Mas­ter your DSLR: Im­age sen­sors

HOW DOES YOUR CAM­ERA TURN LIGHT THAT’S PIPED THROUGH THE LENS INTO A DIG­I­TAL IM­AGE?

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WHILE AUT­O­FO­CUS AC­CU­RACY and speed, con­tin­u­ous shoot­ing rate, buf­fer size and build qual­ity are all im­por­tant con­sid­er­a­tions when choos­ing a cam­era, it’s ac­tu­ally the per­for­mance of the cam­era’s imag­ing sen­sor that re­ally mat­ters most.

How the sen­sor re­sponds to the light col­lected by the lens is key to the qual­ity of your photographs, with the de­gree of de­tail the sen­sor is ca­pa­ble of re­solv­ing, the range of bright­ness it can record and the amount of noise that’s cre­ated lay­ing the foun­da­tions for the fi­nal, pro­cessed im­age.

Your cam­era’s sen­sor is made up of mil­lions of in­di­vid­ual light-sen­si­tive pho­to­sites, com­monly re­ferred to as pix­els. These can be minute — a frac­tion of the width of a hu­man hair in size — and each one gen­er­ates a dis­tinct elec­tri­cal sig­nal in re­sponse to the lu­mi­nance or bright­ness of the light that it’s ex­posed to. The more pix­els that a sen­sor has, the higher its res­o­lu­tion. Im­ages recorded in a higher res­o­lu­tion will be larger, able to hold more fine de­tail and al­low you to crop the pic­ture to change the com­po­si­tion or en­large an ob­ject while still re­tain­ing a us­able im­age size. But cram­ming more pix­els onto a sen­sor can have a detri­men­tal ef­fect on im­age qual­ity too. It’s all down to the sen­sor’s size and the sig­nal-to-noise ra­tio.

On a cam­era with a small high-res­o­lu­tion sen­sor, the pho­to­sites will be smaller and more densely packed to­gether. As a re­sult, they cap­ture less light com­pared with the amount of noise or in­ter­fer­ence recorded. Larger sen­sors en­able the use of larger pho­to­sites, which cap­ture more light. As a re­sult, the sig­nal-to-noise ra­tio is higher. This is why the pic­tures pro­duced by a high-res smart­phone cam­era will look worse, par­tic­u­larly in low light, than those cap­tured by a SLR that shares the same pixel count.

In ad­di­tion to cap­tur­ing more light and hav­ing the po­ten­tial to pro­duce a ‘cleaner’ im­age at higher ISOs, larger sen­sors af­fect your pic­tures in other ways. In gen­eral, the larger the sen­sor, the more likely the im­age will be ren­dered with greater fine de­tail, smoother colours and a broader range of tones. Larger sen­sors also make it eas­ier to achieve a tighter depth of field, too.

The type of sen­sor has an impact too. CCD (charge-cou­pled de­vice) sen­sors used to be the pre­ferred choice for their dy­namic range and han­dling of noise, but these days, most cam­era sen­sors use CMOS (com­ple­men­tary me­tal-ox­ide-semi­con­duc­tor) tech­nol­ogy. CMOS re­quires less power to op­er­ate, so is well-suited to the fast burst speeds re­quired of sporty SLRs.

[ PHO­TOG­RA­PHY MASTERCLASS ]

Mil­lions of pix­els make up a dig­i­tal im­age. The more there are, the more fine de­tail is recorded.

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