The Basics

Digital cameras adjust several features to cope with ambient light conditions, including sensitivity.

When sensitivity is higher (a larger ISO number), photographs look clearer and lighter but less accurate: 'electronic noise' adds blurriness and obscures the small details.  To avoid this phenomenon, you can normally control the sensitivity manually.

There are big differences from one camera to another, notably between those with small sensors (compact and bridge cameras) and those with larger sensors (SLRs).

Getting enough light

Do you remember our guide to exposure?  There, we used the analogy of rainwater pouring into a well to describe how a photo sensor handles light.  Two settings that we can modify are the diameter of the well (the aperture of the lens) and the time we leave it to fill up (the shutter speed).  However, if it's not raining very heavily, you can leave a wide well open for a long time and still not get enough water.


The two photos above were taken with identical settings (aperture: f/4.5, speed: 1/13 s, sensitivity: 100 ISO) but in two very different environments.  The photo on the left, taken in bright sunlight, is slightly overexposed, while the one on the right, taken indoors, is underexposed.  Our photographic well has the same properties in both cases, but on the left it overflows during the storm, while on the right the drought leaves the bottom of the well bone-dry.

What can we do to increase the water level?  One solution is making the well itself narrower: that won't make any more water appear, but will cause the level to rise.  That's a little bit like what we do when we adjust sensitivity.  We can't miraculously create light that doesn't exist, but we can try and adjust things so that there's enough to take a decent photo.


This third image was taken from the same point as the second photo above, with most of the same settings.  Only one thing has changed: the sensitivity, which is now 1600 ISO.  In real terms, that means we need sixteen times less light to obtain the same exposure.  And if that's an exposure that's comparable to the photo above left in the garden, then we can also say that there is sixteen times more light falling on the floors than on the books, although it's difficult for us to tell because our eyes are remarkably able to adapt.

Electronic Noise

Every pixel on your digital camera's sensor produces an electric current which corresponds to the amount of light falling on it.  The problem is that by increasing sensitivity, we are in effect multiplying an electric current that is already very weak and difficult to measure. 

If we double the current, then we double the likelihood of small random variations, some of which are due to the way the light falls on the sensor, which isn't always even, and others by the limits imposed by engineering: when the wires behind the sensor are so tiny, it's hard to guarantee that they'll behave in the same way.

100 ISO	1600 ISO	1600 ISO (blurry)

These irregularities produce 'noise', pixels which don't have the colour that they should do: they can be too light, too dark or an entirely unexpected colour.  That means photos taken at a high sensitivity, like the one in the middle above are grainy and unattractive.  One solution is blurring them, like the one on the right, but that presents its own problems.

12 Megapixel CCD	Traditional CMOS	6 Megapixel Super CCD

SLR CMOS	Exmor R CMOS	12 Megapixel Super CCD

There are a lot of differences in sensitivity from one camera to another.  Those with a big sensor are noticeably better than compacts, where Fuji's Super CCD sensors are clear leaders.  The comparison above is at 1600 ISO: the large sensor (bottom left) easily produces the best image, bating the Super CCD (bottom right) and the new Sony Exmor R (bottom centre).

That's why we always include a demonstration of how each camera performs at different ISO sensitivities in our photography tests.

Setting Sensitivity

Most of the time, your digital camera will choose the correct setting itself.  As long as there's enough light, it will choose relatively low values, around 100 ISO, to produce the most accurate image.  If there is less light, it will choose higher values to maintain a reasonable exposure, as long as you--or your subjects--don't move around too much, which can pose problems when the exposure is too long.

However, there are times when you might want to force your camera to choose a particular sensitivity.  For instance, on a sunny day, if you're photographing some rapidly-moving insects, you can choose a higher sensitivity, which will force the camera to choose a very fast shutter speed, ensuring sharp detail on their beating wings
.

If, on the other hand, you're photographing in dark conditions, you can force the camera to keep the shutter open for longer.  As long as it's still (on a tripod for instance) and the subject isn't moving, you won't need to worry about movement and will be able to minimise the amount of noise.  This can also be a technique that can be used to great visual effect, because car headlights become bright trails of light, and people look like ghosts.


Sensitivity settings aren't found in the same place on all digital cameras, and some don't even allow you to control it at all.  You need to change to P mode, because this isn't a setting you can control in automatic moe.

Then, depending on how your camera is set up, you'll need to either look in the shortcut menu, or the main menu, although some compacts and most SLRs have a button marked ISO to do just that. 

Depending on light levels and your camera's performance, you'll be able to try out different settings.  The latest SLRs allow you to preserve excellent quality up to 1600 ISO, or even 3200 ISO.

With compacts and bridge cameras, the situation is very variable, with the upper limit somewhere between 400 ISO and 800 ISO.  Some cameras in this category produce 'clean' photos at 1600 ISO, but they're very blurry.  While that's acceptable for making small prints, it can be trickier at other sizes.

Finally, don't hesitate to try different sensitivities for yourself: some people like grainy photos.