ISO: the digital photographer’s friend
ISO, in the ancient days of film, represented the film “speed” or how much light was needed to expose the film properly. Film was typically rated from 200 to 800. The lower the number, the more light was needed for proper exposure. Many professionals opted for the lowest ISO film they could find because these images tended to have a better dynamic range of color and was less grainy than their high ISO counterparts. To compensate for needing more light to expose the film, they bought large aperture lenses, also commonly referred to as a “fast” lens or “fast glass”.
In the film days, you were stuck with one ISO rating per roll of film, but with digital cameras, you can change the rating from picture to picture.
Modern DSLR cameras will have an ISO rating of at least 200-1600. Usually in the feature lists of a particular camera, the ISO range will be listed under “Sensitivity” or “Light Sensitivity”. High end cameras can go from 50-6400 (though usually one particular camera won’t be able to use the entire range).
I like to use the lowest ISO number that I need for a given situation. If I’m shooting a landscape, I tend to not need a really fast shutter speed and can get away with using the lowest ISO my camera will allow (which is 200). This helps improve the detail in the shadows and reduces the amount of grain in the picture.
It’s a juggling act between shutter speed, aperture, and ISO. When you need more light, you need to use a larger aperture, slow down your shutter speed, or use a higher ISO. If the situation permits, and I want a large depth of field, I use a smaller aperture and start reducing my shutter speed. If I want a more narrow depth of field, I’ll use a larger aperture. If I can’t go any slower and capture a sharp image, and my aperture is maxed, then it’s time to raise the ISO rating.
By raising the ISO, you can get a shot in a darker environment, at the expense of additional grain or “noise” in your image and a slightly flatter image because the range of color is narrower. Sometimes this is the best you can hope for. I’d much rather have a grainier picture than resort to the on-camera flash. High-end DSLR cameras have in-camera software that reduces the amount of grain in your picture, and you can use Photoshop to adjust the levels of the picture to compensate for flat looking pictures; however, the more time you spend fixing pictures the less time you’ll have to spend actually taking pictures, and you won’t be able to get detail that the camera never recorded.
To give you an example, I have a 50mm prime 1.8f lens. If I bump my ISO to 1600 (the highest my camera goes), and use the largest aperture of 1.8, I can shoot indoors handheld without needing to use a flash as long as there’s a reasonable amount of light available. Usually, in this scenario, I’m shooting candids. It’s a lot easier to get natural looking candids when a flash isn’t constantly firing. People tend to get annoyed if they’re trying to have a peaceful get-together when they’re occasionally blinded by flashes of light.
On the other extreme, if I’m shooting a waterfall, I want to reduce the amount of light as far as I possibly can so that I can keep the shutter open as long as I can. This produces that smooth water effect that most waterfall pictures have on postcards. To do this, I’ll use a small aperture, low ISO, neutral density filter, and shoot when it’s almost dark.
