Now that you have a better idea about aperture and shutter speed, it’s time to take a closer look at ISO and how it affects exposure. ISO sensitivity refers to the “speed” of the film you use in your film camera. Because digital cameras use a sensor in place of film, an ISO equivalent is given.

Basically, ISO conveys how sensitive your sensor is to the light available when taking a shot. The higher the sensitivity, the brighter the exposure.

Depending on your camera, your ISO settings can start as low as 50 and as high as 25,600. Full stop values are usually 100 - 200 - 400 - 800 - 1600 - 3200 - 6400, doubling each time.

To demonstrate the effects of changing the ISO, I took a sequence of shots with a constant focal length (44mm), aperture (f/2.8) and shutter speed (1/60), changing only the ISO in full stops:

I took the photo in an area with low light to demonstrate the range of brightness you can achieve by increasing 5 stops. It’s quite clear how ISO can affect the brightness of your photo, right?

Another important fact to note is how ISO affects aperture and shutter speed. Remember how shifting combinations of aperture and shutter speed one stop at a time all resulted in the same exposure? ISO does the same thing.

Let’s say I set my ISO to 200 and shutter speed to 1/15 and take a shot. To get the same exposure, I can also use ISO 400 and a shutter speed of 1/30. So, if I increase the ISO by one stop, I can increase the shutter speed by one full stop as well.

Sound familiar? The same concept works with changing ISO and aperture. You can use all three (ISO, shutter speed and aperture) in the combination that is most ideal for your shot.

Most ideal? But don’t they all look the same? Well, not exactly. If you look closely, you’ll notice that with increased ISO’s, you will get an increased amount of noise or grain in your image.

The noise isn’t very pretty at the higher ISO’s is it? Now, on a 4×6 print, a noisy image will still look fine. It’s when you have large prints that you will find the noise undesirable. It is possible to smooth it out using a noise reduction software or even in Adobe Lightroom. However, keep in mind that you may lose some of the sharpness in your photo or risk having skin tones look plastic-like.

So then, why would you even want to shoot at a high ISO in the first place?

Let’s say you are enjoying a nice dinner in a darkly lit restaurant. It’s a bit embarrassing to use a flash, you don’t have a tripod (nor would you use one there anyway) and the largest aperture your lens can open to is f/3.5. You want to take a portrait of your dining partner so you take a shot with your aperture set wide open at f/3.5 and shutter speed at about 1/100. Not only did your photo turn out too dark, your subject moved a bit and everything came out blurry.

Your shutter speed was not open long enough to compensate for your small lens opening. If you used a longer shutter speed to allow in more light, say 1/15, you will get a blurry picture due to camera shake from hand holding your camera (no tripod). Because you can’t run out and buy a new lens with a bigger aperture (eg. f/1.4) here is where you want to pump up that ISO to 800 or more, in order to prevent motion blur and expose a brighter picture. Yeah, you’ll have noise, but it’s better to have some noise than no photo worth keeping.

Here’s a blurry shot (at ISO 200) where I wish I had used a higher ISO:

At ISO 400:

“Ahhhhh!” Much better!

So what’s the secret to exposing properly while preventing motion blur? It really comes down to assessing your lighting conditions and adjusting your ISO (try to keep it as low as possible), aperture and shutter speed to get the look you want.

If you’re not sure about finding the right combinations in all lighting situations, try shooting in either shutter priority (when you want to freeze motion) or aperture priority (when you want to control depth of field) so you only have to worry about setting two of the three elements of exposure. In shutter priority, you set the shutter speed and ISO while the camera will determine the correct aperture to use. In aperture priority, you set the aperture and ISO while the camera will find the right shutter speed to use in that situation.

One other thing to note is that the larger your sensor size, the less noise you will get while using a higher ISO. For example, point and shoot cameras have a small sensor and therefore will exhibit more noise at ISO 400 than would a DSLR would at ISO 400.

In Summary:

• ISO tells you how sensitive your sensor is to light. The higher the ISO, the lighter the resulting image.
• Choose the lowest ISO possible according to your lighting situation to reduce noise. Adjust the aperture or shutter speed first. You can also use a tripod or a flash to help keep the ISO low.
• A noisy photo is usually better than a blurry photo, so don’t be afraid to increase your ISO as need be.

Hopefully you now have a better understanding of the role ISO plays in exposure along side aperture and shutter speed. If you have any questions or comments, please do not hesitate to ask. I’d love to hear from you!

Laura is a Vancouver portrait photographer. Visit Dolce Life Photography.