Exposure 102

In Exposure 101, we learned how film speed, shutter speed, aperture, and light intensity work together for proper exposure and how your camera’s metering system evaluate them. If you have not read Exposure 101, please do so now.

Let’s take this one huge step further and get into the guts of exposure. Let’s talk about "tonality". Tonality refers to the brightness of an object or part of an object.

A scene will almost always have a fairly wide range of tonality values in it. This range can vary tremendously and, in extreme cases, can be many many stops wide, from the blackest black of deep space to as bright as the center of the sun. Your eye, however, can only see a range of light values about 14 stops wide. Actually, 14 stops is pretty miraculous when you do the math. A 14 stop range means you can see detail in two objects at the same time even when one of the objects is over 16,000 times brighter than the other object (2 raised to the power of 14). Film, however, is only capable of capturing a fraction of this tonality range. Slide film is typically capable of capturing a 5 stop tonality range while print film is capable of a 7 stop range. This range is what we call a film’s "latitude".

Let’s look at this graphically for an extremely high contrast black-and-white scene. We can do this in black-and-white because tonality refers to "brightness", not color. The actual range of tonalities in a scene might be visualized as such:

However, our eye can only see detail in a 14 stop wide portion of this scene. What your eye sees if it is focused on the middle tone of the scene above might look like this

Everything in the scene to the left or right of this range will show no detail to your eye and will appear pure black or pure white. What the film records, however, is even less than what your eye sees. If it is exposed based on the middle tone of what you see, it will record this 5 stop range

As you can see in the above illustration, there are tones that your eye can see that are outside the limits of what the film can record, and these tones will appear as pure black or pure white with no detail on the film.

Let's take these 3 illustrations and stack them for a moment to discuss one additional effect. The top bar represents the actual tones in the scene, the middle bar those tones seen by the eye with detail, and the bottom bar those tones captured on film with detail.

Look what happens to an object just a little darker than middle tone represented with the "x" above. Notice that if you drop down from the top bar to the middle bar, the object appears slightly darker to your eye (you're computer may not show this, however, depending on your monitor's settings). And that if you drop down vertically to the bottom bar, the object will appear quite darker when captured on film. In effect, the tones in the scene get compressed into narrower bands of tonality for your eye and even narrower bands for the film with the tones at the extremes defaulting to pure black or pure white. Also note that what your eye might see as "a little darker" than middle tone, the film might record as even darker than "a little darker" than middle tone.

It is very important to understand all this and the illustration I provide above is the best way I can think of doing this.

Now, it is also important to realize that, while the range of tonalities your eye and your film can see are narrower than what really exists, you actually do have some control over which portion of the range you want to see and which portion of the range you want to capture on film. Let’s start with what your eye sees.

In the fairly high contrast scene above, your eye will automatically dilate to a certain pupil size depending on what portion of the scene your eye is focused on. This pupil size will determine which parts of the scene your eye will see details in and which parts of the scene will appear completely washed out white or totally black. But you could adjust this by putting on sunglasses for example. Now, some of the stuff that was too bright to see before has been darkened somewhat by the sunglasses and is now visible. Likewise, some of the stuff that was so dark that you could barely see it has now become too dark to see at all and now appears completely black to the eye. In effect, the range of tonalities your eye can see has shifted to the right. For example, you can now see more detail in the snow that was formerly too bright to look at but have lost detail in really dark shadows

In this illustration, I have shifted the "eye with sunglasses" scale, say, 2 stops to the right, which would correlate to sunglasses that only allowed 25% of the light to pass through them (1 stop equals half the light passing through, 2 stops equals a quarter of the light passing through).

How can we apply this to film, which sees even a much narrower band of tonalities. As it turns out, we have much more control over exposure to our film that we do to our own eyes. As a matter of fact, you will see later that we can actually capture things on film that we cannot even see with our own eyes.

The primary ways of doing this are with the selection of shutter speed and aperture. Every one stop change in shutter speed or aperture results in a one stop shift in the range of tonalities captured on film. In effect, each one stop change in shutter speed or aperture moves the film range of tonalities one mark to the left or right in my illustration and selects a different range of tonalities in the scene to appear on the film. You can shift this range, for example, by about 36 stops by adjusting your shutter speed and aperture from f22 at 1/1000 sec all the way to f2.8 at 8 hours (at which time your camera’s battery may go dead from holding the shutter open so long). That’s tremendous flexibility.

So, how do you apply all this theory?

The first thing you must understand is the concept of "middle tone". An object is said to be middle toned when it’s reflects about 18% of the light that strikes it. Examples of middle toned objects would be the green of freshly cut grass or a red ripe tomato. Snow, on the other hand, reflects much more than 18% of the light striking it and would be brighter than middle tone. The dark fur of a buffalo would be dimmer than middle tone.

When you point your camera meter at something, the meter is not capable of recognizing whether the object is really middle toned or not. The meter doesn't know if you are pointing it at green grass, a ripe tomato, a buffalo, or snow. All the meter does is measure how much light is reflecting off of the subject and calculate an exposure setting that would make it appear medium toned on film. The meter is programmed to assume that all objects are middle toned and it will assume that you want the object to appear middle toned in the final image and it gives you a shutter speed and aperture that accomplish this.

