Gray Card

A gray card is a flat, matte card of a known, neutral tone, usually 18% gray, that you photograph or meter in the same light as your subject. It does two jobs. Because it has no color of its own, it tells your camera or editor exactly what neutral looks like under that light, which fixes white balance. Because its brightness is known, it gives your light meter a target it reads correctly, which fixes exposure when the scene itself would fool the meter.

Both jobs come down to the same idea. A camera has no way of knowing what color the light is, or how bright your subject really is. It has to guess from the scene. A gray card replaces the guess with a measurement.

One card, two separate jobs

It helps to keep the two jobs apart, because they need different things from the card.

• Color needs neutrality. For white balance, any surface that reflects red, green and blue light equally will work, whether it is light gray, mid gray or white. The card’s exact brightness does not matter, as long as no channel is clipped.
• Exposure needs a known tone. For metering, the card must be a known brightness. An 18% card reflects 18% of the light that falls on it, which is close to the tone a reflective meter tries to produce: middle gray.

Many cards have more than one side or patch for this reason. A common set is a black patch of about 3%, the 18% gray and a white of about 90%. The white is the better white balance target in dim light, because it gives the sensor about five times as much signal as the gray (90 divided by 18), so the reading is less noisy. The gray is the exposure target.

Setting white balance with a gray card

Every light source has its own color temperature, and auto white balance estimates it from what is in the frame. It does well with ordinary scenes and badly when one color dominates: a red-walled room, a green forest canopy, a stage washed in one color. The result is a color cast that shifts from frame to frame as you recompose. A card in the frame removes the guess.

The raw method: sample it later

1. Put the card where your subject is, facing the camera and lit by the same light. It does not need to fill the frame for color; it only needs to be large enough to click on.
2. Take one frame with the card in it. Keep everything else the same.
3. Remove the card and shoot normally.
4. In your editor, open the card frame and click the card with the white balance eyedropper. The editor sets white balance so the card reads equal in red, green and blue.
5. Copy that white balance to every frame taken under the same light.

This works best with raw files, where white balance is only an instruction stored with the file and can be changed with no loss. A JPEG has white balance baked in. Correcting a strong cast in a JPEG pulls on 8-bit color data that has already been processed, and big corrections can leave banding or muddy color. The raw vs JPEG page covers the difference in full.

The in-camera method: custom white balance

If you shoot JPEG, or want the colors right on the back of the camera, use custom (sometimes called preset or manual) white balance. Cameras do it in one of two ways. Some measure directly: you choose the custom setting, point the camera at the card filling the center of the frame, and press the shutter. Others ask you to take an ordinary photo of the card first, then pick that frame from the menu as the reference. Your camera’s manual names the exact menu, but the idea is the same in both.

Whichever method you use, the card holds only for the light it was in. Shoot it again when the light changes: when you move from window light to a lamp, when the sun goes behind a cloud, when you add or move a flash. Film and video crews shoot a card at the start of every lighting setup for this reason.

Without a card, matching the light means judging it by eye. Try that in the camera simulator: the street is lit by warm lamps, and you have to set a white balance by hand that lands within 400K of them.

Want to see what a few hundred kelvin of error does to a picture? The white balance tool on the white balance page lets you drag the setting across the scale and watch the cast come and go.

Getting exposure from a gray card

A camera’s built-in meter is a reflective meter. It measures the light coming back from the scene and sets an exposure that would make that light come out as a middle tone. It has no idea whether it is looking at a white wedding dress, a black dog or a gray road, so it makes all three the same middle gray. Snow comes out dull, and a dark subject comes out washed out.

See the problem for yourself in the camera simulator: with Evaluative metering in Aperture, Shutter or Auto mode, the meter turns the snow gray. Use exposure compensation to bring it back to white, and notice that you have to find the amount by trial, which is the guess a card reading removes.

Want the other challenges? The full camera simulator has all 21, across eight scenes.

The card fixes this. If you meter a surface of known tone, the reading describes the light, not the subject. Every other tone then falls where it belongs: snow lands near white, dark fur lands near black. In effect a gray card turns your reflective meter into a rough incident light meter, which measures the light falling on the subject directly.

1. Switch to manual mode, so the exposure stays put once you set it.
2. Hold the card at your subject’s position, in the same light, angled as shown in the diagram below.
3. Fill the frame with the card, or switch to spot metering and put the spot on it. Keep your own shadow off the card.
4. Set aperture, shutter speed and ISO until the exposure scale reads zero.
5. Open up by about half a stop (see the next section), remove the card, and shoot. The meter will now disagree with you as the framing changes. Ignore it until the light changes.

