When Should I Change My ISO Setting?


I've always tried to keep my ISO setting as low as possible because I don't want excessive noise in my photos, but there are times when raising this value is simply unavoidable. By the way, if you aren't familiar with ISO as it relates to cameras, you can take a look at these two articles that will tell you almost everything you need to know.

Let me give you a scenario to help illustrate a point. It's dusk and you're sitting on a park bench in a city while people watching. You've got your camera in hand and you're ready to take a shot. The perfect and most interesting person walks by and you lift your camera and begin snapping away. You think, "Great job!" But when you review your photos, you see that while the background of each photo is perfectly sharp, in each and every one of them, the person walking by is blurry. "What the heck happened?" you ask yourself. Something has gone terribly wrong.

Well, I'll tell you what happened. Your shutter speed was too slow. If you had your camera set to shutter priority mode, that's of your own doing. If you had it set to aperture mode, your aperture wasn't opened up enough. If you had your ISO locked down to a very low number, your camera may not have had any leeway anywhere other than slowing the shutter speed to create a well exposed shot. Any way you look at it, the slow shutter speed caused the blur and that needs to be corrected.

Your camera's ISO value is the determining factor when it comes to its ability to absorb light. The higher the ISO value, the more sensitive it will be to that light. Conversely, the lower the ISO value, the less sensitive the sensor will be to light. In the illustration above, since you had your ISO number set to a low value, the camera's sensor wasn't very sensitive to light at all. To make up for this, it could do two things; either open up the aperture as much as it could go to let as much light in as possible or slow down the shutter speed to allow more time to let the light to reach the sensor. If you've got a lens that doesn't offer a large maximum aperture, the only other place for the camera to look is the shutter speed.

Many people look toward lenses with large apertures for their night time photography. This is a very smart thing to do because those large apertures can let a lot of light in to the camera's sensor. The thing is, sometimes those openings, while large, don't allow enough light to come in. If you're working with still life, then sure, feel free to open up those apertures and lengthen the shutter speeds. Since nothing is moving, the shutter speed duration doesn't really matter. The problems arise when there's action in your low-light scenes. When people or things are moving and you'd like them to be captured still and sharp, sometimes the largest aperture can't give enough compensation to correctly capture the scene. At times like these, you'll need to look toward your ISO setting. If you allow this setting to float automatically, your camera will choose the proper setting, so you can feel free to open up the aperture as much as you'd like and then set that quick shutter speed so each and every photo you take is crystal clear and sharp.

At this point, you probably have two questions. First, how do you change the ISO setting and second, what's a good ISO value to use at night for action shots? I'll tackle the "how to" question first. On my Canon DSRL, there's an ISO button right on top. I can push that button and then use the dial to roll back and forth between the different ISO values. When I'm finished with that, I can push the ISO button again to accept the setting change. As for the second question, if you're shooting your low light shots in shutter priority mode (which you should be), I would leave the ISO value set to Auto. Let the camera choose what to use to properly expose the photo. It'll do so depending on how dark your scene is. If you're finding that the ISO number is shooting way up to something like 6400 or higher, it may be time to start looking at lenses that offer larger apertures, such as f/1.8 or f/2.8. Many kit lenses have starting aperture sizes between f/3.5 and f/5.6. These are oftentimes just too small and they don't let the necessary light through.

For low light shooting, I like to use my Canon 50mm f/1.8. The aperture is huge and I can set my shutter speed to whatever I want and have confidence that my ISO won't go up too much. I actually have it set to a maximum of 1600 because I don't want any more noise than I absolutely have to accept. Even 1600 is too high for my taste, but if that's what I have to do then that's what I have to do.

Another question may be, are there other times when it may be necessary to increase my ISO value? The answer to that is definitely yes. If you're engaging in handheld photography and you're taking photos of movement in lower light situations, you'll likely want to crank up the ISO. Also, if you're indoors and there's lower light, you may need to look at your ISO value, depending on what's happening in your scene. If there's a lot of movement and action, raise the value or let it float so the camera can choose something appropriate. If there's no action, keep the value low and then slow down your shutter speed.

Your primary concern when experimenting with different ISO values is noise in your images. You'll need to experiment with the various settings and determine your ceiling, meaning, the highest value you'd accept before telling yourself that there's simply too much noise in your images and that you don't want to take those anymore. Years ago, I said that my ceiling was ISO 800, but I've increased that to ISO 1600 because the quality of these cameras has gotten better.

I've seen scales out there that have been created by various photographers. They say that, in general, you should use ISO 100 in the morning and afternoon when there's lots of available light in the sky and then raise it to 640 to 1600 and all the way to 6400 as there's less and less light available. I like this kind of advice, but it's really overly simplistic. Your scene and your intentions are what should dictate your ISO value as well as the rest of the camera's values.

Here's a question; how do you manage your ISO settings? Do you work backwards and set your ISO first and then allow that to dictate your shutter speed or do you set your shutter speed and keep the ISO set to Auto so your camera can make that decision? I'd love to know. I also want to know your feelings on ISO in general. What do you think about higher values? Lower values? What are your preferences? Thanks!