DSLR starters' guides: ISO

Tue 3 June

Changing the ISO is the DSLR equivalent of changing film speed in a conventional camera.

What is ISO?

You’ve probably seen an ISO or ASA number associated with a roll of film. This tells you how sensitive the film is to light, a higher number indicating more sensitivity to light. In digital photography ISO indicates how sensitive the image sensor is to light.

The most common ISOs are:

100, 200, 400, 800, 1600, 3200

Quick tip

The higher the ISO number = the higher the sensitivity to light

Moving from one ISO to the next value changes the exposure by half or double.

The ability to shoot in low light has a trade-off in the form of grain or noise. In digital cameras it's not the grain of the film that becomes visible at higher sensitivities, but digital noise - the visual equivalent to the hiss you hear when you turn a hi-fi amp up to full volume when nothing is playing.

100 ISO will give you sharp images while 3200 will show quite a lot of noise.

The higher the ISO number = the more noise/grain

The rule of thumb is to select the lowest ISO you can get away with that will allow you to take a picture at a fast enough shutter speed and/or larger aperture.

Flower picture taken at ISO 400, f/2.5, 1/50
Gig picture taken at ISO 3200, f2.8, 1/100

Example situations:

A football match outdoors on a bright day: at ISO 200 you should be able to shoot at 1/250 or faster to freeze action.

Concerts and gigs: always a tough one especially at smaller venues where there’s little lighting and the subject is moving. Best bet is to start at 3200, do a few test shots and then lower the ISO to the lowest you can take successful shots.

Churches and galleries: 1600 might be the minimum ISO you can get away with to shoot at 1/60. 

Top tip 1: faced with the choice of introducing more noise and a fast enough shutter speed, go with the shutter speed.  You can live with noise and make attempts to reduce it with software but you can’t do much with a blurred shot other than bin it.

Top tip 2: purposely use a high ISO for a grainy effect and give  mood to a scene.  This might be harder to do than you think as camera manufacturers are continuously reducing (“improving”) the noise from ISO settings to the point where you might need to add it in post-production

ISO Sensitivity Settings

At ISO 200 sensitivity you can see in this tight crop that there's relatively little noise and the red of the phone box and text on the poster are relatively unaffected Doubling the sensitivity to ISO 400 has added relatively little noise and the effects are still acceptable.
Up the sensitivity again to ISO 800 and now the digital noise is obvious - especially in the red of the phone box and the degradation of the poster headline At ISO 1600 the noise is so evident as to make the image unusable except as an example of how not to set your ISO


It’s your turn

  1. Set your camera on Program Mode to let it choose the aperture and shutter speed.  Set the ISO to 100 and take a picture.  Continue at the other ISOs and take note of how the shutter speed changes.
  2. Block out the light in a room and switch on a lamp or light a candle.  Shoot at the highest ISO possible for your camera.  Look at the noise in the picture, especially in the shadow areas.

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