Help understanding gray card use

336v · May 31, 2025

Hi all,
Need to calibrate my light meter and I keep reading about gray card use, which seem unnecessary for what I'm after.
Pls tell me where I'm missing something.
So, I have an old light meter requiring calibration (Luna Pro in my case, but the discussion applies to any such generic
device allowing calibration). I'll be using Nikon D-200 DSLR's internal metering as a reference. I'm not interested in
knowing exact absolute number of Lux hitting the sensor, converting it to EV or any of that (may be that's what a gray
card is used for). All I need is to point D-200 to any uniformly lit object (a flat wall would be ideal), and illuminate it with
variable light source, dialing light intensity such that D-200 shows, say 1/25" with F/5.6 @ ISO=100 (or any combination
for that matter). The "whiter" the wall, the less illumination it will require to achieve this level of exposure, certainly no
need for the wall to be gray. I then turn adjustments on my light meter to show the same numbers on it. Next, I'll arbitrarily
increase light intensity to achieve next readout point, say, 1/200" @ F/5.6 @ the same ISO=100. Adjust my meter to match
that. Increment lighting a couple of more times to get additional measurement points, I may go back and forth a couple of
times, until Luna Pro readings match those of the D-200 for given lighting. Again, note, I don't care what is actual illumination
in Lux or what EV number is - the meter will sure show me that, but my objective is to just match un-calibrated hardware with
calibrated one - whatever EV happens to be in those steps. This the procedure doesn't care what color the wall is, it certainly
does not have to be gray and in fact doesn't have to be a wall at all - can be any object I can artificially illuminate and point
both meters to. Looks like no need for gray cards of any sort for such calibration. What am I missing?

BrianShaw · May 31, 2025

I don’t think you are missing anything. You are matching 2 light meters rather than directly calibrating to a standard. Your method seems quite logical and your conclusion about target color seems quite correct in that application.

xkaes · May 31, 2025

I don't know exactly what you are trying to do, but if you "Need to calibrate my light meter" you can use the D-200, but they will give you different readings even assuming each is working perfectly.

Your camera has a lens. Your meter doesn't. You're meter's photo cell has a different angle of acceptance than your camera and lens. I could go on. They will very likely give you different readings under any situation.

You don't need a gray card, but that's why they are used -- to try to eliminate as many variables as possible.

I'd suggest you try to match your camera AND your meter to the f-16 rule instead -- to find out how accurate each one is.

Chan Tran · May 31, 2025

336v said:
Hi all,
Need to calibrate my light meter and I keep reading about gray card use, which seem unnecessary for what I'm after.
Pls tell me where I'm missing something.
So, I have an old light meter requiring calibration (Luna Pro in my case, but the discussion applies to any such generic
device allowing calibration). I'll be using Nikon D-200 DSLR's internal metering as a reference. I'm not interested in
knowing exact absolute number of Lux hitting the sensor, converting it to EV or any of that (may be that's what a gray
card is used for). All I need is to point D-200 to any uniformly lit object (a flat wall would be ideal), and illuminate it with
variable light source, dialing light intensity such that D-200 shows, say 1/25" with F/5.6 @ ISO=100 (or any combination
for that matter). The "whiter" the wall, the less illumination it will require to achieve this level of exposure, certainly no
need for the wall to be gray. I then turn adjustments on my light meter to show the same numbers on it. Next, I'll arbitrarily
increase light intensity to achieve next readout point, say, 1/200" @ F/5.6 @ the same ISO=100. Adjust my meter to match
that. Increment lighting a couple of more times to get additional measurement points, I may go back and forth a couple of
times, until Luna Pro readings match those of the D-200 for given lighting. Again, note, I don't care what is actual illumination
in Lux or what EV number is - the meter will sure show me that, but my objective is to just match un-calibrated hardware with
calibrated one - whatever EV happens to be in those steps. This the procedure doesn't care what color the wall is, it certainly
does not have to be gray and in fact doesn't have to be a wall at all - can be any object I can artificially illuminate and point
both meters to. Looks like no need for gray cards of any sort for such calibration. What am I missing?

