Exposure

Photography is the art of capturing light and shadow, on film or digitally.

Exposure is the amount of light hitting the recording medium (film or sensor).

You always aim for the perfect exposure (not over or under exposed), but how do you know what the perfect exposure is, and how do you get it?

It’s a matter of:

1. reading how much light is on your subject
2. adjusting aperture, shutter speed and ISO to expose perfectly in the available light

Settings that Affect Exposure: ISO, Aperture and Shutter Speed

1. ISO/ASA – Sensitivity

The ISO of a film means how sensitive it is to light – the higher the ISO, the more sensitive. Most digital cameras also have an ISO setting.

ISO	64, 100	200, 400	800, 1600, 3200
Sensitivity	low	medium	High (“fast”)
Advantages	Clear, non-grainy pictures	Still fairly clear while more sensitive	Can shoot in low light or at high shutter speed
Disadvantages	Need more light	neither super clear nor super fast	grainy film, “noisy” digital shots
Uses	Daylight, shots for printing in high-res	Exposes well in a variety of conditions	Indoors, low light conditions; high shutter speeds (e.g. sports)

2. Aperture

The aperture is the opening in the lens. A wide or large aperture lets more light in than a narrow/small aperture.

As you adjust the aperture, you’re actually dilating an iris, exactly like the human eye.

Small aperture f/32: iris almost closed	Wider aperture f/11: iris open wide (this is a very old camera – modern lenses have lower f-stops)

The aperture is measured in f-stops (technically the ratio of the focal length to the opening).

A low f-stop number, like f/2.8, is a wide aperture, while f/16 and f/22 are narrow apertures, letting in much less light.

Each f-stop increase lets in half the amount of light as you go through a typical series: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22,32

You either set the aperture manually, using the aperture ring on the lens, or electronically. Automatic modes let the camera set the aperture for you, but often the camera will tell you what aperture it chose. This camera is set manually to f/11, but since it’s in an auto mode (“Pict”), it is automatically choosing f/4.0

Changing the aperture mainly controls the amount of light let in, but also affects Depth-of-Field.

3. Shutter Speed

When you take a photo, a little curtain called a shutter briefly opens, letting light from the lens onto the medium (the film or sensor at the back of the camera).

The longer the shutter stays open, the more light hits the medium.

Shutter speed is measured in fractions of a second like 1/60^th , but usually the numerator is dropped, so a shutter speed of 500 means 1/500^th of a second.

In darker conditions, as well as opening your aperture wide, you have to use a slower shutter speed.

At slower speeds (below 60) you risk getting a blur from camera shake or your subject moving. Action shots and telephoto lenses require faster shutter speeds, like 250 or even 1000. Good photographers sometimes use motion blur creatively.

Shutter speed is set either automatically, manually using a dial on top of older SLRs, or electronically in modern cameras.

The dial will have a series of numbers such as 1,2,4,8,16,30,60,125,250,500 etc., from 1 to 1/500^th of a second. The B setting keeps the shutter open as long as you hold the shutter release button.

Each increase in the shutter speed on a standard dial doubles the speed, therefore halving the amount of light let in.

The shutter on this film camera prevents light from reaching the film. It opens like a curtain, but at very high and precise speeds.

Getting the Right Exposure

As stated, exposure is controlled by ISO, aperture and shutter speed. For a roll of film, the ISO is fixed, so you usually only adjust the shutter and aperture.

In good light, you usually set a fast shutter speed like 125, and then adjust the aperture to match the light conditions.

With low light, you open the aperture as wide as possible, then slow the shutter speed down until the shutter will stay open long enough to let sufficient light in.

How do you know when the exposure is correct?

Guess or Trial and Error

With experience, photographers can sometimes figure out the right aperture/shutter speed combination just by looking at the light. F/16 125^th seconds is a standard starting point for daylight, with a longer speed/larger aperture as the light is reduced. Indoor light, except directly in a sunny window, is always a lot weaker than outdoor light. But you can waste a lot of time and money, or lose photo opportunities, by not knowing how to use some kind of meter to read the light.

Automatic Exposure

When you set your camera to Auto mode, it reads the light through its built-in meter, and sets the shutter speed and aperture to what it thinks is best. Much of the time it does a pretty good job of this, and you get a good exposure without having to think much. You can also choose modes like “action”, where it will select a fast shutter speed if possible. The downside? It doesn’t give you much control over the exposure and settings, which matters for advanced photography.

