2011-11-24

Using an 18% Grey Card

How to article: Using an 18 percent Grey Card.  This article discusses using in-camera reflectance metering with an 18% grey card.  This article applies to both film and digital cameras.

An 18% grey card allows you to set the camera's exposure using a known reference and the card will give the 'correct' exposure for any scene, regardless of the subject.  I use this method frequently.

Grey cards are literally a grey rectangle made of cardboard, plastic or cloth, and can be purchased from any camera store for about $15 to $30.  What they all have in common is they are made of a photographic neutral shade, averaging 18% grey.  Ideally, you want one that can be folded or cut so it fits in your camera bag.    

Contents:
  • Using a Grey Card
  • Why use a grey card (the proof)
  • Interpreting the results
  • Grey Card reading + 0.5 stops
  • Dark Subjects, night scenes overexposed
  • Light and backlit subjects
  • Controversy and exceptions

Why a Grey Card?

With your camera, do this 1-minute experiment to prove why you need a grey-card:

1.  Place a dark sheet of paper or a dark magazine on the table-top, then cover it with a white sheet of paper.

2.  Using the camera, focus on the white paper so it occupies most of the frame.

3.  Meter the scene, noting the camera's recommended shutter speed and aperture.

4.  Remove the sheet of paper and meter the darker scene.  Note the different exposure.

Questions:  
  • ? Knowing both subjects were in the same lighting, why does the meter reading change when  the subject's color is different?  Isn't the subject in the same quality and quantity of light?
     
  • ? Which meter reading should you use if both the dark and white subject are in the same scene?  

The Answer:  Neither the white or black-paper reading is correct. If you were to actually take the photographs, you would see the white paper looks grey, not white, and the dark paper will look grey, not black.  The issue is this: The camera meter is fooled by the color of the scene and will not set the exposure correctly.


The Camera Meter is Usually Wrong

When a majority of a scene is "whiter than normal" (snow, concrete, light colored walls, backlighted) or "darker than normal" (black subjects, theater stages with dark curtains, night shots), all cameras meter incorrectly.

But here is the surprise: It turns out with almost any scene, any subject, under any lighting condition, the camera's meter reading is wrong to some degree.  This is true with landscapes, portraits and still lifes.  Granted, on an average scene, the meter is close and you will get an acceptable picture, but almost always, a more accurate value can be set with an inexpensive grey card.  

To explain this another way, the camera meter expects a scene to have an average number of rocks, trees, sky, people and things; this is how it is calibrated.  According to Kodak, if you took all these items and spun them in a black-and-white blender, you would get something along the lines of an 18% grey.  Since the camera cannot tell what the subject matter is, it assumes grey and sets the exposure with this in mind.  Examples of missed exposures are shown below.

Better exposure means less post-processing.  Better exposures give better colors, with better shadow details and this is true with film, digital and digital RAW.  


(For reasons that are too complicated to explain here, all cameras, film and digital sensors are calibrated for 12 to 14% grey, depending on the manufacturer.  Grey Cards are calibrated at 18%, for black-and-white printing -- meaning the grey card is not quite right.  This article explains how to work around this.  The world really needs a 13% grey card.)


Buying a Card:


Grey cards can be made of cardboard, plastic or cloth and ideally should be waterproof -- but I have found some plastic cards too glossy.  My card is the cardboard variety and because it is not waterproof, I have to replace it every few years.  But being cardboard, I can cut it to any size; mine is about 6x8".  For in-camera metering, it should be at least 5x7" or larger.  The back of the card should be white, which I use for White Balancing.

You will see some credit-card-sized cards.  These are meant for post-processing and are too small to help with in-camera metering.

I do not have any brands to recommend, but here is an Amazon link to get you started:



Using the Card

For in-camera metering, follow these steps
  • For simplicity, set the camera to Av mode (aperture preferred) or Tv (shutter preferred)
  • Hold the grey card in the image area
  • It does not need to be accurately focused but it must be in the same quality of light as the subject.
  • In the view finder, it should occupy most of the image area.  Zoom if needed.