Which way to point the card

The angle changes the reading more than most people expect. Turned square to the camera under side light, the card catches the light at a glancing angle and reads dark, so you overexpose. Turned square to the light, it reads as bright as it can, so you underexpose. The classic gray card instructions give a simple rule: aim it at a point about one third of the way from the camera toward the main light.

Outdoors, decide which light is the main one. In direct sun, it is the sun. In open shade, on an overcast day or with the sun behind your subject, the main light is the brightest part of the sky that the subject can see, so angle the card toward that.

The half stop most guides leave out

Meters are not calibrated to 18% gray. They are built to an international standard that uses a calibration constant rather than a reflectance, and the tone that constant puts at the meter’s zero is usually worked out as roughly 12 to 13%, not 18%. The difference between 18% and 12.5% is log2(18 / 12.5) = 0.53 stop. So a meter reading taken off an 18% card puts the card half a stop darker than its true tone, and the classic gray card instructions tell you to increase the indicated exposure by about half a stop for subjects of normal brightness.

In practice: if the card reads 1/125 at f/8, shoot at 1/100 (a third of a stop more) or 1/80 (two thirds), whichever your subject’s brightest parts allow. Manufacturers’ calibrations differ slightly, so rather than take this on trust, measure your own camera with the exercise in “Try this” below.

Metering the white side

In dim light, a meter can struggle with the gray side. The white side reflects about 90%, five times as much light as the gray, which is log2(5) = 2.3 stops. Meter the white side, then give about 2⅓ stops more exposure to get the gray-side reading. For example, a white-side reading of 1/500 at f/8 becomes 1/100 at f/8, the same reading the gray side would have given. The half stop correction then applies as before. White surfaces vary more in tone than calibrated grays, so treat this as a backup method.

Not sure what a stop is, or why a third of one matters? The stop page shows how shutter speed, aperture and ISO all move in the same steps.

Where the card should land on the histogram

A common belief is that a correctly exposed gray card should sit in the exact middle of the histogram. It should not. Image files store tones with a gamma curve, which spends more of the scale on the darker tones, where the eye is most sensitive. An 18% tone, exposed to its true value, lands at about 118 on the 0 to 255 scale of a standard sRGB image, 46% of the way across, a little left of center.

Reflectance	Stops from 18%	8-bit value (sRGB)	Position on the histogram
3% (black patch)	-2.6	48	19%
9%	-1	85	33%
12.5%	-0.5	99	39%
18% (gray card)	0	118	46%
36%	+1	162	63%
90% (white patch)	+2.3	243	95%

Two cautions when you read these numbers off your own photos. First, camera picture styles add contrast and change the curve, so a JPEG may put the card several levels away from 118. A flatter or neutral picture style comes closer. Second, the histogram on the camera is drawn from a JPEG preview even when you shoot raw, so it follows the same style. What matters most is the relationship: if your card lands near 99, your exposure is about half a stop under its true tone, which is exactly what the meter calibration predicts.

For color, look at the three channels, not the overall value. A neutral card that is correctly white balanced reads the same in red, green and blue, for example 118, 118, 118. If one channel is clearly higher than the others, that is your cast.

Want to practice reading tones on a histogram? The histogram page has a practice round where you match photos to their histograms, with each photo shown from two stops darker to two stops brighter.

When neutral is the wrong answer

A gray card tells you what neutral is. It does not tell you that neutral is what you want. Warm evening light is warm, and that warmth is often the reason for the photograph.

Two ways to keep a look while still using a card:

• Sample, then shift. Click the card to find neutral, then warm or cool the result by a known amount. You get a consistent starting point across the whole set, and the mood you chose rather than the one auto white balance happened to pick.
• Shoot the card under clean light. When a look comes from colored gels or a filter, film crews shoot the reference card under the same lights without the gels. Correcting to that card keeps the gel color in the picture instead of removing it.

Mixed light is the other limit. A card neutralises only the light falling on the card. Under a window and a lamp at the same time, a card near the window corrects for daylight and leaves the lamp-lit side orange, and a card near the lamp does the reverse. You can choose which light is neutral, fix one light at the source, or correct each area separately in editing, as shown in how to fix mixed light in post.

Want the warm light without fighting it? Golden hour photography covers when it comes, how long it lasts and how to expose for it.

