Picking your ISO – the relative ‘speed’ of your digital captures—can be a source of confusion for beginning photographers, as I was reminded recently by a newbie DSLR shooter. He wanted to know how I go about deciding what ISO to use, and whether I changed it frequently during the course of a shooting session.
What great questions, I thought. This is something experienced shooters forget was ever an issue, but it’s something that’s fundamental to obtaining quality results. So here are the factors I consider when setting ISO speed.
ISO is expressed as a number that indicates the relative sensitivity of your camera’s light censor (CCD). Most modern digital cameras have an ISO range from around 100 to upwards of 12,000.
The lower the number you set, the less sensitive to light your sensor will be. This means that you’ll need slower shutter speeds and wider apertures to capture a proper exposure. But your result will be an image with much finer detail and little or no discernible digital noise.
Conversely, the higher you set your ISO, the ‘faster’ your camera will be, very valuable when shooting moving objects in low light (think indoor sports stadiums). You’ll be able to use a fast shutter speed (1/500th/sec or faster) to freeze action in its tracks. But your images will have far more digital ‘noise’than the slower, lower ISO would have.
So it’s all about tradeoffs. No free lunch in the ISO world. Is speed or noise your primary concern? You must choose.
Now that I’ve laid out the parameters, I’ll explain how I set ISO in a couple of real-life situations.
When I go out on a landscape shooting trip, I know I’ll be working with lots of long shutter speeds on a tripod. If there’s any movement in my image, it will probably be some ghostly motion blur that I’m after. I’m not trying to freeze any action but I do want to be able to have the option to make big enlargements of the images. So in this situation I’ll set my ISO for the slowest my camera will allow—in my case, ISO 50. (There’s a custom function on my Canon Mark IV that allows me to boost the ‘low’ end from ISO 100 down to ISO 50.)
Now let’s say I’m going to shoot an NFL football game in an indoor stadium. My primary concern will be properly stopping the action on the field. I want a minimum shutter speed of 1/500th sec. – although 1/800th to 1/1000th is much better. I’ll shoot with the aperture on my lens wide open since I want as little distracting in-focus background detail as possible. A typical exposure for this will be 1/800thsec @ f2.8 or f4, ISO 1200-1600. Even here, I’ll pick the lowest ISO speed that will allow me to achieve my other exposure requirements: in other words, I’ll want to use ISO 1200 over ISO 1600, if there is enough light to allow it.
These are two extreme examples. Where should you set your ISO for more general photography? Is there a happy medium that you can set that will work fine most of the time and usually give acceptable results? I’m happy to report there is! For these normal, run-of-the-mill cases, set your ISO on 400 and forget it! (This is the old film speed of black and white Kodak Trix-X film, the workhorse of newspapers and magazines for decades.) You’ll get perfectly useable images with a bit of speed but without too much noise.
That’s really all you need to know. The lower the ISO, the slower you must shoot, but the finer the ‘grain’ in your resulting image. The higher the ISO, the faster you can shoot, but with the tradeoff being increased digital noise in the results. Evaluate what your needs are and set your ISO accordingly.
Hi, I’m Andrew Boyd, a.k.a. The Discerning Photographer, and I hope this post has been interesting and informative. Please leave me a comment about it, let me know what you’d like to see more of on the site! You can also sign up for email delivery of all future articles or my RSS feed. Or subscribe to our Facebook page or our Twitter feed. Thanks!–DiscerningPhotog