  • Angle the card between the lens and the main light source so it does not glare
  • Do not cast your own shadow on the card
  • Note the exposure (Shutter speed and aperture).  This is the exposure that should be used, plus 1/3 to 1/2 stop, as explained below.
Although the grey card does not need to be in focus while metering, some cameras, such as my Nikon D5100, struggle with auto-focus on a featureless grey card, especially in dim light. You won't be able to use a focus-lock on the edge, because that will also probably lock the exposure.  You may need to temporarily set the lens to manual focus.

Interpreting the Grey Card:

1.  While metering the card, note the exposure.
2.  Then remove the card and re-meter the actual scene.

3.  *If* the exposure changes (either the shutter speed or aperture changes), then exposure compensation is needed.  Use the grey-card's reading for your final exposure, plus a third to half-stop, as explained below.


I find it easiest to set the camera in either Aperture Preferred (Av) or Shutter Preferred (Tv) mode.  This way, only one meter value changes as the card is removed.  For example, if in Av mode, the aperture stays fixed and only the shutter changes.

In the viewfinder illustration above, the grey card metered at
125 @ f5.3 (this is the grey card's recommended exposure).
Without the card,
250 @ f5.3,

indicating a +1 stop (+1.0EV) over exposure is required.

Because the grey card is 18% grey, but the camera is calibrated to approximately 13%, add an additional 1/3 to 1/2 stop to the grey-card reading.  The variation of a 1/3 to 1/2 depends on your camera.  Some cameras jump in third-stop increments and others, especially older cameras, use half-stop increments.  The difference is immaterial.

The final adjustment can be seen on a graph:


Setting the Compensation:

There is no need to count or calculate the number of stops. 

Start with the camera's recommended meter reading.  Then, using your camera's Exposure Compensation controls ("+/-"), spin the dial until it reaches the grey-card's recommended setting, plus one click for the +1/3 (or +1/2) additional. 

Cameras vary in how this is done, but most work similarly.  Illustrated below, is the Nikon D5100's compensation controls.


When adding that last +1/3 to +1/2 stop, use 'one more click of the spin-dial' from the grey-card setting.  If under-exposing, spin the wheel the opposite direction (remember, you are always 'adding' the last-click - even if under-exposing); see the 'dark subject' example, below.  As a reminder, under-exposure means "less light," with shorter shutter speeds or narrower apertures but you still need to add that last 1/3 stop.

See also this handy reference/counting chart: Fractional Shutter Speed and Aperture chart.


Important Note: In order to use 'Exposure Compensation' the camera must be in a non-automatic mode (switch the camera's mode dial off of fully-automatic (the green mode) and place it in either Aperture Preferred (Av), Shutter-Preferred (Tv), Programmed (P).  Most cameras, in the green-squared, fully-automatic mode will not allow exposure changes.

A little history:  For decades, grey cards were simpler to use:  You would use the grey card's reading without any other adjustments.  But with digital cameras, histograms, and a lot of experience, along with the fact that camera makers publicized how their meters were calibrated, photographers came to the realization that additional compensation was needed.  You should now always add 1/3 to 1/2 stop from the grey card's suggestion, regardless if you are over or underexposing.


Re-Metering 

Once the Exposure Compensation has been set for a particular scene or composition, the camera can remain at this setting, even if the light changes.  For example, if a cloud passes overhead, the meter may change but you do not need to reset the compensation because the scene did not change.  Similarly, if you add a polarizing or other filter, you do not need to re-compensate.

However, if you re-compose the picture and point to a new subject,
  • Re-set the compensation to zero
  • Re-calibrate with the grey card. 
It is easy to forget to reset calibration to zero when re-metering the grey card.

Some people like to set their camera to pure-manual and set the exposure compensation that way.  I think this is a mistake and prefer using exposure compensation.  If the quality of light changes, the camera will automatically adjust, but in Manual mode, you would have to re-meter.


Examples: 

Dark Subjects = Under Expose


Camera meters are fooled when a dark or light subject fills most of the frame; especially if you are using a center-weighted metering system.  The subject's color can affect the meter.

You have probably seen metering errors. Here is a great example from Canon Camera's website, showing how a darker than normal subject can fool the camera meter into over-exposing.  Note how the black car is nearly grey.  You would see the same problem taking a night or stage photo where most of the scene is black.  In each case, the camera would overexpose:


To properly render a dark subject, under-expose from the camera's recommended meter reading.