For example, suppose the scene’s tonal range looks like the illustration below and you point the meter at an object in this scene with a tonality marked with the ‘X’. Your meter will help you make this object appear middle tone as seen by the film as shown below.

Notice that the tonality range captured on film by the camera (lower bar) has shifted to the right such that the middle tone of the film’s range is now aligned with the ‘X’. The camera’s meter will help you select a shutter speed and aperture that achieves this and the object you have metered on will appear neutral in tonality. In other words, it would appear gray or middle orange or middle blue or middle green, depending on what color the object is (the meter doesn’t read the color of the reflected light, it just reads the tonality, or brightness, of the reflected light). This means that a subject that appears middle green to your eye would appear middle green on film. Unfortunately, it also means that a subject that appears white to your eye, such as snow, would appear middle gray on film or a subject that appears very dark to your eye, such as a buffalo, would appear gray on film because your meter has shifted the exposure scale to do this. We’ll learn how to deal with this problem using "Exposure Compensation" in Exposure 103.

Let’s look at a little more realistic example. Let’s take a typical scene as your eye sees it and play with this concept. Take the highly simulated scene illustrated below (Note: the effectiveness of this example may be influenced by the settings of your computer monitor).

In this scene, you see a green meadow, a snowy mountain, some dark green trees with brown trunks. The trees are casting a shadow across the meadow and a deer is standing in the shadow. It’s a blue sky day with a white puffy cloud in the sky. Notice that your eye can see detail in the meadow, snowy mountain, trees, deer, and cloud. This is how your eye might see the scene.

However, this is not what the film will capture. What the film will capture depends upon what object you meter on and on what tonal value you assign that object. These are two decisions you must make. You could put your camera on Program mode and let it try to make these decisions but you will then have no control over the result.

If photographing this overall scene, you would probably want the meadow to appear middle tone since the meadow dominates the scene and a green meadow would naturally appear middle tone to your eye. But which part of the meadow should you meter on? The part in the sun or the part in the shade? Since you are trying to capture the entire scene, let’s first try the meadow in the sun. So you will "meter on the meadow in the sun and set it to middle tone".

Let’s take a closer look at this key phrase. The key phrase is "meter on [x] and set it to [y] tone", where [x] is the object and [y] is a tonal value. In this case, the meadow in the sun is the object and you have decided to make this object middle toned on your film. Let's say the meter gave us 1/125 second for shutter speed and f16 for aperture. Here's what the resulting picture might look like on film:

Well, the sunny meadow looks great. But the deer and tree trunks appear "nearly black", the shaded meadow appears "very dark", the sky "a little light", and the snowy mountain and cloud "nearly washed out white". Not a pleasing picture. The problem is that the scene is too high contrast for the slide film to record all of the details in both the dark shade and in the bright snow/clouds, even though your eye can see details there (compare to the first color illustration above). Remember that slide film can only capture a range of light tones about 5 stops wide. Your eye and your film do not see alike and this scene is just too high contrast. This is common when shooting in mid-day sun and is why most landscape photography is NOT done in mid-day sun.

Let's take a closer look at this scene's contrast levels. Let's take some experimental meter readings on various parts of the scene. Let’s leave the aperture set at f16 and let the meter tell us what the exposure would be for other objects in the scene. Let’s say we meter on the shaded part of the meadow and the meter says 1/45 second at f16. That’s 1.5 stops darker than the sunny meadow. We give this the nomenclature -1.5 stop. Let’s say we meter on the brown deer in the shade. We get 1/30 sec at f16, or 2 stops (-2 stops) less bright than the sunny meadow. Continuing, we find the tree trunks also 2 stops less bright (-2 stops) than the sunny meadow, the tree foliage 1.5 stops darker (-1.5 stops) than the sunny meadow, the sky .5 stops brighter (+.5) and the cloud and snowy mountain both 2 stops brighter (+2 stops) than the meadow. These tone values can be illustrated as:

It is important to note that these tonal values are relative to the sunny meadow being assigned middle tone (0).

So what are our options?

What if we metered on the shaded meadow and set that at middle tone? Relative to the shaded meadow, the mountain and cloud are 3.5 stops brighter, the blue sky 2 stops brighter, the tree trunks and deer .5 stops darker and the trees the same as the shaded meadow. These tone values can be illustrated as:

with a resulting slide looking like

The mountain and cloud are now totally washed out, the sky too bright, and the sunny meadow too bright, but the deer, shaded meadow, and trees look good. Still not a pleasing picture. Still too much contrast in this scene.

What else can we do with this scene? Not a whole lot short of waiting for the sky to cloud up and shade the mountain, sunny part of the meadow, and sky.

Or, we could focus on a smaller part of the scene. Let's take out our 300mm lens and zoom in on the deer in the shade (i.e. crop out the sun-lit stuff and only shoot the shaded stuff. If we again meter on the shaded meadow and set that to middle tone, this is what we get:

Looks good (for a cheap simulation). This is a much lower contrast scene and is within the film’s latitude because we've cropped out all the bright stuff in the scene. It’s OK for the deer to be a little dark because deer are a little dark in reality (darker relative to middle tone). We’ll make a print of this one.

So what do you do in bright, sunny mid-day light? You can either work in the sun or work in the shadows. But not both.

Take a look at my Exposure 103 article take the next very important lesson in Exposure, which is dealing with objects that are not middle tone.