Exposure compensation, white increase and black decrease theory

Exposure compensation , White plus black minus principle

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         Correct exposure is one of the necessary conditions for a good photo , How to make the exposure correct ？ Metering of the camera ？ Own experience ？ I believe everyone has his own experience .

         There is a rule that I believe everyone has heard , Namely “ White plus black minus ”！ What does that mean ？ That is to say , When there is more white in the picture , If you want the exposure to be correct, you have to add compensation ; When there is more black in the picture , If you want the exposure to be correct, there is subtraction compensation .

         Some people are surprised , When there is more white, add ？ But it was exposed ？ When there is a lot of black, it is reduced ？ No more exposure ？ Why is it just the opposite ？ But the truth is really so strange , If you don't believe it, you can try .

         The light meter of the camera is very sensitive to it “ see ” Any scenery you see will tell the camera ：“ You adjust this picture to 18% ash .” So the camera adjusts the shutter and aperture to reach that combination .

         for example , To the extreme ,“ see ” To all white paper , The camera also adjusts the shutter aperture to make the white paper 18% Gray image ,（ Because the white paper reflects strongly , Aperture shutter automatically reduces exposure using a small aperture and a high-speed shutter ,） The paper is white “ ash ” paper , Equivalent to under exposure , Want to make it white , You need to artificially increase the exposure compensation .

        “ see ” To all black paper , It will also ask the camera to make black paper 18% ash ,（ Because the black paper has no light , The shutter and aperture should automatically increase the exposure. Use a large aperture and a slow shutter ,） The black paper is also made “ ash ” paper , It is equivalent to overexposure , If you want to make it black , You need to reduce exposure compensation .

        “ see ” To 18% Grey paper , It will tell the camera that this is 18% ash , Gray paper is still gray paper , Of course, no plus or minus exposure compensation . This is it. “ White plus black minus ” The origin of the statement .

         I have read many masters' works recently , Combined with my recent shooting , There is a new understanding of white plus black subtraction and exposure compensation , Of course, these understandings are only in the stage of consciousness , There is no specific quantitative stage . To be brief .

         Let's not talk about white plus black minus , use Baidu Search , The explanation will be very clear . If the environment is very bright , become pale （ Such as snow ）, Then when the camera measures light, it will feel that the ambient light is sufficient , Photometric data （ aperture ＋ Shutter ） It will be lower than the actual level , So we have to compensate when we take photos . contrary , The truth of black subtraction . But in practice , There is no big black and white scene , Compensation is often just 0.3,0.7 Such a small compensation , When can we add and subtract at this time ？ I have summed up some experience in shooting portraits .

         Portraits , So the subject is a portrait （ face ）, And the background , The use of white plus black minus at this time , It should be converted into a portrait （ face ） And background color to determine positive or negative compensation . For example, a backlight scene , Often the background will be very bright , And the face is dark , So many people will say that we should make compensation , Polish your face more . But I think , This is not entirely correct , It depends on the situation , Generally speaking, there are 2 In this case ：1. Give positive compensation . Just as people think , Positive compensation , But there are certain conditions for this situation , Is the general body , Bust , The area of the background is much larger than the area of the human face , This is the time （ It is also a scene that people often shoot ） To use positive compensation ;2. Negative compensation , This situation , The same is true of backlighting , But take a big close-up of the person's face , The area of the face is much larger than the area of the background , At this time, the face of the person is dark , At this point, a little negative compensation is given , Because the dark part of the whole picture is much larger than the bright part , The light metering value of the machine will be too high , We give negative compensation .

         So in conclusion , White plus black minus is not absolute , It still depends on the subject and background you shoot , Their relative area and contrast . let me put it another way ,“ White plus black minus ” Medium “ white ” and “ black ” It is often thought to be “ background ”, But this “ background ” It is often relative , It is possible that the subject becomes “ background ”（ For example, a close-up of the face ）, That is this. “ background ” It refers to the larger part of the whole picture , Maybe the background , It could be the subject . therefore , When applying white plus black minus , The concept of background and subject should be dissected , Instead, you should focus on the brightness of large areas in the photo to use white plus black minus .

         Because exposure compensation is closely related to metering mode , Let's first define the concept of several metering modes ：

1. Evaluation photometry ： In detecting the position of the subject 、 brightness 、 background 、 After smoothing and backlighting , The camera sets the appropriate exposure parameters .
2. Central focus average photometry ： The metering is biased towards the center of the viewfinder , Then average to the whole scene .
3. Local photometry ： It covers the center of the viewfinder about 8％ The area of .
4. Spot metering ： It is used to measure the light of a specific part of the subject or scene . The metering is biased towards the center of the viewfinder , It covers the center of the viewfinder about 3.5％ The area of .

