Shutter priority | Mode where you choose the shutter speed

In this mode, you set the shutter speed and the camera sets the aperture required to give the correct exposure. Shutter priority mode is often chosen for action photography where you need to be sure of high shutter speeds, or pictorial photography where you want controlled amounts of motion blur, for example in moving water.

Shutter speed | Not just for 'freezing' subjects

The shutter speed is the length of time the shutter is open during the exposure, and one of the two methods of controlling the exposure (the other is the lens aperture). The camera's shutter is normally closed, but when the picture is taken it briefly opens to let the light pass through before closing again.

Shutter speeds go in a particular sequence, where each is twice as fast as the one before (this 2x step is used with ISOs and lens apertures to in order to simplify exposure calculations). Here's the sequence (there are slower and faster speeds too):

1sec, 1/2sec, 1/4sec, 1/8sec, 1/16sec, 1/30sec, 1/60sec, 1/125sec, 1/250sec, 1/500sec, 1/1000sec

The shutter can be inside the lens mechanism (compact cameras) or directly in front of the film/sensor (single lens reflexes and other interchangeable lens cameras like rangefinders). Shutter speed is an important creative control because it can be used to 'freeze' movement or introduce deliberate movement blur.

With moving subjects like this fairground ride, the usual technique is to use a shutter speed fast enough to freeze the movement. In this case, though, a very long shutter speed of 1/6sec was used instead. Using an image-stabilised lens helped keep the background relatively sharp, while the spinning horses have turned into an impressionistic blur.

Histograms | The new way to get the exposure right

A histogram is a graphical display of the distribution of tonal values in the image, from the darkest (on the left) to the lightest (on the right). The shape of the histogram can tell you a lot about the image’s characteristics and, indeed, its flaws. If the histogram is ‘clipped’ (cut off abruptly) at either end, this means the photo has missing shadow or highlight detail.

You can use your camera's histogram display, where available, to get the exposure right rather than relying on traditional metering techniques.

There are three places where you'll find histograms:

1. On the camera's LCD when you're composing the shot. Many compact cameras can display a histogram during shooting, and it's worth checking the manual to find out if yours does. Digital SLRs with a live view mode will be able to display a histogram too. You can use the histogram in conjunction with the EV compensation control to get the exposure exactly right.

2. All digital SLRs and some compacts can display a histogram in playback mode too. On digital SLRs which don't have a live view mode, this is your chance to check the exposure. If you need to, you can then adjust the camera's exposure controls and re-shoot.

3. Image-editing programs display histograms too, usually in the Levels dialog (e.g. Photoshop and Elements). The histogram can help diagnose any exposure problems so that you can see what you need to do to fix them. If highlight or shadow detail has been 'clipped', though, it's too late to do anything about it on the computer. You really do have to get the exposure right with the camera.

Handheld meters | Not the dinosaurs you think

Digital SLRs and compacts have fantastically sophisticated metering systems, so why on earth would anyone want to bother with a handheld meter like the Sekonic L-208 (above) any more? After all, it takes ten times the effort and the metering cell is incredibly primitive compared to the sensor in a modern digital camera.

Well, it's true, handheld meters are slow to use. But they have intrinsic advantages which are often overlooked.

• They're separate to the camera. You can walk around, taking meter readings from different things and different angles to build a truer picture of what the lighting is like.

• You take the light readings manually, estimate a suitable average manually, select a shutter speed and lens aperture combination manually, and apply these settings manually to the camera. Yes, it's more work, but it leads you to think properly (and maybe for the first time) about light, exposure, shutter speed and aperture.

• Even a simple handheld light meter like this one does something not even the most expensive D-SLR can, and that is to take an 'incident' rather than a reflected light reading. It measures the light falling on the subject, not the light bouncing off. You can't use it all the time, but when you can, it eliminates one of the single biggest barriers to successful exposure.

This is how simple handheld meters work:

1. First, you need to make sure you've set the same ISO on the meter as you're using on the camera

2. Then you press a button on the side to take a meter reading, which moves a needle on the scale...

3. You then turn the main dial...

4. To line up the pointer with the needle

5. And then you can read off suitable shutter speed and aperture combinations on the dial

6. To take an incident reading, you move a diffuser over the metering cell, then stand by the subject and aim the meter at the camera.

The L-208 is a basic 'match-needle' meter, but it does the job perfectly well. You can also get digital meters which display an EV value which you transfer to the main dial on the same way, and more sophisticated flash meters which can measure flash exposures too.

