Lin­ear capt ure and ga mma CU RVES

Cam­eras don’t work like our eyes – that’s why it’s eas­ier to re­cover de­tails from shad­ows in RAW files

NPhoto - - Nikopedia -

We treat the way we see as nat­u­ral and straight­for­ward, yet it’s any­thing but. Even our ba­sic re­sponse to dif­fer­ent lev­els of bright­ness is com­plex. We see twice the bright­ness, for ex­am­ple, as only a slight in­crease, and we are aware of changes in dark tones much more than in bright ones. In other words, the re­sponse of our eyes to light is non-lin­ear. A sen­sor, how­ever, is lin­ear. Twice the bright­ness is recorded as twice the sig­nal. The re­sult is that a lin­ear im­age cap­ture, which is what your cam­era gets as soon as you take a shot, would look very dark. You never get to see this, be­cause the cam­era au­to­mat­i­cally ap­plies a cor­rec­tion to make it look nor­mal. To match the way our eyes work, this cor­rec­tion is not an over­all in­crease in bright­ness, but an in­crease that favours the dark tones. In the form of a graph it is a steep curve, as shown above, known as a 2.2 gamma cor­rec­tion. Seen as a tonal range from black to white, the cor­rec­tion spreads the tones out evenly. This is why a RAW file al­lows you to re­cover de­tails in an im­age that might ap­pear to have been lost by a less-than-per­fect ex­po­sure, par­tic­u­larly in the shad­ows. Ap­ply­ing a stronger gamma curve will pull more vis­i­ble de­tail out from those densely packed shadow ar­eas. In other words, most of the in­for­ma­tion in a just-cap­tured im­age is clumped in the dark ar­eas, and needs to be pulled out.

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