Gray card and exposure

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#1 Peter Bitic

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Posted 24 June 2014 - 11:55 AM

Hey everyone,

I am an aspiring filmmaker, primarily interested in screenwriting and directing, but since I have a plan to shoot the first couple of shorts on my own, I am also trying to teach myself the basics of cinematography.

I am afraid the following question is probably really thrivial, but I haven't found an answer just googling for it.

18% gray cards have a RGB value of 119, 119, 119, so when you make a picture with correctly exposed gray card in it, the resulting pixels that form the card should have approximately the same values, right?

The first picture below is the photo as I shot it. To me it seems too dark, but when I inspected the pixels of the gray card, the RGB reading indicated that the gray card and therefore the top of the stump was actually overexposed (157,159,159). I had to lower the exposure by 0.85 to get the values of approximately 119. The end result (second picture) really seems way too dark to me.

I am asking because I am not sure if I am really understanding how exposing using a gray card works. So far I have always just used it in combination with spot meter and been satisfied with results, but I have never fixed exposure in post based on the card in the picture.  Am I doing it right (and the stump should look so dark when correctly exposed), or have I misunderstood something?

Thanks a lot for any respones.

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#2 John E Clark

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Posted 24 June 2014 - 12:32 PM

Hey everyone,

I am an aspiring filmmaker, primarily interested in screenwriting and directing, but since I have a plan to shoot the first couple of shorts on my own, I am also trying to teach myself the basics of cinematography.

I am afraid the following question is probably really thrivial, but I haven't found an answer just googling for it.

18% gray cards have a RGB value of 119, 119, 119, so when you make a picture with correctly exposed gray card in it, the resulting pixels that form the card should have approximately the same values, right?

The first picture below is the photo as I shot it. To me it seems too dark, but when I inspected the pixels of the gray card, the RGB reading indicated that the gray card and therefore the top of the stump was actually overexposed (157,159,159). I had to lower the exposure by 0.85 to get the values of approximately 119. The end result (second picture) really seems way too dark to me.

I am asking because I am not sure if I am really understanding how exposing using a gray card works. So far I have always just used it in combination with spot meter and been satisfied with results, but I have never fixed exposure in post based on the card in the picture.  Am I doing it right (and the stump should look so dark when correctly exposed), or have I misunderstood something?

Thanks a lot for any respones.

Some manufacturers recommend that a 18% grey card should fall at about 30-50% IRE. Because camera models have different gammas, and there is sufficient bit depth, the resulting image after post processing, etc. can be placed such that the image reads well.

I've been tending to set my GH-1 to place the 18% grey card at 50% IRE. I've not tested my D600 sufficiently to come to some determination about it in this regard.

It may also be that I prefer slightly 'lighter' values.

Edited by jeclark2006, 24 June 2014 - 12:33 PM.

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#3 Stuart Brereton

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Posted 24 June 2014 - 12:39 PM

As the gray card is not perpendicular to the camera, it's possible that you're getting a false reading due to light falling on the card from an acute angle and causing a sheen.

Also, not all cameras and light meters are calibrated for 18% gray. The true figure may be between 14-18%. As has been pointed out, the gamma of the camera makes a difference. In REC 709, mid gray falls at 41.7 IRE. Variations on 709, and Log Gammas will have a different figure.

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#4 Peter Bitic

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Posted 24 June 2014 - 02:01 PM

As the gray card is not perpendicular to the camera, it's possible that you're getting a false reading due to light falling on the card from an acute angle and causing a sheen.

I see, I didn't know that an angle from which you meter a card can play a role, thanks. What I am puzzled about: as far as I understand, if I metered the card from an ideal angle, getting a potentially more accurate reading, and then returned to the angle I originally shot the picture from, I would still get the same "exposure relationship" between the stump and the card, and I would have the same dillema when inspecting the picture Photoshop - either the card would have a wrong RGB value, or the stump would seem to dark (or both).

I might have misunderstood you though. Is it possible that by "false reading" you don't mean (just) camera meter reading, but RGB reading in Photoshop? Meaning that the RGB values don't necessarily correctly represent the relationship between the exposure of the two objects (i.e. that one was actually more/less lit in reality than it shows in the picture)? Is it possible that the card was exposed differently than the top of the stump based on the angle I shot it from? That would explain the apparent underexposure of the picture when correcting it based on the card.

As for all cameras not being calibrated for 18% gray as you and Jeclark pointed out - I understand this, but wouldn't shooting a gray card for reference and correcting exposure in post like I did, make any such camera differences irrelevant? For example, if my camera is calibrated for 14% gray, then exposing for 18% gray card would overexpose the card. But I would be able to see in Photoshop that the card is overexposed and adjust the exposure.