Metering patterns | Why simpler is sometimes better

A camera's 'metering pattern' is the way it measures the  light values in the scene. In the old days, the meter was a fairly unsophisticated light-sensitive cell that took one overall reading. Modern cameras, though, offer multi-pattern, centre-weighted metering and spot metering.

The diagrams above represent the way in which these three main metering patterns work. From left to right:

1. Multi-pattern metering (the camera's default) breaks the scene down into segments, measures the light in each and then makes an 'intelligent' decision about the type of scene it's looking and the best exposure to reproduce it correctly. Multi-pattern metering is more likely to be right, more of the time, than any other system, but its response isn't easy to predict.

2. Centre-weighted metering is much cruder. It measures the overall light value, like older metering systems, but places more weight on the centre of the image, where the subject (it's assumed) is most likely to be. Centre-weighted metering is more likely to get it wrong, but, if you're an experienced photographer, it's predictable and easier to correct than multi-pattern metering.

3. Spot metering takes a reading from a small area in the middle of the picture. It's useful when there's a big difference in brightness between the subject and the background or between one part of the scene and another. You use it when you want to base the exposure on just a single area. It's easy to get it wrong, though, and not quite meter the area you meant to, or choose an object that's intrinsically light or dark. Spot metering magnifies your mistakes alarmingly!

ISO | High ISOs are fine on SLRs, terrible on compacts

ISO is the measure of a sensor’s sensitivity to light. The numbers are the same as the ISO ratings given to traditional film. The difference is that a film only has one fixed ISO, while you can change the ISO of your sensor from one shot to the next. ISO values go in the following sequence (some cameras offer intermediate values too):

100 • 200 • 400 • 800 • 1600 • 3200 • 6400 • 12800

Each ISO value is twice the sensitivity of the one before, which makes exposure calculations more straightforward because this is the system used for shutter speeds and aperture settings too.

You can increase the ISO in poor light so that you don't have to use slow shutter speeds and risk camera shake. In full auto mode, digital cameras will adjust the ISO automatically with this in mind.

The disadvantages of higher ISOs on digital cameras are not unlike those of high-speed film. You get more noise (similar to film grain) and reduced definition.

The amount of noise you get is directly related to the size of the sensor and the number of megapixels. The small, high-resolution sensors in compact digital cameras produce much more noise at the same ISO than the larger sensors in digital SLRs.

It's like turning up the volume on an old audio cassette because the music is quiet. The music gets louder, but so does the background hiss, and the overall quality is pretty poor.

You can see that in this example where a small section has been blown up to show the image quality at ISO 100 (left) and ISO 1600 (right). Compact camera makers are constantly increasing the maximum ISO values on their cameras, but only at the cost of plummeting picture quality - no matter what the claims!

The two big problems are small sensors and high megapixels ratings. That's why digital SLRs are so much better at high ISOs than compacts. The sensors are far larger, yet the megapixel ratings are much the same.

Nikon's D3 and D700 are perfect examples. Both have full-frame sensors, but 'only' 12 million pixels. The results is truly astonishing high-ISO performance. The picture above was shot at ISO 6400, yet the enlargement shows that the picture quality is still excellent, with very little noise.

HDR modes in cameras | Only half way to the 'HDR look'

High Dynamic Range is a technique for capturing a much wider brightness range than the sensor could normally record. It's done by taking two shots at different exposures and then combining them. This is usually done using software, but some cameras have HDR 'double-shot' modes built in.

The Pentax K-5 is one example, and Ricoh's CX4 compact is another. But there are a couple of provisos:


• The camera mustn't move between exposures. Ideally, you should put it on a tripod

• The effects you get won't be like those you see in magazines. The camera's aiming to record the maximum possible brightness range, not produce that characteristic 'HDR' effect with dark skies and bright shadows. For that, you still need software.

HDR images straight from the camera will tend to look very 'flat', and their aim is simply to record a full range of tones. If you want shots like this one, you still need to do some work on the computer.

i-Contrast | Canon's shadow-lightening tech

Pictures taken against the light or in bright sun often have dense shadows that don't show much detail. They may also have overexposed highlights which come out a blank white. Canon's i-Contrast feature, found of some of its compact cameras, is designed to counteract this effect by selectively lightening dark shadow areas.

This illustration shows how it's done. The camera identifies the darker areas of the picture (shown in red) and brightens them while leaving the rest of the image alone. The latest version of this technology is more sophisticated, too, because it increases the 'gain' (sensitivity) of the darker areas as the image is captured rather than just applying a software 'fix' to the processed image.

