Proper Exposure and How to use a Grey Card

[Big Mike]

The importance of metering and how to use a grey card.

One of the keys to great photography is understanding exposure and knowing how to create a properly exposed photo.

The first thing we need to understand is that our cameras have built-in light meters, more to the point, they have 'reflected light' meters. They measure the light that is bouncing off of the scene and then give us the exposure settings (shutter speed, aperture & ISO).

The problem with this, is that the things we photograph will reflect different amounts light, which we see as different tones and colors. The camera doesn't know what it's looking at, it just knows how much light is reflecting off of it.

For this system to work, the cameras have to be programed for a certain amount of light...we call this middle grey. The camera assumes that the average reflectance of the scene will be the same tone as middle grey, so it gives you exposure settings that would work if your scene was actually middle grey. Fortunately, this is often the case, which is why many of the photos you take, don't look too bad.

However, when our scene (or the part that we meter on) isn't middle grey, we end up with under-exposed or over-exposed photos. This is because the camera is still assuming that our scene is middle gray, so the exposure settings it recommends are likely going to be wrong.



In the example above, the scene is mostly bright white snow, which is a lot brighter than middle grey. The camera doesn't know that it's seeing snow, just that it's bright. Therefore, it recommended settings that caused the photo to be underexposed. This is what you would get in any of the 'auto' modes on your camera. I got this in Manual mode by adjusting the settings (shutter speed, aperture & ISO) until the 'needle' on the camera's meter/scale got to zero (...0...).


One of the best tools to ensure that you do get properly exposed photos, is a grey card. They can be found in most photography shops for about $20. The are specially made to reflect exactly middle grey....which is what the camera is expecting to meter off of.


To use a grey card, you place the card under the same lighting conditions that you are shooting in (that your subject is in). If your subject can hold the card, that's great. You then meter off of the card. To do this, you basically fill the frame with the grey card and see what exposure settings you camera is recommending.



Now you need to lock your exposure at that exposure value. With an SLR type camera, you can use Manual mode and adjust the settings (shutter speed, aperture & ISO), or you can engage exposure lock (check your user manual). With a point & shoot type camera, you should be able to lock the exposure as well. If you are using exposure lock, be sure not to accidentally re-meter the scene once you back up again. That's why it's so much easier to just use manual mode.
The exact combination of settings that you choose, will depend on your artistic goal, which is another lesson.


So now that you have locked in the settings that you got from metering on the grey card, back up and compose the shot how you like and take your photo.





Do you see that my subject is now properly exposed? Also notice how the snow actually looks white in the photo, like it does in 'real life'. In this case, some of the snow is even blown out, which is because it's just that much brighter than proper exposure for my subject. Given the choice between an underexposed photo or a bit of blown out snow, I'll choose the one that gives me a properly exposed subject.


With my camera in manual mode, I now have proper exposure locked in. So I can continue to shoot without having to re-meter for each shot. Provided, of course, that the lighting conditions don't change.





So there you have it. How to get proper exposure using a Grey Card. This will work in any lighting situation, just keep in mind that if you change your lighting (shooting in another direction or moving to the shade etc), that you will have to re-meter to get new exposure settings.

This lesson is also available on my blog.

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Just to add - grey cards can be used for more than just metering. You can also use it to do a custom white balance. For custom WB, you don't necessarily need a 'white' object, just a 'neutral' one. A grey card is neutral.

Many programs allow you to just click on an object with a dropper tool and apply that setting to any number of photos. You can also set a custom WB in camera (usually by taking a picture of a suitable target, then setting that as the 'correct' WB).

Just another reason to have a grey card... They're so useful (and relatively cheap as well) that there isn't really any reason not to have one.

[christopher walrath]

If any of you have the old Kodak Professional Photo Handbooks, the ones with all of the dials for focal distance and lighting and the like, then the back page IS a grey card.

Good write, Mike.

[behanana]

OK, this is the second "gray card" article I've read, and Mike its great thanks, the question I have relates to more outdoor/action shooting. Now, Mike, I know the example you used here was an outdoor shot, but if you are taking landscape shots would you still use a gray card? Likewise if you were shooting a, lets say soccer game, would you use a gray card to help with proper settings before the game starts. The other video I watched basically used the gray card for white balance adjustment in post pro. Just wanted to get some feedback from some of you that have been doing this for awhile. Thanks to all the expieranced photographers for helping out us Noobs!

[O|||||||O]

Likewise if you were shooting a, lets say soccer game, would you use a gray card to help with proper settings before the game starts.

That would be fine. You may need to check it periodically as the light changes though.

