Proper Exposure and How to use a Grey Card

[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.

 

[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?

 

[Helen B]

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

 

[Vinny]

Derrel,

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

Vinny

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.

 

[Vinny]

Thank you for this!

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

 

[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!

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?

 

[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!

 

[Big Mike]

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.

 

[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.

 

[jands]

...trust me on this one...

I'm not sure I believe you.

 

[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

 

[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

 

[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.

 

[Ed Bray]

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.

 

[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