When to Change ISO

Being able to change ISO on the fly is one of the most significant advantages that digital cameras have over their film counterparts.

This brings us to an important question: why should you change the ISO rather than letting the camera decide for you?

More importantly: when should you change the ISO setting on your camera?

The answer will significantly increase the quality of the photos you take.

When the Lights are Low

I'm not going to define ISO here. I've already done that on a different page where I talk about ISO and image noise.

I will provide a short definition: ISO is an indication of how sensitive your camera's sensor is to light. The higher the ISO number, the greater the sensitivity.

Put another way: increasing the ISO of your digital camera helps it to "see" in the dark.

I just tipped you off to the answer to the question I posed above...

You want to manually increase the ISO setting on your camera when you are taking photos in dim light conditions AND you don't want to use the flash.

I'll get into more details about this in just a moment. Before I do, take a quick look at this chart to see the relationship between certain lighting conditions and the ISO setting:

Lighting	ISO
Bright sunshine	100
Mild shade or overcast	200
Deep shade	400
Indoors on a sunny day	800
Indoors at night	1600

The important thing to note: as the light gets dimmer, the ISO must increase accordingly.

It's All About Shutter Speed

Let's delve a bit deeper into the specifics of when you should change the ISO setting.

First, a question for you: don't you just hate it when a prime photographic opportunity comes up and your photo turns out a blurry mess?

I certainly do. It drove me crazy when I first started using digital cameras that half my photos were blurry. So many shots that could have turned out great were immediately deleted.

That's because I didn't know how to properly leverage my ISO setting to get a fast shutter speed.

A slow shutter speed is what causes a blurry photo.

Here's a more detailed description: a shutter speed that is not fast enough to freeze the motion of your subject will result in a blurry photo.

This means that you can take a perfectly clear portrait using a shutter speed of 1/60th of a second (because your subject is still) but a photo of a running horse at 1/60th of a second will be a blurry mess.

Here's a quick table to help you determine the approximate shutter speed required for different subjects:

Subject	Shutter Speed*
Landscape	60
Portrait	125
Children	250
Animals	500
Sports	1000
Race Cars	4000

* Shutter speeds are measured in fractions of seconds, so 60 is really 1/60th of a second.

One factor that contributes to blurry photos is how fast your subject is moving.

The Focal Length Factor

The second key player in any blurry photograph is the focal length of your lens.

Focal length indicates the telephoto capacity of your lens. For example, a lens with a focal length of 100mm is a more powerful telephoto lens than a 50mm. Zoom lenses have variable focal lengths.

Here's the rule:

IF you are holding the camera in your hands and IF the shutter speed is less than the focal length, a blurry photo will result.

But what's causing the blur? If you're taking a portrait of a non-moving subject why should the ENTIRE photo come out blurry?

The problem is that when you hold the camera in your hands the CAMERA is shaking.

Camera shake is magnified by longer focal lengths.

The way to negate camera shake it to always use a shutter speed that is faster than your focal length:

Focal Length	Shutter Speed (for no blur)
28mm	30
50mm	60
100mm	125
200mm	250
300mm	350

The only time that this rule doesn't apply is with cameras that have built-in anti-shake or if you're using an anti-shake lens.

Anti-shake works against the movement of the camera when you hold it in your hands, and allows you to shoot at speeds that are slower than the focal length of the lens and still get clear shots.

Boost ISO For a Faster Shutter

We've just covered two of the most common causes of blurry photos:

A shutter speed that's not fast enough to capture the subject motion
A shutter speed that is slower than the focal length of your lens

Weren't we talking about ISO? What does that have to do with all this shutter speed stuff?

You might have noticed that there's a common problem when it comes to blurry photos: slow shutter speeds.

Here's the thing: as the available light gets dimmer, your camera wants to do one of two things to capture a correctly exposed photo:

Use the flash
Reduce the shutter speed to let in more light

While using the flash works well in some situations, it's not always a practical solution.

Let's take a concrete example: you're photographing a baseball game on an overcast day with a 200mm lens.

First, to negate the camera shake effect, you have to use a shutter speed of 250 or faster. Second, to capture the moment of action when the bat is swung, you'll need something even faster: 500 or 1000 is about right.

But with the dim available light, the fastest shutter speed your camera will let you use is 300 (1/300th of a second), and using the flash when you're photographing action from the bleachers isn't going to do a bit of good.

You're stuck, right? Not quite!

Now's a perfect time to increase your ISO to 400 or even 800.

By increasing the ISO, you'll be able to use a faster shutter speed. Even though the faster shutter speed is letting LESS light into the camera, the higher ISO means the camera is MORE sensitive to light, and everything balances out.

Voila! At ISO 800 you can use a shutter speed of 1000 and can capture all the action of the ballgame.