Depending on what time the games starts and how long it is, the light at the end will probably be different than the light at the beginning.

[Big Mike]

Yes, as Josh said, that would be fine.

The thing to remember when metering with a grey card, is that as long as you are shooting in the same light that you metered in, your exposure will be OK. So for the soccer game, you could meter before the game, but because the light might change during the game, you might have to adjust your exposure as well. And you don't really need to bring out the grey card again, just use common sense. If it gets a little darker, then give it a little more exposure.

To be honest, I don't even own a grey card. This one isn't mine, it's part of the kit I use to teach a DSLR & Metering class. And it's not even a real grey card, it's a piece of construction paper that is pretty close to middle grey. In the above situation, I'd would draw on my knowledge and experience to set my exposure. I know that all the white snow in the scene will likely cause underexposure (provided I'm using a metering mode that will see those parts of the frame). And I know that snow is white, which should be about two stops brighter than middle grey. So rather than using a grey card, I'd go right to +2 on my meter and see what the shot looked like (paying close attention to the histogram). If I didn't like the results, I'd tweak my exposure and shoot again.

Another thing to keep in mind, is that you want to use the grey card at the right angle, which is about halfway between the camera angle and the light source. In this case, the light source is mainly the sky (but also the snow all around) so ideally I'd have the grey card tilted up a bit when metering on it. My son wasn't cooperating with that part so the grey card was vertical or even tilted slightly down, so the reading wasn't as accurate as it could have been.

[Rekd]

Great read Mike, thanks. The picts show the example perfectly and give me a ray of hope as I begin my next phase of learning to shoot.

[Vinny]

Hey Mike,

Thanks for this! I am interested in getting a grey card to play around with exposure/white balance but am a little confused about using it for exposure.

I saw a video where the photographer was close to the subject, pulled out a grey card, took the reading and then walked back a few feet; it also looks like you did this as well. What about the inverse square law in measuring light? Is that law just for lighting a subject as when using flash? If not, how far can you be from the original exposure reading before it is no longer valid?

Due to the inverse square law, I always thought (and probably wrongly) that the further away you were to the subject the less light was being received by the light meter.

Thanks for you help!

Vinny

[klakf]

Vinny, I have the same question, so I'll be waiting to see what the response is. Exactly how far should you be from the subject when using the gray card as that will determine how much light is actually entering the lens.

[klakf]

Mike, thank you for posting this. Wonderful, helpful info.

[kundalini]

Hey Mike,

Thanks for this! I am interested in getting a grey card to play around with exposure/white balance but am a little confused about using it for exposure.

I saw a video where the photographer was close to the subject, pulled out a grey card, took the reading and then walked back a few feet; it also looks like you did this as well. What about the inverse square law in measuring light? Is that law just for lighting a subject as when using flash? If not, how far can you be from the original exposure reading before it is no longer valid?

Due to the inverse square law, I always thought (and probably wrongly) that the further away you were to the subject the less light was being received by the light meter.

Thanks for you help!

Vinny

You might be confusing light fall off with reflective meter reading. The grey card is for reflective meter reading and color balance of the exposure with the lighting in the scene you're shooting. As long as the lighting remains fairly constant, you just carry on. If your lighting changes, take another grey card shot. I use the grey card shot primarily to sync all the shots (same scene lighting) in Lightroom, BAM, you're done.

[Big Mike]

Hey Mike,

Thanks for this! I am interested in getting a grey card to play around with exposure/white balance but am a little confused about using it for exposure.

I saw a video where the photographer was close to the subject, pulled out a grey card, took the reading and then walked back a few feet; it also looks like you did this as well. What about the inverse square law in measuring light? Is that law just for lighting a subject as when using flash? If not, how far can you be from the original exposure reading before it is no longer valid?

Due to the inverse square law, I always thought (and probably wrongly) that the further away you were to the subject the less light was being received by the light meter.

Thanks for you help!

Vinny

The inverse square law has to do with the distance from the subject to the light source....not the distance of the camera to the subject. Specifically, it has to do with the diffuse reflection properties of the subject. In my example here, the light source on the subject is the sky and the snow all around him. So the only way that the inverse square law would come into play, is if he somehow got closer to the snow (or a lot closer to the sky/sun, which isn't practical)...in which case, he would reflect more of the light that he's getting from the snow (which is basically fill, as he's getting most of his light from the sky).

The inverse square law applies to flash and constant/ambient lighting. We tend to think of it as a property of the light falling off...but it's really more a property of the reflectance of the subject. The more properties of diffuse reflection the subject has, the more this law will apply. But if they tend to exhibit properties of direct reflection, the brightness of the reflection does not change with distance (just the size of the reflection).

