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Exposure

An understanding of how to control exposure is probably the single most important technical facet of theatre photography. An exposure meter reading can give a rough clue as to the average light levels of a scene but the photographer then has to interpret those readings.

For any given scene there is an optimal exposure. Sufficient light needs to reach the camera film or sensor to record an image of acceptable quality. Too little light (the more likely case with productions here at Warwick) and the image is under exposed. Large areas of muddy grey or black will dominate, with little detail discernible in the murk, and few bright areas. Too much light and the image is over exposed. Light areas, for example faces and skin tones, are rendered as bright white. Again with little or no detail discernible in the incandescent bright areas of the image. Usually we want to aim for a correctly exposed image, where detail can be distinguished in the shadows and highlights.

Most cameras, above the point and shoot level, allow us to control exposure by altering the amount of light falling on the film/sensor. Broadly we can do this in one of two ways. By varying the aperture (or f-number or f-stop) or by altering the shutter speed. The aperture is the hole through which the light, reflected from the subject, must pass to reach the film/sensor. It is almost always integrated into the lens mechanism. As should be self evident, the bigger the hole the more light is allowed to pass. The smaller the aperture, the less light is allowed to pass.

The second way to control exposure is with the shutter. This is effectively the doorway to the film/sensor. It works by varying the time available for the light reflected from the subject to pass through. A slow shutter speed holds the doorway open for a long time. A fast shutter speed holds the doorway open very briefly.

Numbers

Aperture values and shutter speeds have sequences of numbers associated with them, that apply to all cameras.

A typical range of shutter speeds is like this:

1 2 4 8 15 30 60 125 250 500 1000 2000

A typical range of f-stops is like this:

1.4 2 2.8 4 5.6 8 11 16 22

The shutter speed range above is from 1 second to 1/2000. So 8 is 1/8 of a second, 125 is 1/125 of a second, and so on. The larger the number the faster the shutter speed and the shorter the time that the light has to pass through to the film/sensor. With each increase in shutter speed the time the shutter is held open is halved. i.e. a change from 125 to 250 halves the time the shutter is held open. A change from 125 to 60 (roughly) doubles the time the shutter is held open.

The aperture values or range of f-stops is slightly more complicated. An f-number is not an absolute value in units such as millimetres, but is a ratio. It is the focal length of the lens divided by the size of the aperture. The larger the f-number the smaller the physical size of the aperture and so less light can pass through to the film/sensor. With each increase in f-stop the light gathering area is halved. i.e. a change from f8 to f11 gives half the light gathering area. A change from f8 to f5.6 doubles the light gathering area.

So the two scales are linked. A change of one f-stop value is exactly equivalent to a change of one shutter speed value. By varying f-stop and shutter speed combinations the camera may be set to give the optimal exposure. This, as was stated at the very beginning, is usually guided by some type of exposure metering. Because the f-stop and shutter speeds are linked there are several combinations of f-stop and shutter speed that will give exactly the same optimal exposure. This is known as reciprocity.

Here's an example. An exposure meter might give a reading for an exposure of f5.6 at 1/60 of a second. However, all of the combinations in the table below will deliver exactly the same exposure.

1.4 2 2.8 4 5.6 8 11 16 22
1000 500 250 125 60 30 15 8 4


F-stops Vs. Shutter speed

So, we have two separate but linked means to control the amount of light falling upon the film/sensor. How can we best vary the combinations for different occasions?

If there is a lot of swift action of stage then we need to use a fast shutter speed to freeze the movement. From the exercises undertaken in Workshop One you should have a good idea of how action is frozen at a range of different shutter speeds. The faster the action then generally the faster the shutter speed needed to freeze it. This is simply because in 1/15 second a fast moving element (an actor's arm and sword is a duel scene, for example) will have travelled four times as far as it will in 1/250 second. Sometimes we want a degree of motion blur to impart the impression of action, but generally not so the whole subject is completely blurred.

Another aspect of faster shutter speeds centres on camera shake. With longer focal length lenses and slow shutter speeds there is a much greater likelihood of registering vibration and unintentional camera movement. A faster shutter speed can help reduce the effects of this. The use of a camera brace, monopod or tripod can be essential when using longer lenses for dimmer productions, in which it is just not possible to set a fast shutter speed.

The most important use of varying the f-stop is to control depth of field. This simply is the range between the nearest and furthest acceptably sharp elements in the image, when the camera is focused on a primary subject. Depth of field also varies with lens focal length, distance to the subject and the format size of the film/sensor in the camera being used.

Put simply if you have an actor downstage and another upstage behind her, then you will need much greater depth of field (and thus a smaller f-stop) than if the actors are closer together at the front of the stage.

Here is an example: If I use a telephoto lens of 70mm focal length on my Nikon D700 focused at 5m at f11 then I can expect a depth of field of around 3.8 metres stretching from 3.7m to 7.5m. If I open up the f-stop value to its maximum of f2.8 then the total depth of field is reduced to 0.8m, stretching from 4.6m to 5.4m.

NB the camera is focused at 5m and the depth of field extends both in front of and behind this distance, so sometimes we have to focus in between two actors to get both acceptably sharp. This can at first be counter intuitive, as neither will necessarily appear sharp in the viewfinder, unless the lens is stopped down to the actual taking aperture.

Remember that every time you set a specific shutter speed or f-number to achieve a required outcome then correct exposure requires that you make a reciprocal change to the other, i.e. a small aperture will require a slower shutter speed, a fast shutter speed will require a wider aperture.

ISO sensitivity

Can anything be done if you need a fast shutter speed and a narrow aperture? There is one final variable we can adjust to try and get correct exposure. That is to increase the ISO rating of the film or sensor.

The ISO rating (formerly also known as ASA or DIN) is a measure of the light sensitivity of the film or sensor. These are typical ISO values:

25 50 100 200 400 800 1600 3200 6400 102400

A doubling of the value is a doubling of the sensitivity or the need for half as much light to register a correct exposure. However, you don't get something for nothing. In the case of film a high ISO film (or push-processing a slower film to a higher ISO value) will typically have a much grainier appearance, in which the light sensitive elements of the film start to become apparent in the final image.

In the case of a digital photography the deleterious effect is known as noise, and is an artifact of the electronic functions of the light sensor. As a general rule, larger sensors, with greater space between the light sensitive 'cells' can be set to higher ISO values than smaller sensors where the light sensitive cells are spaced closer together.

Broadly therefore, full frame (FX) and 2/3 frame (DX) DSLRs will produce less noise at higher ISO values than a compact camera, that may struggle with ISO values of 800 and above.

In order to deliver as high quality images as possible the rule is to use the lowest possible ISO value that provides the combination of fastest shutter speed and smallest aperture that are required to get the shot. This can vary throughout a production. So you should be prepared to adjust the ISO value along with the shutter speed and f-number as the lighting on stage changes from scene to scene.