Creative exposure control... without the maths

Introduction

Most cameras boast an automatic exposure function – the exception being some pro models –
but part of the creative journey for any photographer is eschewing automatic exposure and
understanding the ménage-a-trois at the heart of photography: ISO, aperture and shutter speed.
Whether you’re getting to grips with your first DSLR or determined to get the most from your compact
digi, to properly control the creative process you need to understand the relationships between these
three core elements. When I was learning the basics of photography (and still shooting film) I purchased
a book that promised perfect exposure every time… if you followed its method. Great, except that it
treated photography like a maths lesson, sucking out the fun and spontaneity.
Now, if like me, you’re a bit more right brain than left, and you struggle to remember your telephone
number, never mind a whole list of f-numbers and fractions of seconds, fear not! You don’t have to be
a walking calculator to correctly expose your images – after all, this is the digital age.

Terminology

Stops and EV (exposure value)

These are the basic units of photographic exposure, and are, to all but the pedants amongst us, equivalent.
From here on in I shall use EV – it’s the shortest.
EV can be increased and decreased. A change of plus or minus 1 EV is a doubling or halving (respectively)
of the amount of light hitting the camera’s sensor. Many cameras allow you to make adjustments in
increments of ½ or 1/3 EV.
Your job is to find the exposure value that lights the area framed in the manner you wish it to be lit.
Frequently that will mean both your principal subject and the background are lit such that there is detail
in both dark/shadow areas and bright/high light areas.
Sometimes this goal is simply not attainable - it may not even be desirable!
If, for example, you photograph an obsidian statue against a white wall on a bright sunny day, you would
struggle to get detail in both the statue and the wall. This is because your camera is not as sophisticated
as your eyes are, and can only deal with a limited range of brightness values. You can, however, decide
what that range will encompass – the highlights or the shadows.
Received wisdom dictates that a lack of detail in shadow areas (a.k.a. blocking) is usually more acceptable
than a lack of detail in bright areas of the image (a.k.a. blown out highlights). You might think otherwise,
and it is your decision to make, based on the way you interpret the scene you are photographing.
A common example of blown out highlights as an undesirable phenomenon is in landscape photography.
You expose for the foreground on a bright sunny day with a perfect blue sky only to find that your camera has
transmogrified that perfect rich blue into a hideously bland white. This usual occurs when shooting into the
sun (not directly at the sun, of course, which is not good for your eyes or camera!), or when the sun is just
out of frame. Solutions include:

1. changing the angle at which you approach your subject, so the sun is more behind you
2. using a graduated filter in front of the lens
3. experimenting with HDR (high dynamic range) photography, which involves multiple exposures of the same
subject and access to Photoshop or similar software

Sometimes you might not want a ‘perfect’ exposure – you may prefer deep shadows to create a darker mood,
or you may even want to blow out the highlights to create a bleached, minimalist look.
Once you understand the rules, you can break them, creatively.

ISO

Also known as ASA, ISO is the sensitivity of the media recording the image, be it film or digital sensor.
Want to know what it stands for? That’s what Wikipedia is for. All you really need to know is:

1. the lower the ISO value, the greater the amount of light required to correctly expose an image, and vice versa
2. low ISO produces ‘fine grain’ images, high ISO produces a courser grain and can reduce contrast

In the film world, course grain can be used for artistic purposes – to create a gritty atmosphere, or to
emulate vintage. In the digital world, course grain takes the form of noise, and features rather ugly
irregular-shaped artefacts of various colours that rarely improve an image.
So, the first rule of thumb is shoot at the lowest ISO circumstances permit. Most digis offer ‘optimal’
ISO values of 100 or 200, and they are worth sticking with for the majority of scenarios.

When might ISO 100 or 200 not be suitable?

1. shooting handheld in low light (e.g. a concert where tripods may not be welcome) – a higher ISO may be needed to avoid blurry images
2. for long exposures in light environments some cameras offer values lower than their optimal ISO; my Nikon, for example has LO0.3, LO0.7 and LO1, and the manual warns that using them may reduce contrast – that may be so, but it can always be fixed in Photoshop

If you wish to add grain to your image, do it in a controlled fashion using software such as Photoshop.
There are plenty of methods, and you can experiment until satisfied.