【 notes 】 Attention, everyone , The scope of photometry is from the evaluation of photometry 、 Central focus average photometry 、 Local photometry 、 Spot metering , Step by step, focus on the center of the viewfinder . The small circle in the middle of the viewfinder is the coverage of spot metering . The smaller the metering range, the more accurate the metering of the main body , It also gives the photographer more creative space .

One 、 Shoot Ming 、 When the subject is in a dark background “ White plus black minus ” Application of principles .
         The probability of this scenario is very high , such as ： The background is egrets in the woods , A typical dark background with a bright body ; Butterflies on yellow flowers , Typical bright background and dark body .

         This kind of scene is that the coverage of the metering mode you choose exceeds the main body , Conditions brought to part of the background , Even if you choose point metering . At this time, the camera's calculated exposure is the subject covered by the comprehensive metering range 、 The light reflected from the background , That is, a comprehensive average .

         If the exposure is not compensated , The exposure of the subject is not accurate . The egret body in the forest will be too violent , Lose feather level detail （ Need to reduce the blast ）; The butterfly on the yellow flower will be less violent , The beautiful patterns of the wings do not show （ Need to be increased ）.

【 Conclusion 】 Right now “ White plus black minus ” The understanding of should be like this ：“ white ” A background that is bright relative to the subject , That is, yellow flowers （ Not egrets ）;“ black ” A dark background relative to the subject , That is, the woods （ Not a butterfly ）.

Two 、 It is specified that 、 The dark subject is right “ White plus black minus ” Application of principles .
         The metering system of the camera is a reflection meter , Generally, the point metering angle of a handheld reflective light meter is 1 to 5 degree , Very precise . Canon camera spot metering is not strictly called ” spot “ metering , I even doubt 5D The spot metering range will be beyond the small circle in the center of the viewfinder （3.5％ The area of ）. Try Nikon D80, It should be said that the spot metering area of Nikon is smaller than that of canon .

         This scenario generally means that the coverage of the metering mode you choose is within the main body , It mainly refers to point metering 、 Close up in local metering mode . For example, close-up of black cat ; Close up of white rose . If the exposure is not compensated , The picture shows the grey cat ; Grey rose .

         What's the matter ？ This requires understanding the principle of photometry . The photometry system is used to measure the field light conditions 18％ Grey light reflects , It is not the colorful world we see in our eyes . So we should shoot the real world , It is up to the photographer to compensate for the exposure . If you don't want to turn a black cat into a gray cat, you need to reduce the violence , almost 2 files . If you want a white rose or pure white , You're going to increase your rage .

【 Conclusion 】 Right now “ White plus black minus ” The understanding of should be like this ：“ white ” Is a relatively bright close-up of the subject , It is also a white rose ;“ black ” Is a relatively dark subject , That is, black cat .

3、 ... and 、 A close-up of a relatively balanced subject “ White plus black minus ” Application skills of principles .
         The principle of photometry mentioned above is to measure the light at the scene 18％ Grey light reflects . It is said that many professionals will bring a piece of 18％ The grey board is used as a light measuring tool . First measure under the field light conditions 18％ Exposure of gray board , Then take this as the basis to shoot the subject with relatively balanced reflection . For ordinary fans , You can use the back of your hand instead 18％ The grey board is used as a light measuring tool . The amount of skin color reflection on the back of the hands of the yellow people is 18％ The grey board is similar , Of course , Maybe there's a cool guy or girl who spends a lot of time in the wild , The back of the hand is dark . You can use the camera's metering system to compare 18％ Use the gray board to measure whether the exposure on the back of your hand should be compensated .

【 Conclusion 】 Right now “ White plus black minus ” The understanding of should be like this ：“ white ” It refers to the white back of the hand , Especially the white skin beauties who usually sit in air-conditioned office buildings ;“ black ” It refers to the back of the hand with dark skin , Including African friends .

