Nikon Know-how

Michael Free­man un­rav­els the in­tri­ca­cies of the ‘ex­po­sure tri­an­gle’

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It’s not enough to know that there is an ex­po­sure tri­an­gle; you have to make use of it too. Three cam­era set­tings con­trol the bright­ness of the im­age you cap­ture (aper­ture, shut­ter speed and ISO) and to­gether they en­able you to choose a com­bi­na­tion to get any par­tic­u­lar ex­po­sure, hence the term ‘ex­po­sure tri­an­gle’.

Two of the set­tings (aper­ture and shut­ter speed) con­trol how much light en­ters the cam­era to strike the sen­sor while the third, the ISO set­ting, am­pli­fies the sig­nal. Nikon’s 16-megapixel full-frame CMOS sen­sors are ar­guably the world’s top per­form­ers, of­fer­ing a max­i­mum ISO set­ting of 409,600 in the D4s, but most of the credit for this low-light per­for­mance goes to the way in which the sig­nal is pro­cessed and the noise limited.

A flex­i­ble tri­an­gle

■ Aper­ture, shut­ter speed and ISO are in­ex­tri­ca­bly linked in the ex­po­sure tri­an­gle. Rais­ing one of th­ese three means that you’ll need to lower one, or both, of the other two to en­sure the same ex­po­sure. In th­ese ex­am­ples, up means more ex­po­sure (a wider aper­ture, longer ex­po­sure or higher ISO), while down means less ex­po­sure (so a nar­rower aper­ture, shorter ex­po­sure or lower ISO set­ting).

Be­cause noise, which is the vis­ual equiv­a­lent of au­dio hiss, has no re­deem­ing qual­i­ties at all, and just de­grades the im­age, the ex­po­sure tri­an­gle is lop­sided – there are good rea­sons for in­creas­ing or de­creas­ing both aper­ture and shut­ter speed, but rais­ing the ISO is al­most al­ways a last-re­sort op­tion. This makes choos­ing the bal­ance be­tween the set­tings in­ter­est­ing and, well, a skill.

Pick your pri­or­i­ties

Us­ing the ex­po­sure tri­an­gle means know­ing clearly what your pri­or­i­ties are for any given shot. Apart from con­trol­ling the light, aper­ture and shut­ter speed each do an­other job for the im­age: aper­ture af­fects depth of field, while shut­ter speed af­fects mo­tion blur, and you may want less or more of ei­ther in any spe­cific shoot­ing sit­u­a­tion.

1/125 sec at f/4, ISO1600 Dur­ing a fu­neral in Yun­nan, China, light lev­els in the main in­te­rior, lit only by a door­way and win­dow, were low. I set my ISO to 1600, re­ly­ing on the D4’s low-noise

ca­pa­bil­ity, and that left me a 1/125 sec shut­ter speed to freeze any move­ment.

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