Kodak grey card usage

RobC · Mar 22, 2016

Kodak instructions for using a grey card say that the orientation of the card should be as follows.

Looking from above, the plan view from top, angle the card 1/3 of the way from the subject to camera axis towards the main light source.
AND
Looking from the side, angle the card 1/3 of the way from subject to camera axis towards the main light source.

So you need to tilt the card to the side and upwards assuming your light source is higher than the camera to subject axis.

Note: just becasue the lighting is diffuse doesn't mean you can forget about the angles because you can't. The angle the light is reflected from the card makes a big difference. Its not just the angle of light falling on the card that counts.

If you are not adhering to the correct angles then your card is not reflecting 18%. It becomes an unknown reflectance and is no use as a known reflectance.

Furthermore, if lighting is low or high then adjustments need to be made.

Personally I don't use a grey card since doing my own film speed, dev and printing calibration means I get more accurate readings and exposure placement than I would when using a grey card.

markbarendt · Mar 22, 2016

Good info Rob,

What do those instructions say about adding or subtracting from the measured reading?

I do on occasion use a gray card but it is typically when testing a new idea or designing a specific set.

wiltw · Mar 22, 2016

But then there is also the fact that 18% grey only happens to be 'in the middle between the tonal extremes of white and black' and is NOT truly a good metering target for exposure.

As to the variability of angle, this illustrative series shot under overcast conditions when specular reflectivity is less an issue, and exposed per incident meter reading (ISO 400, 1/250 f/5.6 +0.3EV), a series of shots taken with the 18% card at a continuously moving (tilted) angle both horizontally and vertically. Note only frame 7, which is tilted per the Kodak instructions cited by RobC, appear truly similar to the midtone surrounding grey.

BrianShaw · Mar 22, 2016

That's similar to the test I did yesterday, but I only tilted side-to-side and measured at 180 and 45 degrees, for a difference of about .5 stop (I may have rounded up to that from .3 or .4... I can hardly recall yesterday; sorry).

Interestingly, in Professional Portrait Techniques, Kodak Pub O-4 dated 1980, and the Kodak Light-Ratio Calculator, Kodak Pub O-4a, there is mention of angling the gray cards on one axis at 45 degree.

I, personally, did not recall the gray card instructions discussing tilting in two axes. My gray card has not been used for so long that I know I still have it but cant find it. No matter, it appears that I've been using it 50% correctly and 50% incorrectly in the past.

markbarendt · Mar 22, 2016

wiltw said:
But then there is also the fact that 18% grey only happens to be 'in the middle between the tonal extremes of white and black' and is NOT truly a good metering target for exposure.

As to the variability of angle, this illustrative series shot under overcast conditions when specular reflectivity is less an issue and per incident meter reading (ISO 400, 1/250 f/5.6 +0.3EV), a series of shots taken with the 18% card at a continuously moving (tilted) angle both horizontally and vertically. Note only frame 7, which is tilted per the Kodak instructions, appear truly similar to the midtone surrounding grey.

Actually when used in a consistent manner the gray card is a fine tool for judging/metering for exposure. It sets up a very specific relationship to the speed point of the film.

While I fully acknowledge that the angle affects what is reflected: as long as a specific user is consistent, aware, and has done some very basic tests; the exact angle isn't that important.

What I'm getting at here is that the card is capable of high accuracy but with experience (when you know what normal is) it's use can help us bias the meter.

If for example your #7 is "normal", then orienting the card differently like say in #1 can bias the meter to add extra shadow detail to the film, orienting like #4 would protect the highlights more.

RobC · Mar 22, 2016

markbarendt said:
Good info Rob,

What do those instructions say about adding or subtracting from the measured reading?

I do on occasion use a gray card but it is typically when testing a new idea or designing a specific set.

