pull ing apart the pixel s

Many Nikons can shoot RAW files of dif­fer­ent bit depths – here’s why it mat­ters

NPhoto - - Nikopedia -

Com­put­ing works with ‘bits’ – ba­si­cally ze­ros or ones – and most of the images we see on screen use eight bits to de­scribe each pixel, for each of the three colours, red, green and blue. Eight bits, also known as 28, gives 256 pos­si­ble val­ues for each of th­ese colours. When th­ese three colour ranges are mul­ti­plied to­gether (ie 256 x 256 x 256) the num­ber of pos­si­ble colours in an im­age, from pure black to pure white, is 16.7 mil­lion. As th­ese num­bers can get out of hand when we talk about higher bit-depths (well into the bil­lions), it’s nor­mal to use bit-depth as a kind of short­hand. There’s the po­ten­tial for con­fu­sion de­pend­ing on whether we’re talk­ing about bits per chan­nel or the over­all num­ber of bits, but per-chan­nel is usu­ally used (eight bits per chan­nel means 24 bits per pixel – eight for red, eight for green and eight for blue).

As you can see from the ta­ble be­low, even an eight-bit JPEG has more colour op­tions than we need for view­ing an im­age – it records 16.7 mil­lion colours, and we can only see about 10 mil­lion. That would be more than enough were it not for the work that needs to be done on dig­i­tal images. If you make big changes to bright­ness or colour, for ex­am­ple, you are es­sen­tially in­creas­ing the dif­fer­ences be­tween ad­ja­cent pix­els. If the bit-depth is low when you do this (i.e. if there aren’t enough colours and tones to work with), there’s likely to be a no­tice­able jump, and this can ap­pear as a hard line sep­a­rat­ing two ar­eas. This is called pos­ter­i­sa­tion, be­cause the re­sults can look like blocky poster art. it oc­curs across the im­age, but is most ob­vi­ous in ar­eas of sub­tle vari­a­tion in tone, where it man­i­fests it­self as ob­vi­ous band­ing.

There are two oc­ca­sions when this hap­pens. It al­ways hap­pens when con­vert­ing the fresh­ly­cap­tured im­age from lin­ear to view­able (in other words, when ap­ply­ing the 2.2 gamma curve, de­scribed on page 73), and it may hap­pen if you process an im­age ex­tremely, for ex­am­ple when try­ing to save an un­der-ex­posed shot.

This is where hav­ing a higher bit-depth than eight-bit makes a dif­fer­ence. The more colours you have to play with, the less dam­age is done when they’re stretched out by the gamma curve. That’s why some Nikons, like the D810 and D4s, of­fer you the choice of 12-bit or even 14-bit cap­ture. Both take up more space, and can slow down the shoot­ing a lit­tle, but they en­sure bet­ter im­age qual­ity, es­pe­cially in the dark ar­eas. RAW, and in par­tic­u­lar 14-bit RAW, def­i­nitely has the edge with very high-con­trast scenes.

Be­cause a RAW file con­tains more data, it still looks good after boost­ing

When we boost the tones in the JPEG ver­sion, we get hor­ri­ble pos­ter­i­sa­tion

Here’s an im­age straight out of the cam­era – it looks a lit­tle bit flat

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