What do ISO numbers mean?

Ever wondered what the ISO numbers mean on your DSLR? You may know what they do, but what exactly are they?

Photo credit: titicat

The ISO numbers on your DSLR are a throwback to film cameras. Specifically, they’re a throwback to the film itself. If you take a look at camera film in the shops you’ll still see ISO numbers on them. Most I could find seemed to be ISO 200, but there were a few ISO 400 films. What’s this all about, though? And how does it relate to your DSLR?

ISO numbers relate to how quickly a film will react to light. The higher the number, the “quicker” the film. So if you’re in a situation where there’s loads of lights, say bright sunlight, you need a film that won’t react too quickly. Why? Because a quick film will easily overexpose in bright sunlight. Conversely, if you’re in a situation where there isn’t a lot of light (indoor party, perhaps?) you do want a quick film so that you can make the most of whatever light is available. These speeds are regulated by the International Standards Organisation (hence “ISO”) so if you picked up an 200 speed film you should know broadly how it will react in various light conditions regardless of the brand name printed on the box.

Now, mentally make the move to the digital realm and the same principle applies. Higher ISO numbers are for use when you need a more sensitive sensor, generally in low-light situations. The big advantage to digital over film is that you can change the ISO speed on the fly, altering it between images depending on the changing circumstances. This is not an option when your entire film has the same speed.

One thing to bear in mind is that, whilst higher ISO speeds do let you shoot in lower light, they come with a tradeoff. Low-light photography can be quite “noisy”. You may notice that solid blocks of colour are speckled, or even that there are bright pink or blue pixels in your image. There are ways to get around this (noise reduction is a good start), but you just need to be aware that it may happen.

So, what do ISO numbers mean? Technically they refer to the “speed” that the film or sensor will react to light, but you can think of them this way: the darker it gets, the higher an ISO number you will want to use. The beauty, of course, is that if you’re not being particularly choosy, you can just let your camera select the best ISO number for you…

Tags: Exposure, Film, ISO, Light, Speed

Category: The Basics