There's four pages about its usage for metering with a reflection meter, measuring lighting ratios, how to compare with incident meter etc etc and I'm not going to transcribe all of that and don't have a scanner with optical character recognition. So in short, if you want the full instructions you'll need to buy a kodak card which comes with them. But if you aren't going to use one then there's little point. Fact is you don't need one and even if you have one you're unlikely to get all the angles correct so there's little point.
If all you want is a colour reference then any neutral grey card perpendicular to lens axis will do it for you since you can measure its reflectance and the final print reflectance to see if they are the same. And for digital you can use it to set colour balance in your photo app.

Unless someone can find the instructions online but I couldn't find them at kodak site, which incidentally, swears all reflection meters are calibrated to 18% grey density which means precisely nothing as far as I can tell.

Actually its says if your subject is quite light(???) then decrease exposure by 1/2 to 1 stop and if subject is quite dark(???) then increase your exposure by 1/2 to 1 stop. Thats a lot of use to people using slide film and only marginally better for people using negative film.
??? What does quite light and quite dark mean. I presume it measn high or low contrast but how high or how low. Its just another pointless ball park guesstimate approach.

RobC · Mar 22, 2016

using it perpendicular to lens axis is half the sequence of doing a lighting ratio test but I haven't bothered to read all of that.
If you have done proper film,dev, print claibration then you can just use your spot meter directly to get lighting ratios and placement of tones.

Chan Tran · Mar 22, 2016

If you face the card straight toward the camera and use a spot meter to measure it. It would give the same reading as an incident meter with a flat diffuser. Angling the card toward the light source emulates the incident meter with the dome.

DREW WILEY · Mar 22, 2016

Gosh. It can get misleading taking readings off gray cards on copystands where you cross-polarize the light. Seems simple in theory, but woe be to those who don't actually test the exposure result first.

wiltw · Mar 22, 2016

RobC said:
So in short, if you want the full instructions you'll need to buy a kodak card which comes with them. .

For those who have been around Kodak grey cards for a while, it is known that at different points Kodak has neglected to provide instruction about tilting the card! In fact this is all Kodak says on their website currently, something which is highly subjective!...

"Exposure
"When shooting the gray card, it is recommended that you meter the card with a spot meter. Point the meter at the center of the card and use the reading directly. It is a good practice to have the meter reading for the gray card match the shooting stop, which is typically determinded by using an incident meter. In most cases - if the gray card is in the key light - the two readings will be very close. Tilting the card toward the key light may be necessary for them to match exactly. It takes practice, but this method yields accurate and repeatable results."

http://motion.kodak.com/us/en/motion/education/publications/shooting/gray_card/default.htm

Rob, how long ago did you purchase yours, just as reference information?

One version of card purchased by someone in 1999 says only, ""Meter readings of the gray card should be adjusted as follows-
1) For subjects of normal reflectance increase the indicated exposure by 1/2 stop.
2) For light subjects use the indicated exposure; for very light subjects decrease exposure by 1/2 stop
3) If the subject is dark to very dark increase the indicated exposure by 1 to 1.5 stops"

markbarendt · Mar 22, 2016

RobC said:
Actually its says if your subject is quite light(???) then decrease exposure by 1/2 to 1 stop and if subject is quite dark(???) then increase your exposure by 1/2 to 1 stop.

Exactly what I was after, thanks.

IMO that translates to mean "the reading is a suggestion, your judgement is still required to make a good photo".

RobC · Mar 22, 2016

long time ago, approx 20 years.

You need to be very careful about which particular usage of the grey card you are refering to becasue some of the techniques require the card to be perpendicular to the lens axis. But not the reflection meter reading for 18% reflectance.

Its the R27 publication. Publication E152 7795 ©1995

not the newer grey card plus.

wiltw · Mar 22, 2016

A a bit more food for thought, from a web publication available via Adorama, authored by Bob Shell, Dead Link Removed

" If you have done reading about or studied photography, you may have heard that 18% reflectance is a photographic mid tone. This has been stated in many books and taught in photography courses for some time Unfortunately, it is wrong. There has never been an ISO standard created for mid tone reflectance, so by default the older ANSI standard applies. The ANSI standard specifies a reflectance of about 12.5% for a photographic mid tone. This is exactly one half stop less reflectance than 18%.