Make use of exposure compensation , Remember the principle of increasing white and decreasing black
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        You should pay attention to the value in the lower right corner of the display ,0 EV. This value is the exposure compensation . Yes -2.0EV、-1.7EV、-1.3EV、-1.0EV、-0.7EV、-0.3EV、0EV、0.3EV、0.7EV、1.0EV、1.3EV、1.7EV、2.0EV Optional . If the value is negative , Indicates reduced exposure , A positive value indicates increased exposure . The greater the numerical , Indicates an increase in / The greater the reduction .
Everybody knows , Because the camera is based on light 18% Reflection ratio to determine the amount of exposure , Ambient light outside this value , The camera won't work correctly CCD On the right expression , Therefore, the method of exposure compensation must be used .
        For example, when shooting snow scenes , Most of the scenery in the viewfinder box is white , The camera is based on 18% Reflection of , If you think the light is too bright, you will take corrective action , That is to speed up the exposure time （A Pattern , When aperture has priority ） Or reduce the aperture （S Pattern , Shutter priority ）. Therefore, if exposure compensation is not used （ That is, the exposure compensation value is 0）, So the picture is not exposed , The color of snow is not pleasant pure white , It's getting gray , Dark colors .
        Empathy , If you are shooting a black object , The camera is based on 18% Reflection of , If you think the light is too dark, you will take corrective action , That is, lengthen the exposure time （A Pattern , When aperture has priority ） Or increase the aperture （S Pattern , Shutter priority ）. Therefore, if exposure compensation is not used （ That is, the exposure compensation value is 0）, So the picture is over exposed , Details will be lost .
        The extension is ： When most of the objects in the viewfinder are bright colors or have strong reflections , We should use the method of increasing exposure compensation to get the correct exposure ; When most of the objects in the viewfinder are gray or reflective , We should use the method of reducing exposure compensation to get the correct exposure .

        This is just the opposite of what people think . But just remember “ White increases and black decreases ” And flexible application , You can get the correct exposure .

What is the white plus black minus principle

         In physical optics , Think of different colors , Contrast the difference of color scale , Everything comes from the panchromatic absorption and reflection of the object to the light source . Theoretically , Light is absorbed much more on the scene , There will be less reflection ; If total reflection , The absorption is considered to be zero ; If fully absorbed , The reflection is considered to be zero . In the middle of it , There is always a period of light and shadow color scale , In most cases, make the scene most suitable for viewing , This is the middle gray tone . If you use a light meter to measure , The bright colors in the scene （ white ）、 Middle tone （ ash ）、 Dark colors （ black ） After mixing , Calculate the comprehensive light reflectance of the scene , for 18%. Be commonly called ：18% ash , Or say 18 ash . Accordingly , The metering of all cameras is based on 18% The middle gray is the light reflectance reference , Determine the exposure combination of the shutter and aperture value of the camera . As a result, , The metering of the camera is accurate and rigid . Within the photometric range , The reflectance of scene light exceeds 18%, Think the object is too bright , It's too white , Automatically reduce the exposure , Pull the exposure combination back , The shot will be an underexposed photo , A white background looks gray .; If the reflectivity of the scene is lower than 18%, Think the object is too dark , It's dark , Just increase the exposure , Just put the exposure combination up to 18% Exposure . Photos taken tend to be over exposed , The black background will also turn grey .

         thus , When the exposure is close to the reflectivity 18 Grey scenery , Then the camera behaves dutifully , Perfectly reproduce the scenery . But the reality is not so arbitrary , It is often changeable . Make the film , This item is black or not , Is it white . It often feels gray , Whether you have nikon D200 Or canon 5D, And no matter what metering you have in your machine 、 Weighted focus 、 Central evaluation photometry, etc . It's a high-end camera , At this time, it will also show the essence of a papaya head . The final exposure combination , It's still up to you to decide . You can't trust the camera . So according to the photometric data of the camera , Then according to the specific situation at present , And personal light and shadow 、 color 、 To examine beauty concept , To determine the final shutter and aperture . otherwise , Everyone can become a master photographer . The solution is the four character classic ： White plus black minus . That is, to put it more popularly ： Too white scenery , Make it whiter , Add light ; Dark scenery , Make it darker , Dimming . The light and shadow effect of the film , It depends to a large extent on your understanding of 18 Grey's perception and understanding ; Is your judgment correct , Determines your ability to grasp the causes of the film . This requires learning and experience . So when shooting white objects 、 Or white objects 、 Light colored objects 、 When the proportion of bright sky is large , You need to increase the exposure based on the automatic exposure of the camera . Black in the scenery 、 The dark part 、 When dark objects account for a large proportion , It is necessary to reduce the exposure on the basis of automatic camera exposure .

         Now? Digital camera It's much easier to use , Call up during viewfinder or playback “ Histogram ” It's clear at a glance . however , If you can master the basic principles of exposure compensation , It can realize small-scale step-by-step compensation , Greatly improve the success rate of shooting . Here is a general view of the reflectivity of the scene and the increase or decrease of the exposure , Reference data . The list is as follows ：

• Silver polished mirror reflectivity ：92% Exposure compensation upshift ：2 Above file ;
• Pure white cloth Reflectivity ：80~85% Upshift ：2 files ;
• New snow 70~74% Upshift ：1~2 files ;
• White walls 60% Upshift ：1~2 files ;
• wood plate 45% Upshift ：1/2~1 files ;
• Grey paint 20~30% Upshift ：1/2~1 files ;
• tender leaf 20~25% Upshift ;1/2~1 files ;
• Red bricks 20% normal
• Bright sand 、 Rocks 18~24% normal
• Bright green leaves 15~20% normal
• Black paint 10% Downshift ： 1/2~1 files ;
• Dark sandstone rock 6~10% Downshift ： 1 files ;
• Dark green leaves 1~10% Downshift ：1~2 files ;
• Black paper 1~2% Downshift ;2 Above file ;
• Black velvet 0.5% Downshift ：2 Above file . 

