ISO and what it means

How your Nikon’s sen­si­tiv­ity to light is mea­sured, and when you should al­ter the ISO set­ting

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The higher the ISO, the more sen­si­tive it is. This is mea­sured ac­cord­ing to in­ter­na­tional stan­dards, so ISO100 on one cam­era will be ex­actly the same as ISO100 on an­other.

Each ISO set­ting is dou­ble the one be­fore: if you in­crease the ISO from 100 to 200, you dou­ble the cam­era’s sen­si­tiv­ity; and if you in­crease it from 200 to 400, you dou­ble it again. This car­ries on through the ISO scale.

This is de­lib­er­ate. The ISO set­tings are de­signed to dou­ble (or halve) the ex­po­sure in the same way that the lens aper­ture set­tings and shut­ter speed set­tings are, and this is why the lens aper­ture, shut­ter speed and ISO are of­ten de­scribed as the ‘ex­po­sure tri­an­gle’. For ex­am­ple, if you want to use a faster shut­ter speed with­out chang­ing the aper­ture, you could in­crease the ISO in­stead.

This re­la­tion­ship be­tween lens aper­ture, shut­ter speed and ISO could quickly get com­pli­cated, but there are draw­backs to chang­ing the ISO which mean that in prac­tice you tend to change the ISO only when you have to.

ISO draw­backs

When you in­crease the ISO set­ting, you’re not re­ally mak­ing it more sen­si­tive to light, you’re sim­ply

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