# 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 &amp; ISO), or you can engage exposure lock (check your user manual).  With a point &amp; 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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## Josh66

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.


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## 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.


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## 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!


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## Josh66

behanana said:


> 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.


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## 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.


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## 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.


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## 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


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## 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.


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## klakf

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


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## kundalini

Vinny said:


> 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.


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## Big Mike

Vinny said:


> 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).


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## 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...


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## Vinny

Thanks Mike for the explanation!




Big Mike said:


> Vinny said:
> 
> 
> 
> 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).
Click to expand...


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## 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



Derrel said:


> 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...


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## Derrel

Sorry Vinny, if you thought the reply came off as arrogant. It seems that quite a few people here do not understand the Inverse Square Law, and that the statement as Big Mike wrote, "The inverse square law applies to flash and constant/ambient lighting." is not applicable to constant or ambient light being provided by the sun. 

Your comment in Post #8 above [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.]


addressed a misconception, and at least to me, I felt that Big Mike's reply seemed to me to miss the obvious--which is to say that the sun is so far away that there is no possible distance on Earth that one can move to change the proper exposure reading off of a subject....the distance from the light source, the SUN, to the SUBJECT, is what determines the exposure...and not as you wrote, "the further away you were to the subject". So, sorry if my reply seemed "arrogant"; I was merely addressing a fundamental error in understanding that you expressed in post #8.


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## klakf

Hi. I enjoyed reading this article, so I went out and purchased a gray card. I attempted to use it, going by the directions, but my pictures are turning out really dark. When I try to meter the card, it's giving me 1/30 and f4.3 as the settings. I change the EV to get it to the middle, but it doesn't seem to help. To get the reading, I'm getting about 6" in front of the card being careful to place the card directly in front of the subject and halfway angled toward the light illumination. To get the reading I'm using Automatic and then changing the manual to put those settings in.  I'm obviously doing something wrong. What do you suggest?


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## Helen B

Vinny said:


> 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.



I'll put it another way. The intensity of the light from the subject does indeed fall off according to the inverse square law when the subject is in constant illumination (except for specular reflections). In the case of our eyes, a camera or a spot meter (including an in-camera meter) the falloff in intensity is exactly offset by the reduction in area of the image of the subject - therefore the image brightness remains the same (less light, but illuminating a smaller area). In the case of a lensless light meter, the falloff is compensated for by the increasing area of the subject that is illuminating the meter (and if the average reflectivity of the subject changes as more comes into the view of the meter the reading will change).

The inverse square law is simply a matter of light falloff, and there are no magical exceptions to it. You can use it to vary illumination reliably - move a light twice as far away from a subject (making sure that you know where the effective source is) and you will quarter the brightness of the light coming off the subject towards a meter or a camera (with the exception of specular reflections).

Best,
Helen


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## Vinny

Derrel,

Sorry for my misinterpretation of your original response and THANK YOU for taking the time to explain it further!

Vinny



Derrel said:


> Sorry Vinny, if you thought the reply came off as arrogant. It seems that quite a few people here do not understand the Inverse Square Law, and that the statement as Big Mike wrote, "The inverse square law applies to flash and constant/ambient lighting." is not applicable to constant or ambient light being provided by the sun.
> 
> Your comment in Post #8 above [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.]
> 
> 
> addressed a misconception, and at least to me, I felt that Big Mike's reply seemed to me to miss the obvious--which is to say that the sun is so far away that there is no possible distance on Earth that one can move to change the proper exposure reading off of a subject....the distance from the light source, the SUN, to the SUBJECT, is what determines the exposure...and not as you wrote, "the further away you were to the subject". So, sorry if my reply seemed "arrogant"; I was merely addressing a fundamental error in understanding that you expressed in post #8.


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## Vinny

Thank you for this!




Helen B said:


> Vinny said:
> 
> 
> 
> 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.
> 
> 
> 
> 
> I'll put it another way. The intensity of the light from the subject does indeed fall off according to the inverse square law when the subject is in constant illumination (except for specular reflections). In the case of our eyes, a camera or a spot meter (including an in-camera meter) the falloff in intensity is exactly offset by the reduction in area of the image of the subject - therefore the image brightness remains the same (less light, but illuminating a smaller area). In the case of a lensless light meter, the falloff is compensated for by the increasing area of the subject that is illuminating the meter (and if the average reflectivity of the subject changes as more comes into the view of the meter the reading will change).
> 
> The inverse square law is simply a matter of light falloff, and there are no magical exceptions to it. You can use it to vary illumination reliably - move a light twice as far away from a subject (making sure that you know where the effective source is) and you will quarter the brightness of the light coming off the subject towards a meter or a camera (with the exception of specular reflections).
> 
> Best,
> Helen
Click to expand...