If you do that make sure the wall is large enough to cover the entire measureing view of both the D200 and the Luna Pro and the color of the wall are the same overall, no dark or light spot. On the D200 use center weighted. Do not use the matrix as matrix does consider the color of the wall for example it would try to make a yellow wall lighter than a red wall. Also if the scene is very bright it will increase the exposure. Spot is OK but if the wall is not perfectly of same color the spot you measure may be a bit lighter or darker than the others. Measuring the large area with center weighted average the differences. Also the wall must cover the entire view otherwise the weighted factor will become a factor.

RalphLambrecht · May 31, 2025

336v said:
Hi all,
Need to calibrate my light meter and I keep reading about gray card use, which seem unnecessary for what I'm after.
Pls tell me where I'm missing something.
So, I have an old light meter requiring calibration (Luna Pro in my case, but the discussion applies to any such generic
device allowing calibration). I'll be using Nikon D-200 DSLR's internal metering as a reference. I'm not interested in
knowing exact absolute number of Lux hitting the sensor, converting it to EV or any of that (may be that's what a gray
card is used for). All I need is to point D-200 to any uniformly lit object (a flat wall would be ideal), and illuminate it with
variable light source, dialing light intensity such that D-200 shows, say 1/25" with F/5.6 @ ISO=100 (or any combination
for that matter). The "whiter" the wall, the less illumination it will require to achieve this level of exposure, certainly no
need for the wall to be gray. I then turn adjustments on my light meter to show the same numbers on it. Next, I'll arbitrarily
increase light intensity to achieve next readout point, say, 1/200" @ F/5.6 @ the same ISO=100. Adjust my meter to match
that. Increment lighting a couple of more times to get additional measurement points, I may go back and forth a couple of
times, until Luna Pro readings match those of the D-200 for given lighting. Again, note, I don't care what is actual illumination
in Lux or what EV number is - the meter will sure show me that, but my objective is to just match un-calibrated hardware with
calibrated one - whatever EV happens to be in those steps. This the procedure doesn't care what color the wall is, it certainly
does not have to be gray and in fact doesn't have to be a wall at all - can be any object I can artificially illuminate and point
both meters to. Looks like no need for gray cards of any sort for such calibration. What am I missing?

you are missing nothing. that's a perfectly sane approach.

Maris · May 31, 2025

Doing a one point photometric calibration by matching an unknown light meter to a known and correct light meter inspires confidence in getting good exposure information.

I've done this procedure but I still got some exposures wrong. My problem involved a camera with a good TTL meter, Pentax ME-Super, matched to a good Gossen Luna Pro hand held meter.
Faced with a normal outdoor landscape the Pentax would give three different meter readings depending on framing the scene with a bit of sky, 1/2 sky, or a lot of sky. How do I know which reading is correct?
Only one reading is right and I can get it by turning the exposure compensation dial. But this involves either well practiced judgement born of long experience or outright guess work supported by exposure bracketing. Not so easy.

Alternatively how should I deploy the Luna Pro meter to find this "correct" reading? Is it meter the ground and forget the sky, meter the sky and give two stops more, or is it meter land and sky and strike an average?
Again the decision depends on long experience or, failing that, more guesswork. The one point calibration told me that the TTL camera and hand held meter were not contradictory but they do see the world in different ways.
And I still have to make a judgement call about exposure settings.

reddesert · Jun 1, 2025

The gray card is used to represent an average surface brightness, average over a typical scene. It's useful if you're trying to do something like approximate incident metering with a reflective meter - you hold the gray card under the light and point the reflective meter at it.

In your case, it doesn't really matter whether you use a gray card or a white piece of paper or a white wall, so long as the target is large enough to roughly fill the field of both the camera and the meter. There is a caveat, which is that the camera is probably doing matrix metering. That means it is comparing the scene as measured by its multiple sensors to some database of brightness distributions and trying to determine an optimum exposure. It's not as simple as a centerweighted or averaging meter. We assume that if it sees a roughly uniform brightness, it picks an exposure that is roughly the average for that brightness, but we don't know that for sure.

