The Histogram and Picture Brightness

It’s help­ful to review a pho­to­graph after tak­ing it to ensure that your expo­sure is cor­rect. In most cir­cum­stances, you can judge the expo­sure by look­ing at the pre­view pic­ture on the LCD via the play­back func­tion. Nev­er­the­less, there may be occa­sions when the light­ing around you is bright and casts reflec­tions on the small dis­play, which can make judg­ing a photograph’s expo­sure dif­fi­cult. A more objec­tive way to judge a photograph’s expo­sure is to refer to its his­togram.

What is a histogram?

In pho­tog­ra­phy, a his­togram is a graph­i­cal rep­re­sen­ta­tion of the num­ber of pix­els that occu­py each tonal val­ue on a grade from pure black to pure white. When look­ing at a his­togram, the hor­i­zon­tal axis rep­re­sents tonal gra­da­tion, and the ver­ti­cal axis rep­re­sents the rel­a­tive quan­ti­ty of pix­els of a par­tic­u­lar tonal val­ue. Put dif­fer­ent­ly, the height of the graph at any point on the hor­i­zon­tal axis rep­re­sents the num­ber of pix­els of that tone.

In dig­i­tal pho­tog­ra­phy, an image his­togram is always gen­er­at­ed from a processed source image. Your cam­era gen­er­ates his­tograms using the tonal vari­a­tions in the JPEG for­mat images, or the JPEG pre­view images attached to raw files. In most cam­eras and soft­ware suits, his­tograms aren’t gen­er­at­ed from the unprocessed data in raw files.

Some mir­ror­less cam­eras can dis­play a live his­togram in the EVF or on the rear LCD dur­ing shoot­ing mode, which can help you under­stand your scene and expo­sure set­tings before tak­ing a pho­to­graph. 

How the histogram is displayed in Adobe Lightroom Classic CC.
Adobe Light­room Clas­sic CC dis­plays the image his­togram in the upper right cor­ner of the Devel­op mod­ule.

Interpreting the image histogram

In gen­er­al, a his­togram is use­ful only as a con­tex­tu­al guide. In the absence con­text, being either the source image or a good rec­ol­lec­tion of the com­po­si­tion, a his­togram can only tell you how bright the result­ing pho­to­graph is, but not how cor­rect­ly it’s exposed. Although relat­ed, the con­cepts of bright­ness and expo­sure are not inter­change­able. Whether a pic­ture is dark, bright, or aver­age depends both on the subject’s reflectance and the over­all expo­sure.

For exam­ple, a cor­rect­ly exposed pho­to of a black cat in a coal mine (low reflec­tiv­i­ty) will pro­duce a dark image with a cor­re­spond­ing­ly left-skewed his­togram. A cor­rect­ly exposed pho­to of a white dog on snow (high reflec­tiv­i­ty) will cre­ate a bright image with a cor­re­spond­ing­ly right-skewed his­togram. How­ev­er, with­out the source images for ref­er­ence, a his­togram offers no clues about lev­els of reflec­tiv­i­ty or expo­sures.

Dark­er or “low-key” pho­tos pro­duce his­tograms with graphs that are skewed towards the left. Brighter or “high-key” pho­tos pro­duce graphs that are skewed towards the right. Nor­mals pho­tos result in graphs that are spread about the mid­dle because they fea­ture sub­jects with a nor­mal dis­tri­b­u­tion of tonal val­ues from bright, mid­dle, and dark.

To reit­er­ate, a his­togram is not a reli­able mea­sure of over­all expo­sure. Con­trary to many guides for begin­ners, a cor­rect­ly exposed pho­to­graph won’t nec­es­sar­i­ly pro­duce a his­togram with a loose bell-shaped curve extend­ing out­ward from the mid­dle.

