Camera ISO

What is ISO?

ISO, shutter speed and aperture all have an important impact on exposure and all three need to be considered when setting up for a shoot. I’ve talked about shutter speed and aperture already in previous posts, so now I’m going to demystify another one of the photography essentials and answer the question – what is ISO?.

ISO is an international standard that measures the rate at which a photosensitive medium reacts to light in a given environment. The quicker the photosensitive medium reacts, the higher the ISO rating. When we’re discussing film, ISO is often referred to as “film speed”. Whilst that description isn’t really appropriate in digital terms it essentially means the same thing as ISO, even though the manner in which it comes about is entirely different.

Speed of film

Photographic film is basically a thin, transparent plastic sheet coated in a mixture called emulsion which is made up of photosensitive chemicals (silver halides) suspended in gelatin. The chemicals have a crystalline structure and depending on the size of the crystals they will react to light at different speeds and, you guessed it, the speed of this reaction is measured with the ISO standard.

Low ISO films are manufactured using an emulsion with a fine-grained crystal structure that reacts quite slowly to light. The fine crystals provide noise free, high resolution photos that have very sharp lines. In contrast, High ISO films have larger crystalline structures that react with light much faster and the resulting photos will usually have a coarseness or graininess about them.

Sensor speed

With digital photography, we don’t have to worry about changing entire rolls of film to obtain a different level of sensitivity. The reactive speed of the photographic sensor is controlled electrically when we adjust the ISO setting on the digital cameras control panel. I guess you could say that digital ISO is a simulation of how film would react given the same environment.

With digital control over our ISO setting, we have the ability to adjust the sensitivity, and thus reaction time of our cameras sensor on the fly. Being able to adjust ISO as needed gives us more control than we’ve ever had before, but we need to always pay attention to the scene and choose an ISO setting that’s appropriate for our subject.

Environment

Personally, I prefer to shoot with as low an ISO setting as possible for any given environment regardless of whether I’m shooting with film or digital. I find that colours tend to have more depth when shot with a lower ISO, and digital photos suffer quite badly from noise when a high ISO setting is used. High ISO film will present a grainy texture, which is not always unwanted, but most of the time I prefer crisp, clean, sharp lines – again, this is  simply a personal preference.

There are times when using a  higher ISO is unavoidable. For example, I wouldn’t use a higher ISO for shots where I know my subject is moving, such as is sports photography. A fast shutter speed must be used in this scenario to freeze a moving object, and in most cases, a low ISO won’t allow enough exposure for a good photo.

Experimentation

Ideally, the best way to understand ISO on your digital camera is to experiment. With digital cameras, we can experiment very easily! Set your ISO to 100, set a static aperture and shutter speed, then take a few shots while adjusting the ISO setting up with each shot. What results are you getting? Leave a comment below – I’d really like to hear about what you’re learning.