Aperture or f-stop

For practical purposes, f-stops (or f-numbers) are units in the scale of apertures available on a lens.
There is a more detailed and complete description of the term, for which I again direct you to Wiki.
The aperture is the diameter of the lens pupil, and is central to the creative decisions you will be making.
Aperture controls:

1. the amount of light hitting the sensor (along with shutter speed); a larger aperture = more light; unfortunately, the terminology confuses matters as f-stops are fractions (in fact, often fractions combined with decimals), so f5.6 is a wider aperture than f16
2. the depth of field of an image, or in layman’s terms, the zone of sharpness within an image

Depth Of Field (A slight digression)

Depth of field is almost as important as correct exposure, and you may end up having to compromise
one for the other. It refers to the area of an image that is sharp. Regardless of whether you use auto or
manual focus, the precise point within the frame that you are focussing on is the part that should be pin
sharp. Everything the same distance away from your camera as that subject will also be sharp. Anything
closer or further away will become increasingly softer or out of focus.
Generally, depth of field extends 2/3s behind the focal point and 1/3 in front of it. That zone can be
increased by using a narrow aperture or decreased by widening the aperture.
To achieve sharpness throughout an image use very low f-numbers (i.e very narrow apertures) or techniques
such as hyper-focal focussing. You may need to compromise if there is not enough light to shoot with a very
narrow aperture.
You may also prefer a very narrow depth of field. For example, selective focussing is a technique, popular in
recent years, whereby emphasis is given to the subject (or part of the subject) by throwing everything else
right out of focus. Portrait photography also frequently relies on throwing the background out of focus, so that
it does not distract from the subject of the shot.

Depth of field can also be influenced by:

1. the type of lens you use; wide angle lenses provide greater depth of field than tele-zooms at long focal lengths; macro lenses usually produce a very small depth of field, even at narrow apertures
2. the distance between camera and subject; the closer they are, the smaller the depth of field will be

Shutter Speed

The properties associated with shutter speed are more self-evident than those with aperture:

1. a slow shutter speed allows more light onto the sensor than a fast one
2. a fast shutter speed can freeze objects in motion whereas a slower shutter will blur moving objects - and sometimes stationary ones (see below)
3. the range of speeds available depends on your camera, but commonly stretches from thousandths of a second to 30 seconds
4. some cameras also have a bulb setting which allows you to open the shutter for as long as you like (or as long as your power source allows)

You may think you held your camera steady, and the image you see in the camera’s LCD display may look
fine, but when you blow it up to 100% magnification the story can be quite different. Even tiny degrees
of movement will cause loss of sharpness or outright blur in an image. When it is you that is moving
(probably without realising), the phenomenon is known as camera shake. There are various solutions
to this problem:

1. as a rule of thumb when shooting handheld, dial in a shutter speed that is equivalent to or faster than the focal length of your lens; for example, focal length = 50mm set shutter speed = 1/50 or 1/60, or for fl =100mm set shutter = 1/125
2. a better solution by far, is to shoot with a tripod, or monopod, or beanbag, or some other kind of support for your camera

Relationships

ISO, aperture and shutter speed combine to create your exposure. Once you have established the desired
exposure (covered in the next two sections) you must bare in mind that any further adjustments to these
settings must be performed in a balanced manner; changing one setting will usually mean you need to
change one or both of the other two to maintain correct exposure.
Which to change is a question of compromise, and is a purely creative decision – which setting is most
critical to your creative vision for the picture you are taking?

1. if depth of field is most important to you, set the aperture you want and vary shutter speed or ISO to achieve correct exposure
2. if freezing (or blurring) your subject is most important, set the shutter speed required to do so and vary aperture or ISO to achieve correct exposure
3. if you require a noise-free image, a low ISO is required, so vary aperture or shutter speed

The choice is yours, but your camera can help.

Metering

Many digital cameras offer a range of metering options. A proper discussion of metering is a large topic
in itself - only a brief mention of metering methods is relevant here.

matrix metering > the camera uses information from most of the frame; this your all-rounder, good for a large number of scenarios, and will usually produce a balanced image with subject and background exposed reasonably well

centre-partial (or centre-weighted) metering > the camera again uses information from most of the frame, but gives priority to a circle in the middle of the frame; frequently used for portrait photography

spot metering > decisions are based on a very small area of the frame – not necessarily in the centre of the frame; use this to correctly expose your subject (and only your subject) when the background is considerably lighter or darker