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## Vinny

I'll try to explain based on my understanding ... which could be totally wrong :lmao:

When you are measuring the subject (grey card) on automatic the light meter sets the camera up to take the correct exposure. In fully Auto it sets all three parameters - ISO, Aperture and Shutter Speed. If you are keeping 2 of the parameters locked it'll change the last parameter to take the correct exposure. In Auto, any Auto, the readings are the readings and there is no compensation needed. You did mention something about adjusting your EV which should not be needed. In manual you are controlling all 3 parameters so you are using them to get a "0" reading on the light meter. What I've read is if you are shooting in an Auto mode (Aperture or Shutter priority) take a reading and you can switch the camera to manual and use those same settings to obtain the same exposure.

Hope I got it right and I hope it helps!



klakf said:


> Hi. I enjoyed reading this article, so I went out and purchased a gray card. I attempted to use it, going by the directions, but my pictures are turning out really dark. When I try to meter the card, it's giving me 1/30 and f4.3 as the settings. I change the EV to get it to the middle, but it doesn't seem to help. To get the reading, I'm getting about 6" in front of the card being careful to place the card directly in front of the subject and halfway angled toward the light illumination. To get the reading I'm using Automatic and then changing the manual to put those settings in. I'm obviously doing something wrong. What do you suggest?


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## klakf

OK. Thank you, Vinny. So...what you're saying is: I set the camera on Manual, fill the frame with the gray card at about 6" from card (or does distance matter?). Anyway...from that point I look at the exposure dial. If the dial is to the plus or minus, I adjust the shutter speed and aperture settings until the exposure dial reads 0. If adjusting those settings doesn't do it, then I adjust my ISO? If I need to make sure that I have a high enough shutter speed to prevent blurring of moving subjects, then I use the Shutter Priority mode, and attempt to change the exposure dial with the ISO? I'm probably making this much harder than it should be, but it's confusing right now. Thank you!


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## Big Mike

klakf said:


> OK. Thank you, Vinny. So...what you're saying is: I set the camera on Manual, fill the frame with the gray card at about 6" from card (or does distance matter?). Anyway...from that point I look at the exposure dial. If the dial is to the plus or minus, I adjust the shutter speed and aperture settings until the exposure dial reads 0. If adjusting those settings doesn't do it, then I adjust my ISO? If I need to make sure that I have a high enough shutter speed to prevent blurring of moving subjects, then I use the Shutter Priority mode, and attempt to change the exposure dial with the ISO? I'm probably making this much harder than it should be, but it's confusing right now. Thank you!


 The key point that I think you're missing...is that even shutter speed priority is an automatic mode.  So even though you use that when metering on the grey card, the settings will likely change when you back up to take your shot.  That's why you need to (really should) stay in manual mode.
So have your subject hold the card, then get close (make sure that you are not blocking any light if you get too close).  Then you adjust your settings to get to zero.  If you want to maintain a certain shutter speed, then dial that in first, then adjust the aperture and ISO to get the needle to zero.  
Then you can back up and shoot away...and because you are still in manual mode, the setting won't change, thus your exposure won't change and thus should be correct.  The needle probably won't be on zero, once you back up...that's OK...as long as it was on zero when you metered on the grey card.


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## Vinny

^^^^ What Mike said! But I want to add that some (all?) cameras have an exposure lock for auto modes (as Mike said shutter priority, aperture priority and fully auto along with scene modes are auto settings) and in auto you can lock these settings and move away from the subject. I am a little fuzzy if also affects the focus but everything should be in the manual on how to use it; for Nikon it's called AE lock.

Also remember that there is motion blur from the subject and motion blur from the movement of the camera. If you keep the shutter speed at least 1/focal length you can eliminate the camera movement. VR will help but IMO it is better to learn how to get sharp photos without VR and use VR only when needed.


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## jands

Derrel said:


> ...trust me on this one...


 
I'm not sure I believe you.


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## samdamico

Excellent discussion. 

Instead of using a grey card, what about applying a loose interpretation of the zone system?

For example; meter off the subject, with the understanding that the subject will be represented as a mid-value. Then using the zone system place the subject tonally where you want it. 

In my work as a photojournalist, I found this approach more practical than using a grey card as a reference. I would  use a spot meter to get the  tightest area of influence.

Sam 

samdamico.com

beingabetterphotographer.com


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## Helen B

Welcome to the forum, Sam. 

In the field there are two other simple, time-honoured metering methods that are similar to a grey card reading: using the palm of your hand as a reference in place of a grey card and adjusting by half a stop or so increase; and using an incident meter*. 

My preference for documentary work is usually to use a reference tone method with the in-camera meter - the simplest possible.