This is counter-intuitive.  Remember, you are compensating for errors made by the camera meter.  To 'preserve the black, under-expose'.  In this case, you could guess at (-1 2/3) stops or use a grey-card to get the exact value.

For example, with a dark subject, the camera might recommend 1/100 @ f8 (over exposing) while the grey card might recommend a 1 and 2/3's stop under exposure.  As before, set the exposure compensation to the grey card's reading -- under exposing to a shorter shutter speed -- then spin the dial the other direction, adding that last +1/3 (or +1/2) stop.  

Adding the (+1/3) stop compensates for the camera's 12-to-13% vs 18%) calibration difference. 

For night landscapes, or dark night-time sky photos, a grey card will not work well.  Set the exposure compensation to -2 stops (2EV under exposure) and let the camera meter normally. 



Light Subjects = Over Expose


With snow scenes, the camera assumes the subject is likely about 14% (18%) grey and it will try like the devil to make it so.  It does this by (accidentally) under-exposing the snow, making it obviously grey.  You correct the faulty meter reading by manually "over-exposing."  This is also counter-intuitive: On a bright snowy day a good rule of thumb is to over-exposure the meter reading by 1 3/4 to 2.0EV stops.  This keeps the snow white.  Naturally, you can meter off a grey card to get the exact value.



Scenes where most of the image is white will demand a +2.0 stop over exposure -- provided the white fills most of the scene.  You will see this with white or light-colored walls, concrete, etc.. If other subjects occupy space in the same frame, you will need less of a value and it is even more important to meter the card.

Of interest, when deciding to compensate (say for snow), it is the color of the subject that determines what you need to do -- not the brightness of the light.  In the middle of the night, the same snow scene needs to be over-exposed the same +2 stops as on a bright or cloudy day.


Backlighting

Backlighting adds some complexity when using a grey card.  In this next example, the sun is in the top-center of the frame, the bright sky, the nearly white aircraft, and the nearly-white concrete runway all occupy most of the frame.  The camera incorrectly metered, under-exposing the scene (note especially the near silhouetted person, who humorously appears to be the real subject).  To compensate, meter with a grey card (or guess at a 1.0 to 1.5 stop over-exposure).

Greycarding in a backlit scene is more complicated.  In this example, hold the grey card, as you were facing the scene.  Literally, place the card right in the scene, as shown above, but you won't be able to angle the card into the sun.  The backlighting will cast the card in its own shadow.  If you were to use that exposure, the highlights in the scene would be blown into near pure white and the shadows would be as-if high noon.  In this instance, I would back-off the exposure 2/3rds of a stop (shorter shutter speeds or narrower apertures), allowing the shadows to remain.

(This photo is a good candidate for an HDR shot, as long as nobody were moving.)  If you were actually taking a picture of the woman in front of the plane, move in closer and take her picture, not the planes.  Then I would meter the grey-card normally.


Grey Cards and Normal Pictures

Even with seemingly normal landscapes and portraits, the meter is likely off.  For example, in a portrait, caucasian skin is easily 1 stop whiter than grey and if a person occupies a significant portion of the viewfinder, this will throw-off the meter.  A grey card will give a better meter reading.

Similarly, landscapes with dark foliage, expansive skies, wheat fields, etc., are all out of the norm and a grey card can help set a more appropriate exposure.  You would be surprised in how often a better meter reading can be found.


White Balancing

Exposure compensation adjusts for metering errors and for artistic reasons.  With digital cameras, there is also a separate problem with "white balancing," also known as color correction.  See this ImageLiner article: White Balancing.

With my photographs, I often do the following:
1.  Set the White Balance
2.  Meter a grey card and compensate as needed


Using a Grey Card in a Post Processing

During a shooting session, where you are taking multiple pictures of your subject, have a model or assistant hold the grey card while you take their photograph in the same quality of light as the subject.  In post processing, use the photo editor's brightness and contrast tools to hand-pick the center grey-scales in the image.  At the same time, most tools will white-balance.  Once set, apply this curve to the remaining photographs from the photo-shoot.



However, setting the grey-scale mid-tone curve in this fashion is an after-the-fact process.  Better results, with the the best quality highlight and shadow details, are found when the exposure is set properly during capture. 

