera and film are far more limited in what they see compared to the human eye. We may look on a scene with deep shadows and brilliant highlights and barely notice the difference. Film, however, sees a much narrower range light, and we need to be aware of this, and expose properly so we see the detail we want.

Black and White Film

One reason professionals prefer Black and White (or B/W) film is that its latitude is far greater than colour. B/W film might have a latitude of 7 stops, while colour print film is closer to 3 stops and colour slide film is about 2 stops.

Does this mean that if you have a scene with 7 stops between the darkest and lightest areas, you should pick the middle and expose for that? Unfortunately, it’s not quite that simple. We’ll discuss this more later, but print film (whether B/W or colour) has less latitude for shadows. B/W film might have 2 stops latitude below your exposure and 5 stops above. We’ll see how to expose for this shortly.

Colour Print Film

As just discussed, colour print film usually has about three stops of latitude. In its case, this gives it 1 stop latitude for underexposure and 2 stops for overexposure.

Colour Slide Film

Slide film has the narrowest latitude of all, but is more forgiving for underexposure than overexposure. It will forgive about half a stop for overexposure, and 1 _ stops for underexposure.


Calculating the Correct Camera Settings

OK, so we’re finally ready to set the camera settings. We have a beautiful girl, a sunny day, ISO 200 B/W film, and our 35 mm camera with built in meter. As you’ve learned in the module on Lighting, you position your subject so that she is in open shade, under a large tree or at least facing away from the sun.

If you simply point your camera at this scene and let it calculate your exposure, your subject will be very dark since there will be a lot of light behind her which will fool your camera into stopping down. So you might be tempted to use your gray card and measure the light in the shadow of the tree. Now, if you set your camera based on this exposure, the bright sunlight will wash out your background.

First, you should meter the light falling on the girl. Let’s say it specifies a shutter speed of 1/125 and a lens opening of f/2. Now meter the sunlit portion of your scene. The meter suggests a shutter speed of 1/125 at f/22. That’s a 7 stop difference, which is the same as your film’s latitude. So where do we set the lens opening? At f/4. Why? With print film we expose for the shadows. In this case, the film has two stops latitude for underexposure. Since our darkest shadow in this scene is f/2, we will set our camera to f/4, which is two stops less than f/2. That means that the shadows at f/2 will be recorded, but the highlights at f/22, which is 5 stops less than f/4, will also be captured.

For those of you who don’t already understand f/stops and the mathematics behind all of this, don’t worry. Just remember, when you are taking a photograph of a scene with a wide range of highlights and shadows, and if you’re using colour print film, to meter the darkest part of the picture in which you want to see detail, then close down the lens about on or two stops. If you’re using slide film, meter the highlights of your scene and open up the lens about one stop.

As with anything in photography, practice these techniques, and eventually they’ll become easy, even second nature. And if you’re not sure, take a few shots with different exposures. Make a note of the exposure you used for each shot, and next time you’ll be that much more confident.


In this module, you’ve learned two more key elements of success that will set you apart from the amateur:

  1. Interpret your meter reading based on the reflectivity of your subject.
  2. Adjust your settings in a high-contrast scene to properly expose for shadow detail while not washing out the highlights in your picture.

You’re now ready to take some great pictures.

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