What is ISO?
Film speed is measured in ISO (and occasionally in ASA - however this is an older name for it so you'll probably only come across it when using cameras that are about 50 years old). 

The ISO refers to how sensitive your film or digital sensor is to light. A low ISO represents a lower sensitivity to light and so indicates that the film or digital sensor will absorb less light. Whereas a high ISO indicates a higher sensitivity to light, so your film or digital sensor will absorb more light. 

The lower the ISO the clearer and sharper the image will appear, the higher the ISO the more noisy (in digital photographs) or grainy (in film photographs) your image will appear.

ISO goes up in double increments, called stops.

e.g. 100, 200, 400, 800, 1600, and so on.

Increasing the sensitivity of your ISO means you can allow light to hit your film or sensor for less time. An increase in one stop of ISO means you can decrease your shutter speed by one stop.

So if 400 ISO with 1/60 shutter speed was the correct reading, then 800 ISO with 1/30 shutter speed would also be a correct reading. You can chose which combination to use depending on the environment you're in and what effect you're looking to achieve in your photograph. 

Why is ISO also known as 'film speed'?

A 'fast' film refers to a film with a high number ISO, e.g. 1600. Using an ISO of 1600 in normal daylight conditions would allow you to use a fast shutter speed, e.g. 1/1000. These settings would allow the photographer to capture fast motion whilst avoiding blur. 

A 'slow' film refers to a film with a low number ISO, e.g. 50. Using an ISO of 50 in normal daylight conditions would require a slow shutter speed, e.g. 1 second (thus requiring a tripod). Any movement that crosses the camera in this time would appear as a blur, so these settings would be best for photographing still objects, like architecture or landscapes.

What's the best ISO to use?
In order to achieve the sharpest image possible, it is best to use the lowest ISO you can get away with. 

If you are using your camera with a tripod and photographing a still object then you can use a very low ISO to achieve the sharpest image possible. 

If you want to capture motion in your shots, but do not want there to be any blurring, you can use a high ISO in order to then select a very fast shutter speed. 

When using film...
Digital cameras are great because you always have the option of switching between ISOs depending on what/where/when you're shooting. However, if you're into film cameras you have to be prepared to commit to a certain ISO for as long as your film lasts you. 

ISO 400 is a great everyday film, it's kind of a medium in the ISO scale, good for general outdoor shots (especially in the unpredictable British weather with it's changing sunny/cloudy/gloomy/bright days).

If you're going to be shooting somewhere that's likely to be really bright, like on a sunny holiday, go for a film with an ISO of 200.

For shooting in darker conditions, like in the city at night or indoors where there's low lighting, choose as high an ISO as you can get. ISO 800 is more commonly found, but if you can get it, a film of ISO 1600 or ISO 3200 is ideal. However, bear in mind that shooting at these higher ISOs will result in more grainy photographs than the lower alternative, again see the aforementioned article for examples. Not necessarily a bad thing! But good to know.

For examples of the differences between ISOs in film, check out the Film Stock page!

See some examples of various ISOs below

1/500 - f18 - ISO 200
As I shot towards the sunlight and conditions were bright I needed to decrease the sensitivity of the camera's sensor so I chose a low ISO of 200.

1/5000 - f9 - ISO 1600
To capture the motion of this bird in flight I used a high ISO. Making the sensor more sensitive to light meant I could use a fast shutter speed of 1/5000 to prevent blur.

Fun Facts!
'ISO' is not an abbreviation, as may be assumed as it's written in capitals.  It is called ISO as this translates the same in all languages. It is set by an organisation called the 'International Organisation for Standardisation', which is confusing as that is almost an acronym for ISO, but not quite.

You may come across the term ASA on old film cameras or light meters. ASA and ISO are pretty much the same thing, ASA is just an older term for the same measuring system of light sensitivity.


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