What makes a good gray card

• Matte, not glossy. A shiny surface reflects the light source and its surroundings straight back, which raises the reading and can carry color from nearby walls.
• Spectrally neutral. A card sold for this job is made to reflect the visible spectrum evenly. A gray that only looks neutral, such as paint, card stock or a printed patch, may lean green or magenta under some lights.
• Not printer paper. Most office paper contains optical brighteners that absorb ultraviolet light and glow blue. In daylight, which is rich in ultraviolet, the paper reads bluer than it is, and the correction makes the photo too warm.
• Clean and flat. Fingerprints, fading and creases change the tone and the reflection. A card that has spent years in sunlight or a camera bag is worth replacing.

For work where color must match the original, such as photographing artwork or product photography, a target with many color patches lets your software build a profile for the camera under that light, correcting more than the neutral point. A single gray card corrects white balance and exposure only.

Common mistakes with a gray card

• Believing the meter is calibrated to 18%. It is closer to 12 or 13%, which is why an 18% card reading needs about half a stop more exposure.
• Holding the card square to the camera, or square to the light. Use the one-third angle. Under strong side light the difference between these positions can be a stop or more.
• Metering the card in different light from the subject. A card held in the sun for a subject in the shade gives an exposure for the sun. Take the card to the subject.
• Letting your own shadow, or color from a nearby wall, fall on the card. Both change the reading. Check the card is lit only by the light on your subject.
• Expecting the card to fix every frame. It gives an exposure for the light. A very bright subject in strong light can still clip, so check the highlights with clipping warnings after you set the exposure.
• Looking for the card in the middle of the histogram. With the standard curve it sits at about 46%, and a little lower with the meter’s half stop.
• Sampling the card and keeping the result, every time. Neutral is a reference point. Golden light, candlelight and blue hour are meant to have color.
• Using one card reading under mixed light. It neutralises only one of the lights.

Try this: measure your own camera’s half stop

This takes about ten minutes and answers the 12% versus 18% question for your camera, with your own numbers.

1. Set the camera to raw plus JPEG, a neutral or standard picture style, base ISO and manual mode. Place the card in steady, even light, such as open shade or a window on an overcast day.
2. Fill the frame with the card (focus does not matter). Set the exposure so the meter reads zero. Shoot one frame.
3. Shoot two more frames: one third of a stop brighter, then two thirds brighter. Use shutter speed for the steps so nothing else changes.
4. Open the JPEGs in any editor with a color sampler, and read the card’s value in each.

How to judge it. With the plain sRGB curve, a meter whose zero sits half a stop below the card’s true tone gives about 99 for the first frame, 110 for the next and 123 for the brightest, so 118 falls between the last two. With the plain curve each third of a stop adds about 11 to 13 levels. Note where 118 falls in your own set: that is your camera’s correction from a card reading. A contrasty picture style moves all the numbers and stretches the steps, so with anything but a neutral style treat the result as approximate. Then check the three channels of the zero frame. If they are not equal, you also have a measurement of how far your auto white balance missed in that light.

Want to check you can tell a reflected reading from an incident one? Try the light metering quiz.

Gray card FAQ

Is a gray card still useful with modern auto white balance?

Yes, when the color must be right or must match across many frames. Auto white balance is good in ordinary scenes but guesses from scene content, so a dominant color shifts it, and it can change from frame to frame. One card frame gives every photo in that light the same, measured starting point.

Can I use white paper instead of a gray card?

Not reliably. Most printer paper contains optical brighteners that glow blue in daylight, so it reads cooler than it is and pushes your correction too warm. For exposure, paper’s tone is unknown, so you cannot meter from it with any accuracy.

Does a gray card work with a phone camera?

Yes, for color. Most phone editors have a white balance picker, and many camera apps with a pro or manual mode can lock exposure on a tapped point. Shoot the card in the same light, then sample it or tap it to set exposure, and add a little brightness for the half stop.

Do I need a gray card if I already shoot raw?

Raw lets you change white balance freely, but it does not tell you what the right setting is. Without a neutral reference you are adjusting by eye on a screen, under your room’s light. A card frame gives you a correct value to start from in a single click.

Is an 18% gray card the same as middle gray?

Close, but not exactly. Middle gray is a perceptual idea, a tone that looks halfway between black and white, and 18% reflectance is the usual figure for it. Meters place their zero about half a stop darker, near 12 to 13%, which is why the card and the meter disagree by that amount.

Want a full system for placing every tone in a scene, not only gray? The zone system builds on exactly this: meter one area, then decide where it should fall.