ISO – “I-S-O”, “i-so”
- Sensitivity of photographic negative materials (formerly expressed as ASA).
What is ISO?
ISO within photography is the measurement of sensitivity of the photographic material used to capture an exposure. In the days of film this was the physical film stock, but in today’s digital world the ISO is the sensitivity of the CCD or CMOS image sensor which captures the light (the “eye” of your camera).
Prior to the ISO system of measuring film/sensor sensitivity it was often measured as ASA, which stands for “American-Standards-Association”. This was just another standard method of measuring the film. The key historic fact with ASA speed was that the scale was arithmetic, which meant that a film with rating ASA200 was twice as fast as film with rating ASA100, and so on. ISO was a combination of ASA and another standard called DIN in 1974.
For modern amateur/professional photography, and for digital photography here at photography 101 you just need to understand the two facts that ISO denotes the sensitivity of your sensor to light, and that doubling your cameras ISO from say ISO400 to ISO800 will double the sensitivity.
How ISO affects you
How this affects your photography is that it is one of three factors which makes up an exposure of an image. The other two are aperture and shutter speed. Varying the ISO of your camera with vary the exposure of your final image, over and underexposing a scene just like changing the aperture or shutter speed.
If your camera was set with a wide open aperture (e.g. f/1.8) and the shutter speed was set to 1/50s – the only way you could affect the final exposure of the image would be to increase or decrease the ISO value. A lower ISO value means less sensitive to light and therefore a “darker” image, and likewise a higher ISO value means more sensitive to light and therefore a “lighter” image. This can be seen in the graphic below:
A Trade off with light and noise
As you increase your ISO value to increase your cameras sensor to light, physically you are amplifying the digital signal the sensor is receiving. This increases the visibility of fainter light hitting the sensor, but also increases the fainter background electrical noise caused by the sensor. This background signal is amplified along with the actual light signal and causes your image to become noisy, with artefacts “mixed” in with your final image.
This is a trade off caused by increasing your ISO. You need to choose an ISO level which will allow you to capture correct exposures with adequate shutter speeds and aperture values, but not allow too much noise into the image to spoil it.
This trade off was also common in film photography, but with grain instead of noise. Film stock with increased ISO sensitivity would produce a correct exposure in fainter light, but the image would also have a lot of “grain” visible with it, as the film chemicals would react to the faintest of light photons.
Modern digital cameras have an ISO range (equivalent to changing a film roll in a film camera on the fly!) from anything from ISO50 up to ISO32000 and above. More expensive DSLR cameras are capable of handling noise better than others, which include sensor noise reduction and in camera algorithms when saving. To reduce possible noise further you can also use software such as Photoshop, Lightroom etc.
There are situations when an image works well with a slight grain texture, especially in black and white. In this scenario increasing your ISO will add more noise and produce a more natural grain to the image. The below image was taken for SEO Creative Movember charity event.