ISO and Image Noise

You can't have one without the other

ISO and image noise are like Hansel and Gretel, Lewis and Clark or peanut butter and jelly: they just go together.

An increase in ISO also causes an increase in image noise.

Having said that, modern digital SLR cameras are getting better and better about suppressing noise even at high ISO settings.

If all this sounds like a lot of jibberish to you, don't worry. I'll explain myself in a moment.

By the end of this article you'll be well-versed in the intimate relationship between ISO and image noise.

ISO Defined

Let's begin this discussion by defining what ISO means.

The letters ISO come from the Greek work for "equal" (isos). These three letters have been adopted by the International Organization for Standardization, but ISO is not an acronym for the organization.

Sounds good, but what does the greek word for "equal" have to do with your digital SLR?

The ISO number indicates how much light your camera's sensor can absorb.

ISO numbers follow a scale that is common on virtually all digital SLR cameras:

100

200

400

800

1600

3200

As the number increases, your camera's sensor becomes more sensitive to light.

The Purpose of ISO

Being able to change the ISO on your digital camera is most useful in situations where there isn't a lot of ambient light.

Cameras have four ways of gathering more light:

Increase the opening of the lens (aperture)
Increase the time the sensor is exposed to light (shutter speed)
Increase the sensitivity of the sensor (ISO)
Create artificial light (flash)

The most common thing that happens when there isn't enough ambient light is that your shutter speed slows down to let more light hit the sensor.

Here's the problem: this leads to blurry photos.

When the shutter stays open for long periods of time, it captures motion as blur.

ISO 3200 let me capture this shot at night with overhead lighting

If your subjects move when you take the shot, they appear blurry. If you're holding the camera in your hands, the entire photo is blurry (due to camera shake).

ISO solves this problem.

Since the sensor becomes more sensitive to light, the shutter doesn't have to stay open for long periods of time.

Voila! Clear photos even in dim light - without using a flash.

Want an extreme example? With your camera set to ISO 1600, you should be able to take clear photos indoors at night using overhead lights holding the camera in your hands.

Like many other things in life, you don't get this amazing benefit without some consequence.

Let's Make Some Noise

No, I am not talking about what goes on when a garage band is rehearsing for their next big gig.

That's audio noise. In this case we're talking about visual - or image - noise.

Image noise is the by-product of using a high ISO setting on your digital SLR camera. It is also commonly referred to as "grain" since that is its appearance.

When a photo has image noise, areas of color that should look smooth have a mottled or speckled appearance.

Image noise is easiest to see in areas of shadow and in large areas of consistent color - like blue sky.

A noisy photo - click for large version

At ISO 100, you'll be hard-pressed to see digital image noise. This is the setting of choice for landscape photographers who want to make large prints (the more you enlarge a print, the easier it is to see noise).

At ISO 400, you might be able to see some grain in your photos and at ISO 3200 (if your camera goes that high) you'll definitely see some noise.

We arrive at last to a point of great debate: how different digital SLR cameras handle image noise at high ISO settings.

Noise Suppression

While this may sound like popping in some earplugs, it's actually something that your digital camera does every time you take a photo.

As mentioned above, a high ISO setting will add noise to your photo.

At the same time, your digital SLR is trying to compensate: it's "filtering" out the noise so that the image doesn't look like a grainy mess.

This is why you might still have a very hard time seeing image noise even at ISO 800 with some cameras.

Nikon in particular has been praised for the ability of its cameras to reduce noise at high ISO, which gives photographers a lot of flexibility when shooting in low light.

A Nikon photo at ISO 800 - click for large version

If you've been to some online photo forums or have friends who are digital photo geeks, you've heard a lot about this.

Every time a new digital SLR camera is released, the immediate debate begins: how well does it handle noise at high ISO?

Pundits and amateurs alike go back and forth about the threshold: the ISO setting where noise becomes visible in your digital photos. Some claim the noise starts at ISO 400, while others will swear it's 800.

All debating aside, here's the takeaway point: not all digital SLRs handle noise in the same way, and some are better at reducing it than others.

If you are going to take a lot of photos in low-light conditions, ISO and image noise will be important factors to consider before you make a camera purchase.