Manual Exposure - Internal Light Meter

If you look through the lens of many SLR film cameras, you’ll see an indicator that floats up and down according to how much light it sees. (If it never moves you need a new battery!) As you adjust the aperture and shutter speed, there’s always some indication of the perfect exposure – often when the light meter needle lines up with another indicator.

This system is great – simple to use, but gives you control over your settings.

Better digitals also let you use their internal light meters to determine your own settings.

Manual Exposure - External Light Meter

A light meter like the one pictured here is necessary if your camera doesn’t have one built in, or if you want to take a precise reading. For example, you might take a reading right off your subject’s face for a portrait.

With this meter, you point the light sensor at your subject and push the button on the side. The red needle tells you how much light there is, and you line up the white needle with the red one before releasing the button. Once this is done, you can use any of the lined up pairs of shutter speeds and apertures. These are called “equivalent exposures”. e.g. with the setting shown, and ISO 400 film, you could shoot at f/11 with a 1/30^th shutter speed, or to freeze the action more, f/4 at 1/250^th would be an equivalent exposure.

Equivalent Exposure

As mentioned, each f-stop increase lets in half the amount of light, while each decrease in the shutter speed lets double the amount of light in.

So if you have an exposure that you know is right, you’ll get the same exposure by going up one f-stop and down one shutter speed.

The same is true for going down one f-stop and up one speed stop. In fact going up or down any number of f-stops gives the same exposure if you set the speed down or up the same number of speed stops.

Any pairs of f-stops and shutter speeds that give you the same exposure are called equivalent exposures.

The light meter above gives you a lot of these pairs e.g. f/22 1/8 = f/16 1/15 = f/2.8 1/500^th

Why does this matter? Once you have a good exposure, you might want to get the same exposure again, but at a different spped or aperture – e.g. a much faster speed, to freeze the action. If this means going up 4 speed stops, you simply go down 4 f-stops and you’ve got the same exposure i.e. the same amount of light will hit the film and you won’t over/under-expose.

Equivalent Exposure Chart

This appears on Flikr.com, a good place to store and share your photos. Any diagonal stripe of the same colour and number shows pairs of equivalent exposures. For instance, looking at the turquoise (5) diagonal, we see that f2.8 ½ second will expose your film the same as f5.6 for 1 second, or f1.4 for 1/15 second .

Equivalent Exposure Table – how to find Equivalent Exposures

Table of Apertures and Shutter Speeds

Use this to figure out Equivalent Exposures (pairs of F-stops and Shutter Speeds that give the same exposure).

The speeds and F-stops shown match the ones on any standard manual camera, so you can apply this method if you have a camera and know one F-stop and Speed pair that works.

<== Slower speed, more light let in Faster, less light let in ==> Each increase halves the amount of light
Shutter Speed	1	2	4	8	15	30	60	125	250	500
Aperture (F/stop)		22	16	11	8	5.6	4	2.8	1.8
<== Narrower, less light let in Wider, more light let in ==> Each decrease in F-stop doubles the amount of light let in

To get equivalent exposures, like the 3 pairs shown in colour above:

Every time you increase or decrease the shutter speed, you have to increase or decrease the size of the aperture by the same number of “stops”, so…

From an F-stop and Speed known to be correct for a given amount of light, move the same # of spaces left or right on both scales.

e.g. if you know F/4 at speed 15 works, and you want a faster shutter speed,

Move 2 to the right on both scales, and you find that F/1.8 and speed 60 will give you the same exposure.

Or another equivalent exposure pair can be found by moving 3 to the left, F/11 at speed 2.

Equivalent Exposure Wheel

You may be given the components to construct the Equivalent Exposure wheel shown below. It’s simple to use, but very handy. Here’s what to do:

1. determine a correct exposure for a shot, using the shutter speed and f-stop readings from a camera, or a light meter.
2. rotate the inner dial until the correct f-stop and shutter speed align.
3. without moving the inner dial relative to the outer one, find the f-stop or shutter speed you’d like to use. It will be aligned with the correct corresponding shutter speed or f-stop to give the equivalent exposure. In other words, any pair of f-stop and shutter speed that’s aligned gives you the same exposure.

In the first example, 1/30 and 2.8 were found to give a good exposure for a particular situation. If the photographer wanted a faster shutter speed, they’d have to go to 1/60 and f2.0 to get the same exposure. If they wanted a narrow aperture like f16, they’d have to expose for a full second to get the same amount of exposure of the film or sensor.

Think about the 2^nd example compared to the first. In which situation was there more light?