Cam­era Col­lege: His­tograms

What’s that graph shape try­ing to tell you about your ex­po­sures?

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When you’re as­sess­ing the ex­po­sure of your pho­tos, what you see on your cam­era’s screen or elec­tronic viewfinder (EVF) isn’t nec­es­sar­ily what you’ll get. The bright­ness of the dis­play in­flu­ences how bright an im­age looks – with the bright­ness of the am­bi­ent light that you’re view­ing your pic­tures in also hav­ing an ef­fect. You may find that your pho­tos are ac­tu­ally un­der­ex­posed or over­ex­posed when you later check them on a com­puter.

This is where the his­togram comes in. This small graph, which can be viewed along­side your im­ages, al­lows you to quickly make a more ac­cu­rate ap­praisal of the ex­po­sure. We in­tro­duced the his­togram in last is­sue’s guide to me­ter­ing, but we’re go­ing into more depth here, as be­ing able to un­der­stand what the his­togram is telling you is one of the fun­da­men­tal skills of mas­ter­ing dig­i­tal photography.

You can check the his­togram when you play back an im­age, or in real time when you use the Live View feed on your cam­era’s rear screen or EVF. A live his­togram means you can check the ex­po­sure and make any ad­just­ments be­fore you take a photo.

Both of these bright­ness his­tograms do the same thing: they show you the ‘tonal range’ of an im­age – from black on the left through to white on

the right, with a spread of grey tones in be­tween. The height of the his­togram shows you how many pix­els have reg­is­tered at each level of bright­ness, and the his­togram’s shape and po­si­tion on the graph helps you to gauge whether or not your im­age has been prop­erly ex­posed.

There isn’t re­ally a ‘per­fect’ his­togram shape. The spread of tones changes ac­cord­ing to the scene or sub­ject that you’re pho­tograph­ing and the ex­po­sure set­tings on the cam­era. You would ex­pect the his­togram for a pre­dom­i­nantly bright or pale scene to have a his­togram that peaks to­wards the right side, while the his­togram for a very dark sub­ject should be closer to the left side of the graph. The ma­jor­ity of scenes, how­ever, are a mix­ture of shad­ows, mid-tones and high­lights, and are more likely to show a his­togram that rises and falls across the width of the graph.

If the spread of the his­togram doesn’t match the bright­ness of the scene or sub­ject you’re pho­tograph­ing, then the ex­po­sure may be in­cor­rect and you may want to take steps to fix it. For ex­am­ple, if you take a pic­ture of a snow-cov­ered land­scape and the his­togram isn’t to­wards the right side of the graph, the snow will ap­pear grey in the photo rather than white. You can use ex­po­sure com­pen­sa­tion to rec­tify an in­cor­rect ex­po­sure, with pos­i­tive com­pen­sa­tion push­ing the his­togram to­wards the right (bright) side of the

There isn’t re­ally a ‘per­fect’ his­togram shape. The spread of tones changes ac­cord­ing to the scene or sub­ject that you’re pho­tograph­ing and the ex­po­sure set­tings on the cam­era

graph, and neg­a­tive com­pen­sa­tion pulling it to­wards the left.

The his­togram on the cam­era is small, so it can be dif­fi­cult to see when only a few hun­dred or so pix­els are over­ex­posed; and it’s highly un­likely that the high­light alert or ze­bra stripes fea­ture avail­able on some cam­eras will in­di­cate smaller over­ex­posed ar­eas. If record­ing del­i­cate de­tails in bright ar­eas is im­por­tant to the pic­ture, re­duce the ex­po­sure to pre­serve the high­lights, as it’s eas­ier to brighten darker parts of an im­age when you edit your im­ages than it is to try to claw back de­tail in over­ex­posed ones.

While it might seem sen­si­ble to rou­tinely un­der­ex­pose shots in or­der to avoid clip­ping the high­lights, if you then try to brighten up a dark shot in photo-edit­ing soft­ware, you can in­crease the amount of noise. In fact, if you shoot raw files there’s an ar­gu­ment for rou­tinely over­ex­pos­ing your shots so that the his­togram is close to the right edge with­out be­ing clipped, then re­duc­ing the ex­po­sure back to the cor­rect level when you process the im­ages. The the­ory is that more tonal in­for­ma­tion and de­tail is recorded in the right side of the graph, and you also re­duce the chance of noise in the shad­ows.

This tech­nique shouldn’t be used for JPEG im­ages though, and it’s best to aim for a spot-on ex­po­sure in the cam­era. Good job there’s a his­togram to help you…

The his­togram on the cam­era is small, so it can be dif­fi­cult to see when only a few hun­dred or so pix­els are over­ex­posed

End re­sult The peaks and troughs of a his­togram al­low you to judge the ex­po­sure – and in some cases, the in­ten­sity of the colours you’ve shot…

Mar­cus Hawkins Pho­tog­ra­pher and writer Mar­cus is a for­mer ed­i­tor of Dig­i­tal­Cam­era.

His­tograms

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