Understanding Exposure: Part 3

ISO is the final piece of the exposure triangle. Understanding how ISO works will allow you to take shots where it normally wouldn't be possible without working knowledge of how ISO actually works. ISO is basically the light sensitivity of the camera's image sensor (or film). ISO is usually rated at 100, 200, 400, 800, 1600 and even 3200. In this article we'll take a look at an easier way to think of ISO, how ISO can help with faster shutter speeds and ultimately how it affects your exposures.

To better understand how ISO is going to affect your exposures, lets think of ISO as a worker. If your ISO is set to 200, there are effectively 200 different workers. If your ISO is set to 400, there are 400 different workers. Got it? Okay, now what exactly do these workers do? The workers job is to catch and gather any incoming light that passes through the lens and aperture (Understanding Exposure: Part 1) and produce the image. Lets say that I have an ISO of 200 (meaning I have 200 workers to produce my image). We can also say that you have an ISO of 400 (meaning you have 400 workers to produce your image). If we both shoot the same exact scene at the same exact aperture, who is going make the image faster, you or me? Obviously you will, you have twice as many workers as I do. The more workers there are, the faster the image can be made. Relating this to ISO means that the higher the ISO number, the faster the image can be made.

The image below is to help you visualize how ISO will affect your images. The higher the ISO the brighter the image becomes, however the more noise is visible. Some high end digital cameras do a better job at reducing noise than others. However most of the time these tend to be more expensive digital SLR cameras (DSLR). Some prosumer digital SLR's do a nice job at this, such as the Nikon D50.

Make sense? Good. So how does having a higher ISO relate to having a faster shutter speed? Lets look at another example to help us better understand this. Remember in our previous example, my ISO is set to 200 and yours is set to 400 and we're both using the same exact aperture, lets say f/8 (if you?re not sure what this means go back and read Understanding Exposure: Part 1). Lets say for me to obtain a correct exposure using my current setup (f/8 and ISO 200), I need to use a shutter speed of 1/60. If you were to shoot the same exact scene using your current setup (f/8 and ISO 400), then you would need to use a shutter speed of 1/125 to obtain a correct exposure. Here comes the concept of halving and doubling again. You need to double your shutter speed to achieve a correct exposure as compared to me because you have double the ISO (workers) to produce the final image.

So what are we seeing here? A higher ISO allows you to shoot at a higher shutter speed. For example, if you're out in the middle of the woods on a cloudy day, it's pretty dark. If you're trying to snap an action shot of a bird for example, you need to use a high shutter speed, but unless you have a professional lens that goes to say f/4 you?re probably going to have either an under exposed image or a blurry picture. This is where we can use ISO to our advantage. Using a high ISO in a situation where there?s not much light will allow us to achieve a correct exposure with a fast shutter speed. So why not just use a high ISO in any low light situation? As with anything, there?s always a good and a bad. When a higher ISO is used, noise or grain (the presence of color speckles where there should be none) becomes more apparent. For example, if you?re shooting a landscape scene at night with a high ISO, there may be faint pink, purple or other colored speckles amongst the otherwise dark blue sky. This usually degrades from the overall image and makes the image less pleasing to the eye.

• Using high ISO on dark days may not always be needed because if a fast shutter speed is not needed. Instead using a tripod will allow you to achieve a brighter image on a dark day by using a slower shutter speed. The tripod will keep the image sharp as compared to hand holding the camera and will allow for properly exposed images on dark days without having to use a high ISO.

Now that we know we can use a faster shutter speed with a higher ISO, we can also use a smaller aperture (larger number). The smaller the aperture is, the greater the depth of field gets. With a higher ISO we can achieve great depth of field and still achieve a correct exposure.

To review, we know that ISO usually comes in ratings of 100, 200, 400, 800, 1600 and even 3200 on some high end digital cameras.

We also know that using a higher ISO means that the image sensor is more sensitive to the light coming in. This will allow us to use a faster shutter speed or a smaller aperture (larger f number) for greater depth of field. However, a higher ISO also has the consequence of having more noise in the image as the ISO number gets larger.