"What happened to that missing half stop? I found it. In 1999 Eastman Kodak decided to revise the Kodak Gray Card since the old text and packaging had become dated, and they came to me and asked me to write new text for them. This gave me the incentive to delve into just why the Gray Card was made with 18% reflectance (actually it is 17.7XXX) when 12.5% was the standard and made much more sense. It came out after a lot of digging that the last revision of the Gray Card had been in 1979, and at that time a mistake was made in the instructions and this mistake was not noticed in twenty years. The previous instructions had included the statement that you should open up 1/2 stop from the reading obtained from the Gray Card, which would compensate for the difference perfectly, but this was somehow left out of the 1979 instructions.

"To make a long story short, I put it back into the revised instructions in 1999, so if you have an older Gray Card you should remember this correction, and apply it, and if you buy a Kodak Gray Card you should make sure it is the newer version. For a lot of photography an error of half a stop will not be a problem, such as many types of black and white film and color negative films. But when shooting color slide film a half stop error can ruin an image."

RobC · Mar 22, 2016

Well no wonder its causes such a massive amount of confusion.

Can't find my densitometer, i think itspacked somewhere pending move. But I swear it used to read 0.7 when zeroed on the white back 90% reflectance, and not .75 as some are saying they get.
But my memory may be wrong.

markbarendt · Mar 22, 2016

RobC said:
Well no wonder its causes such a massive amount of confusion.

Yes, people expect it (as they expect many other things) to be "the answer" and it is not.

Stephen Benskin · Mar 22, 2016

wiltw said:
[QUOTE="RobC, post: 1772248, member: 70691" So in short, if you want the full instructions you'll need to buy a kodak card which comes with them. .

For those who have been around Kodak grey cards for a while, it is known that at different points Kodak has neglected to provide instruction about tilting the card! In fact this is all Kodak says on their website currently, something which is highly subjective!...

"Exposure
"When shooting the gray card, it is recommended that you meter the card with a spot meter. Point the meter at the center of the card and use the reading directly. It is a good practice to have the meter reading for the gray card match the shooting stop, which is typically determinded by using an incident meter. In most cases - if the gray card is in the key light - the two readings will be very close. Tilting the card toward the key light may be necessary for them to match exactly. It takes practice, but this method yields accurate and repeatable results."

http://motion.kodak.com/us/en/motion/education/publications/shooting/gray_card/default.htm

Rob, how long ago did you purchase yours, just as reference information?

One version of card purchased by someone in 1999 says only, ""Meter readings of the gray card should be adjusted as follows-
1) For subjects of normal reflectance increase the indicated exposure by 1/2 stop.
2) For light subjects use the indicated exposure; for very light subjects decrease exposure by 1/2 stop
3) If the subject is dark to very dark increase the indicated exposure by 1 to 1.5 stops"[/QUOTE]

I've been unsuccessfully looking for those instructions. Thanks for posting it.

BrianShaw · Mar 22, 2016

While witw was typing post #13 I was clawing through a pile of old documentation and found my ancient gray card and the instructions (1978):

Exposure Determination
... Hold the card straight up and down, close to and in front of the subject, and facing halfway between the camera and the main light. Make the light reading with the meter not more than 6 inches away. ... If made on the gray side of the Test Card, the meter reading will, on the average, be the same as a reflected-light reading of an average indoor scene, taken from the camera position. ... (and the same language as wiltw cited in post 10 about subject with unusually lighter/darker...

Similar for Adjusting Lighting Ratio: Hold card straight up and down...