 

Histogram of a photograph with average tonal distribution.
An aver­age expo­sure of an aver­age scene with an aver­age dis­tri­b­u­tion of tonal val­ues pro­duces an aver­age-look­ing his­togram.
Histogram of a photograph with average tonal distribution.
This is the sort of aver­age his­togram most online guides and cam­era man­u­als say rep­re­sents a cor­rect­ly exposed pho­to­graph.
Example of a high-key or bright photograph and histogram.
This high-key or light-toned pho­to­graph is cor­rect­ly exposed and doesn’t result in an aver­age-look­ing his­togram.
Example of a high-key or bright photograph's histogram.
The his­togram of a high-key or light-toned pho­to­graph should be biased towards the right side when cor­rect­ly exposed.
Example of a low-key or dark photograph and histogram.
Like­wise, this low-key or dark-toned pho­to­graph, inten­tion­al­ly exposed to cap­ture the appro­pri­ate mood, results in a his­togram with a sig­nif­i­cant vol­ume of dark-toned pix­els.
Example of a low-key or dark photograph's histogram.
Since the pho­to­graph is inten­tion­al­ly exposed to cap­ture dark areas of the scene as dark, the his­togram has a sig­nif­i­cant bias towards the left side.

Histograms and highlights

His­tograms are use­ful for judg­ing the expo­sure (and over­ex­po­sure) of high­lights. High­lights occu­py the far right por­tion of a histogram’s tonal range. This is the por­tion of the his­togram rep­re­sent­ing cor­rect­ly-exposed bright sub­jects, such as white clouds, wed­ding dress­es, and fluffy Samoyeds. When cor­rect­ly exposed, light-toned sub­jects should look bright while retain­ing details and tex­tures. When a light-toned sub­ject is over­ex­posed, it will appear com­plete­ly white and fea­ture­less, and no amount of edit­ing to reduce image bright­ness will recov­er the lost infor­ma­tion. Over­ex­posed high­lights are fre­quent­ly called “clipped” or “blown.”

Gen­er­al­ly, the pres­ence of some clipped high­lights is no cause for wor­ry, and they’re often unavoid­able. For exam­ple, pho­tos that inci­den­tal­ly include car head­lights and bare light­bulbs, open flames, or the sun and its reflec­tion on a pol­ished sur­face will have some blown high­lights, and that’s both accept­able and (most­ly) unavoid­able. Over­ex­posed high­lights are con­cern­ing when they occur on the main sub­ject, such as a face in a por­trait.

 

Contrast

Contrast

You may deter­mine the amount of con­trast in a pho­to­graph by ana­lyz­ing its his­togram. Con­trast describes the vari­a­tion between the light and dark areas of a pho­to­graph. A low con­trast image will result in a his­togram with a large vol­ume of pix­els con­cen­trat­ed along a rel­a­tive­ly nar­row range of tones. A high con­trast image will often pro­duce a his­togram with a broad dis­tri­b­u­tion along the tonal range, or sev­er­al nar­row promi­nences set far apart.

 

Contrast Media

Histogram of a low contrast photography.
Exam­ple of a low con­trast pho­to­graph. Notice how the his­togram has sev­er­al promi­nences that do not spread wide across the full tonal range.
Histogram of a high contrast photograph.
Exam­ple of a high con­trast pho­to­graph. Notice how the his­togram is spread through­out the tonal range rep­re­sent­ed by the graph.

Brightness and RGB histograms

Many cam­eras fea­ture two types of his­tograms: bright­ness and RGB (red, green, and blue). Bright­ness his­tograms assign a bright­ness val­ue to every pix­el in a pho­to­graph and use that to pro­duce a graph. The colour of indi­vid­ual pix­els is ignored in favour of show­ing total scene bright­ness. How­ev­er, these can be deceiv­ing in the pres­ence of vibrant, sat­u­rat­ed sub­jects. 

Dig­i­tal cam­eras pro­duce colour images by a clever com­bi­na­tion of red, green, and blue light. Each dig­i­tal image can be sep­a­rat­ed into its indi­vid­ual red, green, and blue con­stituents or “colour chan­nels.” Each colour chan­nel is a mono­chro­mat­ic rep­re­sen­ta­tion of the bright­ness val­ues for that colour.

An RGB his­togram presents three sep­a­rate his­tograms that plot the bright­ness val­ues of the red, green, and blue colour val­ues for every pix­el in the pho­to­graph. RGB his­tograms are use­ful for trou­bleshoot­ing over­ex­po­sure in indi­vid­ual colour chan­nels.