If you use the in-scene grey-card in post-processing, you may loose some of the shadow details, which were never captured because of exposure problems. 

Consider the black car example, above.  No amount of post-processing can salvage the glossy paint reflections captured by the over-exposure -- even with an in-scene curve adjustment. And with the under-exposed example, there are simply no details to recover in the shadowed areas because pixel values were never recorded.  In this example, even after brightening, the subject will be blocky and ill-formed.



Ideally, if you metered with the grey-card and properly white-balanced, you would see no difference in post processing with this technique.


Controversy 

Using an 18% grey card reading for the actual exposure has been the gold-standard for metering and it has become somewhat of a religious issue.  The change in procedure, adding 1/3 to 1/2 stop to the card-reading, is a relatively new idea.  You will find conflicting advise on this topic, with most  referencing out-of-date sources that do not take into account modern cameras and published specifications (references at the end of this article). 

After much studying, and with my own photographs, it seems clear that a grey card was originally designed to solve printing problems - not necessarily to calibrate light meters. But using the card in this fashion is clearly better than allowing the meter to automatically gauge the scene.  By making a minor, last minute adjustment of +1/3 to +1/2 stops (due to camera differences), the grey-card is right on the mark. This is an admitted pain and it would be much simpler if we had 13% grey cards

Before 1980, Kodak recommended an even more nuanced approach:  If a light subject, they recommended a half-stop more light than the grey-card's setting, per the recommendations above.  But if it were a 'dark' subject, Kodak said to under-exposing a half-stop from the grey-card; this is counter to the current recommendations of increasing a half (third) -stop, compensating for the difference between 18 and 13%).  Kodak dropped the idea because it seemed overly complicated but this could accentuate the darker shades.   

Then there is this problem.  In a high-contrast scene, such as a white building in bright sunlight, where there are deep shadows in the doorways and overhangs, and open, lighter shadows on the walls, where should the grey card be placed?   None of the locations give satisfactory results and the contrast may exceed the camera's (film's) ability.  In this case, you have to make an artistic decision: If you meter for the dark shadows (exposing for details in the shadows), the highlights will be lost.  If you meter in the sunlight, the shadows will fall into dark nothingness.  An incident light meter would have the same problems -- where do you place the meter?

Keep in mind, "proper" exposure is not carved in stone, even with a grey card.  There are subjective and artistic points to consider.  In the high-contrast building example, deep black shadows may be exactly what was intended from an artistic point of view.  In this case, meter in the brighter areas.  But if you want both shadow details and usable highlights, you have these choices:  Wait for less contrasty light, such as in the early morning or add fill-lighting to the shadowed areas or use HDR and manipulate the photo in software.



Conclusions

I often use a grey card, especially in landscapes and it provides a solid starting point for the exposure.  Frequently I find the camera's meter is off by 2/3rds of a stop or more and this is easily seen in the final print. By exposing properly, I have less post-processing work on the computer and it is nice to begin with an image or negative that has the best details.

But don't blindly follow the grey card's recommendation.  Artistic license sometimes dictates a change from the "proper" exposure.  There are times when a picture should be under-exposed, deepening shadows, accentuating light sources, etc.  And there are times when a photo might benefit from over-exposures. 

Related articles:
ImageLiner: White Balancing
Reference chart: Fractional Shutter Speeds and Apertures

Discussion thread: 18% grey card vs 12-13%
Wikipedia article on Light Meter Calibration
Reflective Light Meters explained
Histograms and Gamma

Calibration Reference:
Kodak 18% Grey Card: 18%
Canon, Nikon, Sekonic: 12.5%
Pentax, Minolta, Kenko: 14%



Humorous Update 2012.01.03:

For the imageLiner Vanguard Tripod Review, I needed to photograph a white sheet of paper and a coin.  This is literally the photograph taken at the camera's recommended exposure 320 @ f5.6, with no RAW corrections.  Notice how the white paper isn't.  I should have used a grey card and forgot to overexpose 2 stops.  I laughed when I saw the picture. 

Click for larger View; click "X" to return
Keywords:
night scene over exposed overexposed.  night shots overexposed. nightshots grey gray.  exposure compensation, gray card greycard graycard gray-card grey-card.

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