Gray card: Why your meter may be lying to you!

[PhotosGuy]

Target influenced differences in exposure readings.
This might also be titled, “Why I shoot in M with custom WB.”

Background: It used to be that all reflective meters thought that they were looking at an 18% gray target. So if you pointed it at a white house, a transparency slide would come out looking 18% gray. If you pointed it at a black sculpture, a transparency slide would come out looking 18% gray. Examples here.
So we used incident meters which would read the light falling on the meter, not the light reflected toward it. Note that I said “transparency slide”. That’s ‘cause negative film had more exposure latitude, & the guy in the lab would adjust exposure for your prints, so you probably didn’t notice the difference very much.

I took some pics in Full-Auto-Mode intending to illustrate to a class how exposure reading on different color values will throw off the exposure readings, & was surprised to see that there wasn’t as much difference as I'd expected. The light subjects weren't as dark, & the dark subjects weren't as light as past experience predicted.

Then I realized that there was a constant involved – the highlights. It didn't work the way I expected because the images had highlights & digital exposure meters key off the highlights to some extent, trying to “expose to the right”, just like video will try to set 100 IRE.

1st result: Full-Auto-Mode



===========================
So, in the same lighting, I first shot flat, colored, construction paper targets without highlights. I filled the frame with the paper for each shot, so nothing else would influence the exposure.

The top group below shows the resulting colors under manual exposure set with a gray card, & custom WB. The last color swatch, outlined in gray, is black background cloth. Exposure was set at 1/30 sec @ f-8 for all.

The middle group below were Aperture Priority exposure, lens at @ f/8. The final range covered 4+ stops, just what I’d expected from my “days in film”! This shows the value of setting exposures with a gray card in situations where there is no reflected light to assist the meter. This may explain why sometimes you get the right exposure, & sometimes not when you’re still in the same apparent situation. Maybe you walked in, or zoomed in, & eliminated a highlight from the frame?

The last group below was “Green Box”, fully automatic mode including WB. Note that the cam assumes that you're hand holding and did not go below 1/60 @ f/5.6. It did want to use the flash on the last exposure of the black background cloth, but I wouldn’t let it.

Finally, notice that the very last exposure was of a gray card. Full auto WB didn’t do much good there, did it?



Edit 2/7/05
I recently used this info to take a pic in a dimly lit expo (pics below) where the lights above threw off the histogram, so I just took a pic of a white paper without including anything else, just as you would for a white balance shot, except in this case I bumped the exposure up just short of blowing out the whites.
(For WB, you need about a 2X underexposed pic so the cam has some gray info to work with.)

Let me clarify this. To use white paper to set exposure:
1. Take a reading of the white paper.
2. Increase the exposure by 2X. Actually, from experience, I open up about 8 clicks ( 8 times 1/3 stops = 2-2/3 stops.)
3. Take a test pic which includes the white paper & NO reflective highlights from anywhere else.
4. Chimp, which means to look at the Histogram and see how close you are to the right side. You want to be close, but not touching it. This will give you a good, general "start exposure" for most situations.
5. I would then enter that exposure into my "M" settings, & usually I'll delete the test pic.
At first, you might want to keep it to use for a post-processing WB setting. With Custom WB, I usually just adjust by eye in RSE.

More on that is in Need an exposure crutch?

Another example That's what I used for my exposure, & the highlight on the top of the tank gave me a reading of about 220 in PS. The lights in the ceiling were completely blown out at 255 in the pic below, so if I'd only relied on the histogram of the overall scene to set the exposure, the result would have been a pic about 2+ stops underexposed. RAW probably would have saved me (again!), but why take the chance!
See the pic below for a comparison. There's another "final" version at:
http://photography-on-the.net/forum/...ad.php?t=57033

Mike-Yeysch_2up_34.jpg

More on this method is in Post #9, below, or look at: Need an exposure crutch?