Or do these camera differences render an image in such way that I can't rely on a reference card to determine the correct exposure in post?

(I hope I am making any sense.)

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#5 John E Clark

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Posted 24 June 2014 - 02:23 PM

I see, I didn't know that an angle from which you meter a card can play a role, thanks. What I am puzzled about: as far as I understand, if I metered the card from an ideal angle, getting a potentially more accurate reading, and then returned to the angle I originally shot the picture from, I would still get the same "exposure relationship" between the stump and the card, and I would have the same dillema when inspecting the picture Photoshop - either the card would have a wrong RGB value, or the stump would seem to dark (or both).

Part of the 'art' of metering includes assessing what the camera 'will see', and what values will be recorded.

While the surface of the grey card is theoretically 'lambertian', that is 'equal reflectance at all viewed angles'... in practice real materials do not have equal reflectance in the 180 degress that one can view that surface. The more one meters at 90 deg to the surface plane, the better...

As for 'why use a grey card when the camera could be calibrated for some other criteria'... well, the use of the grey card, and other calibration test targets, is simple so you the photographer can 'know' what the camera will record, given the subject reflectivities of the scene you are shooting.

With a step wedge, one can 'see' how the low and high values fall in the test image, which will give you the idea of how similar values will fall in your scene.

Since for DSLRs the 'curves' that the camera produces are not particularlly well documented, one has to reverse engineer them by shooting test targets and then analyzing them.

For Digital Film I'd recommend using the IRE scale rather than photoshop and 'pixel' values.

Premiere is the NLE I use, and it has a wave form monitor display to allow me to evaluate the test targets.

I've tuned out of Final Cut, but the Pro version had such a monitor. Don't know what Final Cut X has, but I'm sure there's a plugin. I think Vegas, at least some versions, have such as well.

(I also use a image analysis package called Image-J, which allows me to compute various statistics like noise from an image or a series of images... but who normally goes that far...).

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#6 Jim Ritscher

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Posted 03 August 2014 - 10:52 PM

There is something about exposure you don't understand, but I'm not quite sure what it is. Do you understand that the object of the game is to place all important values (say perhaps excluding a glint off a bumper) so they range from dark to light in the image without clipping or losing detail?

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#7 Jim Ritscher

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Posted 03 August 2014 - 11:53 PM

Let me start this again. There is something about exposure you don't understand, but I'm not quite sure what it is. Do you understand that the object of the game is to place all important values (say perhaps excluding a glint off a bumper) so they range from dark to light in the image without clipping or losing detail? It's fundamentally an artistic process, not a scientific one, though there is a large amount of science involved. An 18% neutral gray card is designed to be a sort of geometric middle ground between light and dark values in a typical scene. And it's actually amazing how well it does, particularly if the scene has a fairly normal distribution of lights and darks. But if the scene you are shooting has either a lot more light values, or a lot more dark values, you will probably want to adjust the exposure to compensate, regardless of what the neutral gray card says.

The problem gets tricky when the scene you are shooting has more dynamic range than your camera can record. Then you either try to adjust the lighting, or you make compromises. For example, if you're shooting a wedding you want faces and the white dresses to look really good – to have lots of detail and no clipping. You are probably willing to let go of some of the detail in dark suits. On the other hand, if you're doing an ad about dark suits, then there better be a lot of visible detail in the dark suits. For a given camera and picture profile, the placement of neutral gray can be a little arbitrary. One person might say, "I want to record 5 stops above neutral gray, and 6 stops below it, all with good detail and no clipping." Another person might say, "I want to record 6 stops above neutral gray, and 5 stops below it, all with good detail and no clipping." It depends on the picture profile, and what you're trying to do.

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#8 Chris Millar

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Posted 04 August 2014 - 03:03 AM

As the gray card is not perpendicular to the camera, it's possible that you're getting a false reading due to light falling on the card from an acute angle and causing a sheen.

Yip, and to complicate matters we're not sure of how perpendicular the meter was to the card at the time of reading.

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#9 David Mullen ASC

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Posted 04 August 2014 - 11:00 AM

Metering a grey card is the beginning of deciding what the exposure should be, not the end.

In the case of a grey card sitting outside in the sun, you'd generally overexpose it about a stop so that the shot "feels" sunny.  Whereas that same grey card in a night interior would probably be exposed more like how it looks in this test if not darker.

(Actually, in how I use a grey card, I'd expose it as metered so that the telecine can be balanced for it and then expose the scene that follows the grey card with my creative exposure. In this case, the darker version of the card on the tree stump looks about right in terms of an 18% grey card. But then after that I'd shoot the actual shot with a bit more exposure.)

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