In addition, at the extremes of the brightness range, it might deviate. For example, if you show a uniform very bright card to a matrix metering camera, its database might compare that to a snow or beach scene and deliberately overexpose a stop or 2 (because it thinks the scene is supposed to come out bright, not 18% gray). Similarly if you show it a very dark card it might think you have a night scene and underexpose compared to an averaging meter. This would be the right decision for pictorial exposure, but wrong for meter calibration. I have no idea if this is a serious issue in practice, but if you can switch the camera to spot or averaging metering, which you probably can, I would try that.

F4U · Jun 1, 2025

The best calibration standard, whether it be electronic test and electronic gear or photographic gear is to pick the newest piece of gear in your house, and use THAT as your standard. The factory that made it presumably owns the best equipment when they made your product. If your neutral gray card is clean and not faded, then whatever piece of gear you chose for your "standard", then simply duplicate the measurement position and angle of view for the piece of gear you are calibrating. If you can set a linear response from light-to-dark within 1/3 f/stop, which was at one time the Kodachcome/Ektachrome standard, then you have done an excellent job. What you really need to keep in mind is color temperature. The standard is generally accepted to be a neutral gray card, lit with 5000-6000K light. If you can get your meter to be accurate from 3-18EV at ASA100 you win the lucky prize, properly calibrated meter. GL

wiltw · Jun 1, 2025

1. When I point my Canon dSLR at an 18% grey card, its reading matches the reading I get with a one-degree spotmeter pointed at the same grey card, and both match the incident reading I get with my Minolta incident light meter
2. Subject brightness can vary widely, between black and white...an 18% grey card is merely the midpoint of that range of brightnesses, so that your reflected light meter is not biased by an unusually bright/dark subject...what Kodak used to call 'subject failure'
3. Different colors can be darker or brighter than mid-tone, as illustrated by these two photos

4. One CAN be fooled in reading an 18% grey card due to it being held at the wrong angle to the light source, as demonstrated by this illustrations...

...but the primary reason to use an 18% grey card is to meter to the mipoint of the range of brightnesses, between black and white extremes.

Nevertheless, a very bright background can fool a meter to overexpose a scene so that an 18% grey card is recorded darker than it should...

...so one needs to use an 18% grey card in an intelligent manner!

gary mulder · Jun 1, 2025

Some things seem to be that obvious people forget. If you take a picture of a 18% grey card and your film development and light meter are functioning properly you will get a negative that wil transmit 18% of the light. If you print that negative you will get a print that reflects 18%. That’s the way a photographic can be calibrated.

Chan Tran · Jun 1, 2025

Maris said:
Doing a one point photometric calibration by matching an unknown light meter to a known and correct light meter inspires confidence in getting good exposure information.

I've done this procedure but I still got some exposures wrong. My problem involved a camera with a good TTL meter, Pentax ME-Super, matched to a good Gossen Luna Pro hand held meter.
Faced with a normal outdoor landscape the Pentax would give three different meter readings depending on framing the scene with a bit of sky, 1/2 sky, or a lot of sky. How do I know which reading is correct?
Only one reading is right and I can get it by turning the exposure compensation dial. But this involves either well practiced judgement born of long experience or outright guess work supported by exposure bracketing. Not so easy.

Alternatively how should I deploy the Luna Pro meter to find this "correct" reading? Is it meter the ground and forget the sky, meter the sky and give two stops more, or is it meter land and sky and strike an average?
Again the decision depends on long experience or, failing that, more guesswork. The one point calibration told me that the TTL camera and hand held meter were not contradictory but they do see the world in different ways.
And I still have to make a judgement call about exposure settings.

You're talking about 2 different things. To calibrate the meter is to make it accurate even if it's accurately wrong for the exposure you want.

cowanw · Jun 1, 2025

gary mulder said:
Some things seem to be that obvious people forget. If you take a picture of a 18% grey card and your film development and light meter are functioning properly you will get a negative that wil transmit 18% of the light. If you print that negative you will get a print that reflects 18%. That’s the way a photographic can be calibrated.