Similar for Use with an exposure meter in daylight; SUNLIGHT - frontlighted or sidelighted scenes with the additional advise, "Note that under most sunlight conditions, it is not necessary to tilt the card because of the elevation of the sun... except at noon in the equatorial region. And similar for Other outdoor lighting conditions; Backlighted subjects, subjects in shade, overcast days.

markbarendt · Mar 22, 2016

Stephen Benskin said:
One version of card purchased by someone in 1999 says only, ""Meter readings of the gray card should be adjusted as follows-
1) For subjects of normal reflectance increase the indicated exposure by 1/2 stop.
2) For light subjects use the indicated exposure; for very light subjects decrease exposure by 1/2 stop
3) If the subject is dark to very dark increase the indicated exposure by 1 to 1.5 stops"

I've been unsuccessfully looking for those instructions. Thanks for posting it.[/QUOTE]

Those are what I remember from when I bought mine but haven't been able to find my copy or any online other than the Motion Picture one.

BrianShaw · Mar 22, 2016

Stephen Benskin said:
For those who have been around Kodak grey cards for a while, it is known that at different points Kodak has neglected to provide instruction about tilting the card! ...

It's worse that that... they weren't just silent about tilting the card, but in the instructions I posted they quite intentionally specified NOT to tilt the card. I wonder what made them change their mind?????????????????????

RobC · Mar 22, 2016

could it be that a grey card is pretty useless as a known reflective percentage and the wind was in the East on a Thursday morning when they re-tested it. Or maybe they got someone else to write the instructions who had a differeent opnion.

Has it actually changed colour and reflectance or has only the angle of the dangle changed to get a different reflection percentage.

RobC · Mar 22, 2016

I think you need a highly complex angle dangle machine to set it to the right orientation and then it'll still be guess work as to whether its quite light or quite dark and whether its quite a 1/2 stop or quite 1 stop quite to get the right reading.

Anyone still thinking of using a kodak grey card to get their exposure spot on

p.s. don't forget that kodak think your meter is calibrated to an 18% reflectance density just to confuse the issue even further

Old-N-Feeble · Mar 22, 2016

spot meter... no gray card

wiltw · Mar 22, 2016

RobC said:
I think you need a highly complex angle dangle machine to set it to the right orientation and then it'll still be guess work as to whether its quite light or quite dark and whether its quite a 1/2 stop or quite 1 stop quite to get the right reading.

Anyone still thinking of using a kodak grey card to get their exposure spot on

p.s. don't forget that kodak think your meter is calibrated to an 18% reflectance density just to confuse the issue even further

Reminds me of these schools of thought, in recent discussions

"I have this 18% (variable brightness surface, see earlier series post 3), which is or is not the grey tone used for reflected light meter readings"
"I have this 12% (darker variable brightness surface than the above series post 3), which is really what should be used as the grey tone used for reflected light readings"
"I have computed that 9% is really the surface brightness which ought to be used as the grey tone for reflected light readings"
"I have conducted tests and densitometer readings and drawn intricate charts and none of you are correct, I am right"

In truth, any one of them could be right at any point in time, and all of them can be equally wrong at all the other points in time. It is no wonder than reflected light readings sometimes match incident readings, but then at other times they don't match, and of the do not match they can vary in the amount of disagreement, too, based upon when the distance from the moon to Earth coincides with the refractive index of the polluted air in the sky and the amount of humidity.

We know with certainty that 18% grey only happens to be 'in the middle between the tonal extremes of white and black'.

BrianShaw · Mar 22, 2016

RobC said:
I think you need a highly complex angle dangle machine to set it to the right orientation and then it'll still be guess work as to whether its quite light or quite dark and whether its quite a 1/2 stop or quite 1 stop quite to get the right reading.

Anyone still thinking of using a kodak grey card to get their exposure spot on

p.s. don't forget that kodak think your meter is calibrated to an 18% reflectance density just to confuse the issue even further

Kodak did not appear to intend to take the brain out of exposure determination!

wiltw · Mar 22, 2016

BrianShaw said:
Kodak did not appear to intend to take the brain out of exposure determination!

And as the typical brain is highly subjective, one can restate the above as, "Kodak did not appear to intend to take the subjectivity out of exposure determination!"

Kodak grey card usage

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