You can use the inverse square law to test the reflectance type. Put a light on a subject and take a meter reading, then move the light twice as close to the subject. According to the I.S.L, it should get 4 times as bright (two stops). Even something that is strongly characterized by diffuse reflection, won't be 100%...so you'll likely get a reading that is shy of that two stop difference. As an example, lets say that it changes 1 2/3 stops. That would mean that your subject is characterized by about 83% diffuse reflection.

When you see a photographer using a grey card, the idea is to measure the light that is falling right at the spot where the subject is....and you want to meter just on the grey card, which is why we get close. Then we back up because we don't want our photo to be taken from that close to the subject. In this case, the light is virtually identical in either his location or mine, so I could have just held the grey card out in front of my camera, and got the same results. (but if my subject was in shade, and I wasn't, then that obviously wouldn't work).

[Derrel]

Maybe this will help those wodnering about the inverse square law and the light reflecting off of a gray card: think about this: Okay....the sun is very far away...how much farther away do you think you'd need to move the sun so that it casts a harder shadow and has substantially more fall off than it has at its current source to subject distance? Ten million miles? Fifty million miles? Twenty light years? Thirty more light years?

For all intents and purposes, in terms of metering light, the sun is at the "same distance" from your gray card, and your camera is the "same distance" from the sun, no matter if you are 3 feet from the gray card or 12 feet from the gray card, zoomed in, with a telephoto lens...trust me on this one...

[Vinny]

Thanks Mike for the explanation!

Hey Mike,

Thanks for this! I am interested in getting a grey card to play around with exposure/white balance but am a little confused about using it for exposure.

I saw a video where the photographer was close to the subject, pulled out a grey card, took the reading and then walked back a few feet; it also looks like you did this as well. What about the inverse square law in measuring light? Is that law just for lighting a subject as when using flash? If not, how far can you be from the original exposure reading before it is no longer valid?

Due to the inverse square law, I always thought (and probably wrongly) that the further away you were to the subject the less light was being received by the light meter.

Thanks for you help!

Vinny

The inverse square law has to do with the distance from the subject to the light source....not the distance of the camera to the subject. Specifically, it has to do with the diffuse reflection properties of the subject. In my example here, the light source on the subject is the sky and the snow all around him. So the only way that the inverse square law would come into play, is if he somehow got closer to the snow (or a lot closer to the sky/sun, which isn't practical)...in which case, he would reflect more of the light that he's getting from the snow (which is basically fill, as he's getting most of his light from the sky).

The inverse square law applies to flash and constant/ambient lighting. We tend to think of it as a property of the light falling off...but it's really more a property of the reflectance of the subject. The more properties of diffuse reflection the subject has, the more this law will apply. But if they tend to exhibit properties of direct reflection, the brightness of the reflection does not change with distance (just the size of the reflection).

You can use the inverse square law to test the reflectance type. Put a light on a subject and take a meter reading, then move the light twice as close to the subject. According to the I.S.L, it should get 4 times as bright (two stops). Even something that is strongly characterized by diffuse reflection, won't be 100%...so you'll likely get a reading that is shy of that two stop difference. As an example, lets say that it changes 1 2/3 stops. That would mean that your subject is characterized by about 83% diffuse reflection.

When you see a photographer using a grey card, the idea is to measure the light that is falling right at the spot where the subject is....and you want to meter just on the grey card, which is why we get close. Then we back up because we don't want our photo to be taken from that close to the subject. In this case, the light is virtually identical in either his location or mine, so I could have just held the grey card out in front of my camera, and got the same results. (but if my subject was in shade, and I wasn't, then that obviously wouldn't work).

[Vinny]

Derrel,

Not sure if it was your intention or not but this reply comes off to me as arrogant. I asked a question because I didn't understand the concept of reflected light and moving away from a subject but I do understand the concept of the inverse square law - obviously not 100%; Mike was kind enough to explain it further. If it bothered you that someone asked the question, I don't know what to say.

Vinny

Maybe this will help those wodnering about the inverse square law and the light reflecting off of a gray card: think about this: Okay....the sun is very far away...how much farther away do you think you'd need to move the sun so that it casts a harder shadow and has substantially more fall off than it has at its current source to subject distance? Ten million miles? Fifty million miles? Twenty light years? Thirty more light years?

For all intents and purposes, in terms of metering light, the sun is at the "same distance" from your gray card, and your camera is the "same distance" from the sun, no matter if you are 3 feet from the gray card or 12 feet from the gray card, zoomed in, with a telephoto lens...trust me on this one...