QUESTIONS?
Do I ever use AutoWB? Only for tests!
Do I ever use a Preset? Sometimes for "walk around" shots I'll use the Sunlight one. For dawn or evening shots I'll use the Cloudy one as I like the warm colors & a CustomWB will destroy them.
Where do I use CustomWB the most? Always in the studio, or in other color critical applications.
Do I tweak after in RSE? Usually a bit if my eye says to, or a lot if I've screwed up! We used to use filters more often than not to tweak Ektachrome to get the results we wanted. Why not tweak with Digital?

I hope this is a help to some of the beginners out there. Any questions or comments?

EDIT:
There’s an update/continuation of this at:
Gray Card…White Paper. What’s best?
What’s best for exposure, Gray cards, white paper, expensive attachments for the lens?

EDIT 7/19/05:
Curtis has expanded on this in the thread
Comparisons of metering modes - an eye opener!
I recommend that everyone take a look at his results & join the ongoing discussion!


EDIT 8/23/05
Another informative thread by Curtis that everyone should see:
How NOT to expose to the right

[PacAce]

I have a feeling that if you had shot your M&M tests in Manual mode and adjusted the exposure based on the meter reading you would have gotten the results more in line with what you were expecting. In manual mode, the DRebel uses the center-weighted metering mode. In full auto mode, the camera uses Evaluative metering and, based on experience, I know it doesn't always expose for 18% gray because of the complex algorithm used to derive the "proper" exposure.

[Tom W]

bump for a re-read after my coffee. Nice job, Frank. Interesting.
The M&M's are making me a little hungry.

[scottbergerphoto]

Welcome to the Zone System of Photography. Your camera meter gives a Standard Exposure for 18%(12%) Grey. To get to a Correct Exposure you need to add/subtract light based on the difference in reflectivity between your subject and 18% Grey. That's why to get white snow you add two stops of exposure and for a dark tuxedo you reduce by two stops. It all depends on what tone you take a reading off of.
Nice post.
Scott

[Tom W]

Re-read - very good illustration. Two variables, rather than one - color balance and exposure.
I guess I need to get a gray card.

[PhotosGuy]

Thanks for the comments. Hope it helps everyone out.

Scott:
Zone System? I was hoping to avoid bringing that in - a subject for a whole 'nother post. But, Pandora having spoken I will say that, if you shoot film, it's something that you should Google & become very familiar with. At times I make the occasional adjustment based on experience, but for me, shooting almost exclusivly RAW, it's not something that I bother with very much any more. OTOH, I should have adjusted when I shot the '53 Ford, but I had no time & RAW saved me again! If you're interested, see:
Why I love RAW - '53 Ford Sunliner

Leo:
Although I wasn't going to illustrate the various metering modes, your comment kept nagging at me, so I decided to have some M&Ms & clean up the 'paperwork'.
OK, metering in M & adjusting the exposure to match the meters recommendation. I put the M&Ms on the gray card to minimize background influence.
All shots were custom WB. Top #1 shot was with the gray cards recommended exposure, 1/10 sec @ f-8. (meters recommendation was 1/15 @ f-8.) Notice that the gray card background is gray.

Centerweighted average metering:
2nd shot on the white ones was the meters choice, 1/40sec. @ f-8. & the gray card background is darker.

3rd shot on the black ones was the meters choice, 0.3 sec = 1/33 sec @ f-8 & the gray card background is much lighter.
Pretty consistent readings, probably the highlight influence again, but more than a stop & a half off the gray card reading.

Here are the visual results:

[PacAce]

Originally Posted by PhotosGuy

Thanks for the comments. Hope it helps everyone out.

