ISO, Shutter Speed and Aperture

The sensor of your camera needs just the right amount of light to record an image.  If too much light reaches the sensor the picture will look pale and washed out because it's over-exposed.  If too little light reaches the sensor the picture will be too dark to see properly because it's under-exposed.

Your camera has three controls to ensure the right amount of light hits the sensor: ISO, shutter speed and aperture.  By adjusting these the photographer can control the exposure of the photograph and the appearance of the final image.

ISO

Setting the ISO is the simplest of the controls.  Use a low ISO in sunny conditions and a high ISO in darker conditions.  Using the lowest ISO possible at all times is a good idea for maximum image quality.

(The lowest ISO setting available on your camera represents the native sensitivity of the sensor.  To make it possible to use higher ISO settings the sensor output is electronically amplified.  This amplifies unwanted noise along with the picture information so the higher the ISO setting the lower the quality of the final picture.  This applies to all cameras, but is only significant on compacts -  I'd happily use any ISO setting available on an SLR, but limit myself to 200 or 400 ISO on a compact, dependant on the camera model in use.  I always use the lowest ISO setting possible, though, regardless of camera.)

Halving the ISO halves the sensitivity of the sensor, doubling the ISO doubles the sensitivity of the sensor.  ISOs usually run in a sequence like this:

50 ISO    100 ISO     200 ISO     400 ISO     800 ISO     1600 ISO

Halving or doubling the ISO changes the exposure by one stop

You may see intermediate values appearing on some cameras, particularly as lowest ISO setting on a camera, or if your camera has an Auto-ISO selection function.  These often represent one-third of a stop increments, like this:

50 ISO     64 ISO     80 ISO     100 ISO     125 ISO     160 ISO     200 ISO

Shutter Speed

The shutter has two functions, to determine the exact moment you take the picture and to control the amount of time for which light is permitted to strike the sensor.  Shutter speeds are usually expressed in fractions of a second, and your camera will have a maximum and minimum shutter speed it can deliver.  Old cameras showed these as a series where each shutter speed was twice the preceding setting and half the next, like this:

 1/30th sec      1/60th sec      1/125th sec     1/250th sec

(For example: A shutter speed of 1/60th second allows light to strike the sensor for half the time as a shutter speed of 1/30th second, or for twice the time of 1/125th second)

Halving or doubling the shutter speed changes the exposure by one stop 

Digital cameras are more pernickety about exposure, so you'll now see two intermediate values between each of these steps, so you can set shutter speed in increments of one-third of a stop.  For example:

1/60th sec      1/80th sec      1/100th sec      1/125th sec

Long (Slow) Shutter Speeds:  If the shutter is open for a significant period of time the camera can move or shake enough to blur the final picture or the subject can move during the exposure.  As a very rough rule of thumb - keep shutter speeds above 1/60th sec to avoid camera shake, and above 1/125th sec to avoid subject movement.

Camera shake - the shutter speed was 1/8th sec despite using 320 ISO!

In the dim light of a dull day at the Farne Islands my shutter speed was 1/8th second, enough for the seal to have moved some distance.  Panning the camera to follow it has rendered the seal fairly sharp, but the long white blurs of the background give a sense of movement

Fast (Short) Shutter Speeds:  Will freeze subject movement and help make pictures sharper by minimising the effects of camera shake.

A fast shutter speed - effectively the 1/1000th sec burst of light from my external strobe - has frozen the movement of this fish and produced a sharp image

 Aperture

The aperture setting determines how much light is allowed to flow through the camera lens to the sensor. 

The aperture is the 'hole' in the lens through which light can flow.  Every lens has a maximum aperture determined by the physical size of the 'hole', and will also have smaller apertures created by moving a set of metal blades to restrict the size of the 'hole'.  The size of an aperture is indicated by an f-number, such as f2.8 or f8. 

The bigger the number, the smaller the aperture. 

Large apertures (Small f-number, eg f2.8) allow a lot of light into the camera.  Smaller apertures (Big f-number, eg f8) allow less light into the camera.

Like shutter speeds, apertures fall into a sequence where each aperture allows half the light to enter the camera of the preceding aperture, and twice the light of the next.  Here's a typical sequence found on an SLR, the figures in black show the range you'll usually find on a compact:

f2     f2.8     f4     f5.6     f8     f11     f16     f22

(For example: An aperture of f4 allows half as much light into the camera as an aperture of f2.8, but allows twice as much light into the camera as an aperture of f5.6)

Halving or doubling the aperture changes the exposure by one stop

Also like shutter speeds, because digital cameras are so finicky about exposure internmediate apertures at intervals of on-third of a stop are necessary.  For example:

f4     f4.5     f4.8     f5.6     f6.3     f7.1     f8

Aperture setting determines Depth of Field

The depth of field is the area in front of and behind the subject which appears to be in focus.  Depth of field with a compact is really only a concern when taking macro shots , and then you simply need to use the smallest aperture you can (Biggest f-number, eg f8 or f11) to give the maximum depth of field.

 Using f11 on a fisheye lens for this scenic shot on an SLR has given an image sharp from front to back

SLR users need to be much more aware of depth of field, and can use limited depth of field creatively to distract attention from an untidy or cluttered background, for example, and will also find depth of field in macro work shallower than equivalent shots made on a compact.

Using f11 on a 105mm macro lens on an SLR for this close-up has given very little depth of field - the eye is sharp, but the mouth is a bit blurry and everything  infront of and behind the  eye is out of focus

Choosing ISO, Shutter Speed and Aperture Combinations

In any situation there will be a range of possible shutter speed and aperture combinations that will deliver good exposure, it's your job to select which would be best for the results you want to achieve.

The simplest way to illustrate this is to look at the output from a hand-held exposure meter, such as this (Very) elderly Leningrad 4. 

(Note that your camera has a built-in light meter and will set both aperture and shutter speed, and probably ISO as well, automatically, whether it tells you what it's doing or not.  You don't need an external light-meter but you can get a functioning meter on eBay for pennies and it's a great way to really understand the relationship between ISO, shutter speed and aperture.)

To use it, set the ISO number here...

Meter set to 100 ISO, a good starting point in most conditions

...measure the amount of light available and set the reading opposite the arrow at the bottom of the meter here...

Meter reading was 7 - it was a dull day and I used the high-sensitivity scale, not visible in the photograph

...and you can then read the range of shutter speeds and apertures you could use on this scale here...

Shutter speed and aperture scale

Any of these shutter speed and aperture combinations will deliver a correctly exposed picture

To stop a fast moving subject, choose 1/250th sec at f2.8 - this is what Sports Mode does

If maximum depth of field is important choose 1/30th sec at f8 (And find somewhere to support the camera so that it can't shake and spoil the shot!) - this is what Landscape Mode does

For general snapping, use 1/125th sec at f4 or 1/60th at f5.6, this is what Program or Auto modes do.

In darker conditions (And it'll always be darker when you're diving, especially in temperate waters), increase the ISO to, say, 400 ISO, and here's the result:

Meter set to 400 ISO

The light meter reading is still 7, but by using an ISO of 400 you can now use a  faster shutter speed at any given aperture, so it's easier to stop moving subjects, or use a smaller aperture at any given shutter speed to get more depth of field without risking camera shake.

Thanks to Chris (Uwila from Yorkshire Divers) for his input