There's nothing particularly remarkable about Canon's system, and other makers have similar features. Nikon uses 'Active D-Lighting', while Sony has a 'Dynamic Range Optimizer'. Similar techniques can be applied later using software, like Photoshop's Shadow/Highlight command.

The advantage of in-camera contrast adjustments is that you can see straight away if the effect has worked and adjust the strength if not, and they mean you don't have to do any image-editing work later on. Newer systems are more sophisticated than software tools, too, because they can adjust the camera exposure at the time the shot is taken to make sure highlight detail is recorded too. This is the difference, for example, between the 'Active D-Lighting' system Nikon uses now and simpler 'D-Lighting' system used before.

The disadvantage of in-camera contrast adjustments is that what looks right on the LCD might not look right later on the computer screen or in a print. Sometimes the results can look a little artificial – you do need good contrast between highlight and shadow areas for the image to look realistic and vibrant. In-camera contrast adjustments can also cause difficulties later on if you do any image-editing. Nikon's D-Lighting system, for example, can produce a broad,barely-visible 'glow' around dark objects which doesn't show up in unadjusted images, but which can become quite prominent if you manipulate them later.

Canon's i-Contrast and similar systems are good solutions if you don't want to do much editing work later, but a combination of careful exposure (or shooting RAW files) and software adjustments are likely to produce better pictures.

Neutral density filters | Can be used for good and evil

Neutral density filters reduce the amount of light entering the lens without altering it in any other way. It sounds an odd sort of thing to do, but ND filters have two important uses: (a) cutting the amount of light in daylight to allow slow shutter speeds; (b) darkening bright skies (graduated ND filters).

Graduated ND filters can be positioned carefully over the lens to darken the sky without affecting the rest of the scene (see the example above). They come in various strengths, usually expressed as a number. A 2x ND filter darkens the image by one stop, a 4x ND filter darkens by  two stops and an 8x ND filter darkens by three stops. It does take a bit of experience to gauge what strength of filter you need in any given situation.

Graduated filters slide up and down in the filter holder so that you can get the position exactly right. The filter holders have extra slots which allow the use of two or even three filters in combination.

You can also find 'neutral density' filters in some compact cameras. Here, they're used to simulate the effect of a smaller lens aperture (presumably it's cheaper than actually including a lens aperture mechanism in the lens). It does the job, adjusting the exposure just like a real lens aperture adjustment, but it will not produce the varying depth of field effects you might be expecting from aperture adjustments.

Light meters | Why all in-camera meters are flawed

In-camera meters measure the light reflected from the subject. It's all they can do. The trouble is that different subjects reflect different amounts of light. A white wedding dress will reflect a lot more than a black cat. But the camera can't tell white from black (that's a cognitive thing, not a measurement) so it has to assume everything is a sort of mid-grey. Which is why, unless you override the exposure, it'll give you grey wedding dresses and grey cats. And this is why a simple handheld meter and a pair of feet can deliver exposure accuracy that no camera can, be it an EOS 1Ds Mk III or a Nikon D3x...

What you need is an 'incident' light reading, where you measure the amount of light falling on the subject, not the amount it's reflecting. This ignores the brightness of the subject itself and delivers pictures where wedding dresses do come out white, and black cats really are black.

So the next time a camera maker trumpets an all-new and even more 'intelligent' multi-pattern metering system, just remember that it's still measuring the wrong thing.

EV (Exposure Value) | How exposure works

EV, or Exposure Value, is a numerical measurement of the amount of light in a scene. We all tend to think of exposure in terms of shutter speeds and apertures, but in fact it's the exposure value that it all starts from.

When you use a handheld light meter like the Weston Master V below, it quotes an EV number which you then translate into shutter speed and aperture combinations using a rotating dial. When the camera measures the light level, it too starts with an EV number, which it then uses to decide shutter speeds and/or apertures, depending on the mode you're working in.

If you want to use manual exposure control, or take more control over what shutter speeds and apertures the camera uses, a basic knowledge of exposure values is really useful. In fact, you might not need an exposure meter at all, because it's possible to predict the EV values for different scenes surprisingly accurately. The table below gives some examples:

Once you know the EV number, either from a handheld light meter or using the table above, you can use it to work out what shutter speed and aperture you're going to use with the table below.

One thing to be aware of is that the EV values change at different ISOs. It's not something you need to worry about if you're using the camera's internal light meter because if you change the ISO the camera will allow for this automatically. But if you're using EV values from the table above, or transferring them from a handheld meter, you will need to match the EV to the ISO - you can do this with the table.

What you can see from this table is that for any one exposure value there are lots of different shutter speed and aperture combinations you can use. When you use the 'program shift' function on a camera, this is what it's doing.