*A grey card reading is an incident reading, albeit one that has more directionality than a meter with an incident dome in the raised position - which has a cardioid directional response - sensitivity to light from the sides and slightly behind but not from directly behind. A grey card has roughly the same directional response as a dome receptor in the lowered position, or a flat receptor - a 'cosine' response. 

There are, of course, many other metering methods, but how far do we want to open the floodgates here...?

Best,
Helen


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## Ed Bray

Having just read this thread and whilst I applaud the OPs intention to instruct people in how to meter correctly unfotunately his summation and theories are slighty incorrect.

He is correct in that the camera does not know what it is being pointed at and also in that the camera is reading reflected light. Where he is mistaken is that on using an 18% grey card will give him a correctly exposed image, it will not, and his own photographic examples bear this out!

Why am I suggesting that this is so? Well the meter in a digital SLR is calibrated against 12% grey as opposed to the 18% grey of a grey card, thus taking the reading directly off the grey card without using any compensation for the difference will lead to over-exposed images by about 1/2 a stop. This is reflected (pun intended) in the OPs snow shots where the snow has been burnt out.

I agree that the direct use of a grey card will give better images than those without (or learn how to meter off known tones and allow for them) but if no correction is made to the grey card reading then the images will still not be correctly exposed.


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## Ed Bray

Vinny said:


> 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 InverseSquare Law applies to any 'electromagnetic wave that originates from a (andhere is the important bit) *point source* (these electromagnetic waves include sound, light, radio, gamma, x-ray, UV & IR)

The ISLstates: The intensity (or illuminance or irradiance)of lightor other linear waves radiating from a point source (energy per unit of area perpendicular to the source) is inversely proportional to the square of the distance from the source; so an object (of the same size) twice as far away, receives only one-quarter the energy (in the same time period), In free space i.e. where there are no surfaces for the light to reflect off!.
 
So from a point source infree space, if we double the distance from the light we reduce its intensity by a factor of 4 and if we half the distance we multiply the intensity by a factorof 4.

So what does this meanin practical terms in lighting for photography:

The Sun, is a point source of light (amongst other things) and gives out an amazing amount, but it's distance from the Earth is approximately 93 million miles, thus to lower the amount of light received from the sun by a factor of 4 then we would have to move the Earth another 93 million miles away from the Sun. Therefore the direct light from the sun is pretty constant regardless whether you take a reading from the ground or even from the top of a skyscraper as the difference between them is negligable in the grand scheme of things.

Now, still using ambient light but shooting inside a room (with black walls to prevent reflections), the light from the window now becomes a point source. If you take a light reading on a meter at 10 feet from the window, and then again at 20 feet from the window you should see a decrease in the reading by about 2 stops (factor of 4).This same analogy can be used for artificial light sources whether they be from Strobe or Constant light.

The main reason the ISL does not seem to work is when you introduce large sources (in comparison to the subject) or areas where light from point sources can be reflected.


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## samdamico

Thank you for the welcome Helen, 

Good advice. 

At the risk of opening the floodgates further, for the way I work I've always found it more accurate to meter off of the subject and then adjust for tonality via exposure. I'm ALWAYS using  my in camera meter with the tightest area as possible (spot). I also have a handheld one degree spot meter. 

The first time I realized that metering off of my hand (or something else near me that I perceived as the same tone as the subject) would not work was when the subject was in different light than I was in. 

Sam

P.S. Here's an example. In this case I metered off of the leaf and then increased my exposure. 

samdamico.com

beingabetterphotographer.com


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## Rosy

Hi Mike THANK YOU for this - Are Gray cards used for any type of setting?  Indoor and Out?  I read Bryan Peterson's Exposure and he meters off the sky in snowy situations - but i guess if it's on overcast day then the gray card is more reliable


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## Josh66

You can use a gray card any time - as long as it's in the same light that the subject is in.

On an overcast day, the sky is pretty much gray, so you can use that instead.  On a sunny day, green grass will work instead of a gray card too.

In snowy situations you don't want to meter off of the snow, because if you did the photo would be underexposed.


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## Rosy

Thks Josh - i guess outdoor sunny days are quite challenging as well - you meter off the grass, then?  If your subject is in a shady area then you can meter off the subject, correct?


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## Big Mike

You can meter off of grass, because (for the most part) grass is about the same reflectance as a grey card.  It's certainly not fool proof, as grass can look many different ways.  



> If your subject is in a shady area then you can meter off the subject, correct?


That's not really the idea.  That would only work if your subject (or the part that you meter off of) is the same reflectance as middle grey.

The point of a grey card, is that it works in all situations....sunny, shady, snowy, indoors etc.  The key is that you have to read the grey card in the same light that your subject is in.  
So if you subject is in shade, then you put your grey card in that shade and take your reading.  If your subject is in the sun, then you put your grey card in the sun and take your reading.


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