The following figure shows the relationship between reflectivity and image brightness ：

Reflection and absorption of light  

         The reflection effect of light enables us to see and know the shape of objects 、 Color and texture , This is the basic principle that we take pictures with cameras . Of course , Use the reflection of light creatively , You can also get unusual scenes . Absorption of light , Is when light passes through or reaches certain objects , The light is attenuated , The disappearance or disappearance of certain chromatograms . We often see dark colored objects , They have a very strong ability to absorb light . 
In optical principle , Suppose there is an object , It is absolutely black , It doesn't reflect light at all , We say its reflectivity is 0; At the other extreme , An object is absolutely white , Then it will reflect all the light , We say its reflectivity is 100%.

The reflectivity is 18% The gray tone of  

         In real life , Absolutely reflective （ Or non reflective ） The object of does not exist , All objects are at some point between these two extremes . After expert research , The usual bright hue in the subject 、 After the middle tone and dark tone are mixed , Will produce a reflectivity of 18% Medium grey tone , Based on this, we decide to expose and take photos , It can really restore the original appearance of the scene in most cases . So the reflectivity is 18% The medium gray tone becomes the benchmark of the camera light meter . 
Camera auto exposure restores scene light reflectance 18% 
         The processor in the camera will be based on the light meter , And reflectivity is 18% And determine the exposure combination of the camera aperture and shutter . Under normal light conditions , The camera can restore the scenery we see correctly . Because under the uniform light , The comprehensive reflectance of these scenes is close to 18% The gray tone of . If the light reflectance of the scene exceeds 18%, The camera thinks the scene is too bright （ white ） 了 , Automatically reduce the exposure , Let the scene appear as light with a reflectivity of 18% The gray tone of ; conversely , The light reflectance of the scene is lower than 18%, The camera thinks the scene is too dark （ black ） 了 , Automatically reduce the exposure , Let the scene appear as light with a reflectivity of 18% The gray tone of

White plus black minus “ Black and White ”

         No one wants to put their furry and cute white cat , Make it a dirty grey cat , But according to the camera's internal light measurement and automatic exposure will cause this phenomenon . Need basis “ Baijia ” Theorem , Increase exposure , Show cute “ white ” cat . The same white wall has a very high reflective rate “ Baijia ” Theorem , Need to increase exposure significantly , To restore the “ white ”.

“ Baijia ” set The reason is

         When shooting white objects , It is necessary to increase the exposure based on the automatic exposure of the camera .

“ Baijia ” Theorem inference 1>

         When the white objects occupy a large proportion of the scene , It is necessary to increase the exposure based on the automatic exposure of the camera . 
“ Baijia ” Theorem inference 2>

         In the scene taken , When the bright part occupies a large proportion , It is necessary to increase the exposure based on the automatic exposure of the camera . 
“ Baijia ” Theorem inference 3>

         When the light color objects occupy a large proportion of the scene , It is necessary to increase the exposure based on the automatic exposure of the camera . Such as the ginkgo tree after the rain in late autumn , The leaves change from dark green to bright yellow , meanwhile , The water droplets attached to the leaves after the rain have increased the brightness . If you want to show this bright autumn , Reason according to Berger's theorem 3, It is necessary to increase the exposure .

“ Black subtraction ” set The reason is

         When shooting black objects , It is necessary to reduce the exposure based on the automatic exposure of the camera .

“ Black subtraction ” Theorem inference 1>

         When the proportion of black objects in the scene is large , It is necessary to reduce the exposure based on the automatic exposure of the camera . 
“ Black subtraction ” Theorem inference 2>

         In the scene taken , When the dark side accounts for a large proportion , It is necessary to reduce the exposure based on the automatic exposure of the camera . When shooting in backlight , The shadow often takes up a large proportion , In especial The lens There is no intake of bright in space time , The shadow of the dark part will affect the auto exposure of the camera , Cause overexposure . So we need to follow “ Black subtraction ” Theorem inference 2 Reduce exposure . 
“ Black subtraction ” Theorem inference 3> When dark color objects take up a large proportion of the scene , It is necessary to reduce the exposure based on the automatic exposure of the camera .