Since the print can be made over a range of enlarger time choices, this seems like a circular argument. Any negative transmitting any percentage can be printed to the print of 18% reflection. what am I missing?

xkaes · Jun 1, 2025

As usual, we certainly have gotten far afield. All 336v wants to do is calibrate his light meter so that it works correctly. He thought of comparing it to the results from his camera -- which is one way -- but it's much simpler to calibrate it to match the f16 rule. No camera or gray card or average scene or gymnastics of any kind are needed.

Chan Tran · Jun 1, 2025

xkaes said:
As usual, we certainly have gotten far afield. All 336v wants to do is calibrate his light meter so that it works correctly. He thought of comparing it to the results from his camera -- which is one way -- but it's much simpler to calibrate it to match the f16 rule. No camera or gray card or average scene or gymnastics of any kind are needed.

In fact if you calibrate for the f/16 rule you will indeed need the gray card.

xkaes · Jun 2, 2025

Chan Tran said:
In fact if you calibrate for the f/16 rule you will indeed need the gray card.

That's one way of doing it.

wiltw · Jun 2, 2025

xkaes said:
As usual, we certainly have gotten far afield. All 336v wants to do is calibrate his light meter so that it works correctly. He thought of comparing it to the results from his camera -- which is one way -- but it's much simpler to calibrate it to match the f16 rule. No camera or gray card or average scene or gymnastics of any kind are needed.

But the Sunny 16 rule of thumb is only an APPROXIMATION on any given day, and one could be 'calibrating' one's meter outside the actual range of exposures specifiied by the ISO standard formula!

stability of 'sunny 16'

I normally see a 1/2 to full stop between a reflective and incident and meters depending on the scene, how much sky, or very bight and large reflective surfaces may be present. It is my understanding that Nikon optimized their version of matrix or evaluative meting for slides, not sure if true...

www.photrio.com

...so you know your meter is 'approximately right' if depending upon Sunny 16.

xkaes · Jun 2, 2025

Much depends on how micro-milli-accurate you want to get. For some odd reason, when I check my camera or hand-held meters for accuracy (every so often, but not routinely) I never have a problem.

Chan Tran · Jun 2, 2025

wiltw said:
But the Sunny 16 rule of thumb is only an APPROXIMATION on any given day, and one could be 'calibrating' one's meter outside the actual range of exposures specifiied by the ISO standard formula!

stability of 'sunny 16'

I normally see a 1/2 to full stop between a reflective and incident and meters depending on the scene, how much sky, or very bight and large reflective surfaces may be present. It is my understanding that Nikon optimized their version of matrix or evaluative meting for slides, not sure if true...

www.photrio.com

...so you know your meter is 'approximately right' if depending upon Sunny 16.

A lot of people say 1/3 of stop is close enough and that's about 25% off and I would call that approximately.

xkaes · Jun 2, 2025

Add up an approximate meter reading, and an approximate f-stop setting, and an approximate shutter speed accuracy, and an approximate ISO rating and you can forget about getting to home plate! You're lucky to land in the ball park.

Chan Tran · Jun 2, 2025

xkaes said:
Add up an approximate meter reading, and an approximate f-stop setting, and an approximate shutter speed accuracy, and an approximate ISO rating and you can forget about getting to home plate! You're lucky to land in the ball park.

In that case I do better without a meter.

MARTIE · Jun 2, 2025

I thought the point of a light meter was to have something to blame when the exposure's wrong!

Chan Tran · Jun 2, 2025

MARTIE said:
I thought the point of a light meter was to have something to blame when the exposure's wrong!

Nah! I hate it's more when I get good exposure with a bad meter.

MARTIE · Jun 2, 2025

Chan Tran said:
Nah! I hate it's more when I get good exposure with a bad meter.

gary mulder · Jun 3, 2025

cowanw said:
Since the print can be made over a range of enlarger time choices, this seems like a circular argument. Any negative transmitting any percentage can be printed to the print of 18% reflection. what am I missing?

Yes you can always overexpose your film to be at the save side.

pentaxuser · Jun 3, 2025

MARTIE said:
I thought the point of a light meter was to have something to blame when the exposure's wrong!

Isn't the point of light meters and a question such as the OP asked to have a couple or s to have read his post and answer his actual question accurately and boringly while most want a more exciting debate?

pentaxuser

Help understanding gray card use

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