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What Is... ISO

  ISO Sensitivity Auto; ISO 100, 200, 400, 800 equivalent  

ISO sensitivity expresses the speed of photographic negative materials (formerly expressed as ASA).

Since digital cameras do not use film but use image sensors instead, the ISO equivalent is usually given.

What ISO denotes is how sensitive the image sensor is to the amount of light present. The higher the ISO, the more sensitive the image sensor and therefore the possibility to take pictures in low-light situations.

And, where you would have needed to physically change to a different roll of film if you wanted a different ISO speed, digital technology allows you to simply dial one in. In this way, you can record images taken at different ISO speeds on the same memory card.

ISO Speed & Exposure

ISO speed affects the shutter speed / aperture combinations you can use to obtain correct exposure.

Suppose your digital camera's light meter warns you there is not enough light to correctly expose a scene. You could use the on-board flash, but let's suppose again it's not allowed (like in a concert or indoors recital).

You would then need to use a higher ISO. Set on "ISO Auto" mode, your digital camera will automatically select a higher ISO. Otherwise, you can manually select the next higher ISO and see if the increased sensitivity allows you to obtain a correctly exposed picture. If it does, you can now take a correctly exposed picture.

Similarly, if you find the camera is using a shutter speed that is too slow (1/60 sec. and slower) to handhold the camera steady and shake-free (thus resulting in blurred pictures), and you cannot open up the aperture anymore, and you do not have a tripod or other means to hold the camera steady, and you want to capture the action, etc. etc. -- then you might select the next higher ISO which will then allow you to select a faster shutter speed.

ISO Speed & Noise

However, all this increase in sensitivity does not come free. There is a price to pay with your image appearing more noisy.

See, when you boost the sensitivity of your image sensor by selecting a higher ISO, the image sensor is now able to record a fainter light signal. However, it is also true now that it will record fainter noise, where noise is any signal that is not attributed to the light from your subject. Remember that an image sensor is still an analog device and it generates its own noise, too! The increased sensitivity allows the image sensor to record more light signal and more noise. The ratio of light signal to noise (S/N ratio) determines the "noise" in your resultant image.

An image sensor is usually calibrated so that it gives the best image quality (greatest S/N ratio) at its lowest possible ISO speed. For most consumer digital cameras, this value will be expressed as ISO 50, ISO 64 or ISO 100. A few digital cameras use ISO 200 as their lowest ISO speed.

Just as with its film counterpart, an image sensor will exhibit "noise" (comparable to "graininess" in film) at the higher ISO speeds. Unlike film, where graininess can sometimes contribute to the mood of the image, noise produced by an image sensor is undesirable and appears as a motley of distracting coloured dots on your image.

ISO Speed & Image Sensor Size

The size of the image sensor determines the ISO speed range that a digital camera can use without suffering from undue noise. One reason for this is because the pixels on the larger image sensor can be larger and therefore receive more light, and thus have a greater signal-to-noise (S/N) ratio (for more information on noise, see our tutorial: What Is... Noise?).

If we take two image sensors, each with 4 megapixels resolution, but of different sizes, the 4 megapixels image sensor that is smaller will exhibit more noise at higher ISOs than the larger one.

1/1.8 in. (7.2x5.3mm)

APS-sized (23x15mm)
4 million tiny pixels crammed into a 1/1.8 in. image sensor cannot compete in image quality with 4 million large pixels on an APS-sized image sensor.

Most consumer digital cameras use 1/1.8 in. (and smaller) image sensors, so noise at high ISO is a problem. Though they will allow you to use a high ISO (such as ISO 400), the resultant image will usually be very noisy.

Click for original image [1.3 MB]
A 100% crop of a picture taken at night ny a Point-and-Shoot digicam at ISO 400 (1/1.8 in. image sensor)
Click on image for original size [warning: large file]

A digital SLR (dSLR), on the other hand, uses a large image sensor, usually full frame (24x36 mm) or APS-sized (half-frame). Noise is rarely a problem and the use of a high ISO 400 results in images with barely noticeable noise.

ISO 400 ISO 800 ISO 1600
Click for original image [3.4 MB] Click for original image [3.0 MB] Click for original image [4.2 MB]
Pictures taken by a DSLR at different high ISOs
(APS-sized image sensor)
Note that here noise really starts to be a problem at ISO 1600
Click on image for original size [warning: large files]

Practical Considerations

Should You Use A High ISO?

Until consumer digital cameras start adopting the larger image sensors, noise will continue to be an ever present fact of life at high ISOs.

Another category of digital cameras for advanced amateur photographers -- commonly known as "prosumers" (professional consumers) -- attempt to bridge the gap between consumer and professional digital cameras by using a slightly larger image sensor (at 2/3 in. or 8.8x6.6mm).

However, the "megapixels race" has meant that ever more pixels are being crammed into a small area. Where before there were 5 million pixels on a 2/3 in. image sensor, now we see 8 million pixels crammed on the same sized image sensor. It is therefore not surprising that noise remains a problem. And which is why you should not be fooled by the "more megapixels is better" mantra.

A little bit of noise may not be a problem depending on the size of your prints or images for display. There are also a number of noise reduction software (Noise Ninja, Neat Image) that you can use to clean up the noise, though there's quite a bit of post-processing work involved, and you might want to reserve this for the special pictures you want to print large format.

If it is a matter of choosing between not being able to take a picture and suffering a noisy image, I'd rather be able to take the picture at a high ISO and then try to clean up the noise afterwards in a noise reduction software.

But remember, to be able to do this, your digital camera must allow you to select a high ISO (ISO 400, ISO 800). Some entry-level digital cameras have only one ISO, usually ISO 64 or ISO 100.

Is There A dSLR In Your Future?

If you definitely must take noise-free low-light pictures and therefore need to be able to use a high ISO speed, then you need to consider digital SLRs with their large image sensors. Just remember, though, that a dSLR usually requires a higher level of proficiency in using the camera and adjusting exposure than a point-and-shoot consumer digital camera does.


[Editor's note: We list the image sensor size in our QuickFact™ Sheets if the manufacturer makes this information available.]

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