Scott:
Zone System? I was hoping to avoid bringing that in - a subject for a whole 'nother post. But, Pandora having spoken I will say that, if you shoot film, it's something that you should Google & become very familiar with. At times I make the occasional adjustment based on experience, but for me, shooting almost exclusivly RAW, it's not something that I bother with very much any more. OTOH, I should have adjusted when I shot the '53 Ford, but I had no time & RAW saved me again! If you're interested, see:
Why I love RAW - '53 Ford Sunliner

Leo:
Although I wasn't going to illustrate the various metering modes, your comment kept nagging at me, so I decided to have some M&Ms & clean up the 'paperwork'.
OK, metering in M & adjusting the exposure to match the meters recommendation. I put the M&Ms on the gray card to minimize background influence.
All shots were custom WB. Top #1 shot was with the gray cards recommended exposure, 1/10 sec @ f-8. (meters recommendation was 1/15 @ f-8.) Notice that the gray card background is gray.

Centerweighted average metering:
2nd shot on the white ones was the meters choice, 1/40sec. @ f-8. & the gray card background is darker.

3rd shot on the black ones was the meters choice, 0.3 sec = 1/33 sec @ f-8 & the gray card background is much lighter.
Pretty consistent readings, probably the highlight influence again, but more than a stop & a half off the gray card reading.

Here are the visual results:

Frank, the gray background in the top picture (metered off the gray card) has a gray value of between 65% to 72% depending on where on the gray card you place the eye dropper (in PS).

The middle picture, although it looks like it's on the lighter side has an average gray value of 70%. And the last picture which looks darker than the middle one, has an average gray value of 71%. In other words, all the images averaged to about the same gray value, as I would have expected. Now, the only thing that confuses me is why all the gray values are so high. I would have expected them to have an average gray value of 50% (mid-tone gray). Did you have EC turned on?

[PhotosGuy]

Did you have EC turned on?

No, it's always been right in the center. Wouldn't matter anyway, on the 300D it doesn't work in M mode (pg. 75).
I first shot the last test series at the start of a session in which I shot a lot of other things. Near the end I replaced the battery, then noticed that I was on Av mode, so I shot the test again on M just to be sure. The results were the same. I get about 50% off the center of a gray card shot on the meter, & 52-53% across one of the middle white M&Ms. The Black ones are about 65-75%.
The construction paper results showed what I wanted to illustrate re: using a gray card & custom WB for the best results. If you figure out what caused the difference, I'd be interested in the reason.

[PhotosGuy]

There’s an update/continuation of this at:
Gray Card…White Paper. What’s best?
What’s best for exposure, Gray cards, white paper, expensive attachments for the lens?
&
Don’t have a gray or white card, or hand held meter with you? “Film tricks” can help you out.
Need an exposure crutch?

Be sure to look at how the subject affects the exposure: Post #47

[Curtis N]

Frank,

I appreciate your work here and the concepts you're trying to illustrate, but I think it would be more easily understood by changing only one variable at a time.

At this point, I'm mostly curious about the differences between metering modes. I have the feeling that evaluative metering might be the culprit ruining a lot of my shots. I think I'll get some M&M's and do some test shooting mysfelf. Stay tuned!

[PhotosGuy]

but I think it would be more easily understood by changing only one variable at a time.

You're right & thats what I started out to do, illustrate the effects of WB & a gray card. I wanted to contrast it with Auto results, & then the metering thing reared it's head!

Feel free to add your results to this thread. If you start a new one, I'd appreciate your adding a link in the "Tips" sticky.
I'm sure you'll enjoy your M&Ms after. I know I did!

[Curtis N]

More M&Ms pictures and an examination of the various metering modes in this thread:
http://photography-on-the.net/forum/...886#post664886

[slin100]

One of these threads (can't seem to find the right one, so I chose this one based on the title), mentioned that color space can have a big influence on evaluating the meter. To show this, I took one of my RAW images and developed it twice, once to sRGB and once to Prophoto RGB. Shown below are the Luminosity histograms of each RAW conversion. sRGB is on top.

As you can see, the sRGB histogram is shifted to the right. People should keep color space in mind when calibrating one's meter.

[CorruptedPhotographer]

good job Frank!

[sexyyoyi]

I am just wonder whether WB metering would be necessary if I am shooting in Raw. Since RAW provides a wide leverage for adjusting the WB to you taste.

Any insight?