Grey card wrong?

[swmcl]

Hello all.

I got out my Kodak grey card and decided to try something different. I had gone to the local craft shop and purchased a 150mm poly whatever (styrene I think) foam ball. You know, the white ones. I also purchased some cheap acrylic paint in tubes - one white, one black. I mixed up what I thought might be a good shade of grey and painted the ball with it. As it turns out it darkened a little upon drying.

Then I sat my newly painted but dry (not glossy) ball on a stick and had my daughter hold the Kodak grey card alongside in even but afternoon light ...

Guess what ? The centre of the ball came up with the same exposure as the incident reading yet the grey card - which does appear lighter and a little glossy I must say - gave a reading 2/3rds of a stop different. I was up pretty close to the ball and card so that the meter was reading in the centre of each.

I'm using a very new Sekonic L758DR mind you which talks in 1 tenth stops.

What do you think ? Could the grey card be out ? Should I now paint the grey card and the ball ?!?! Should I just get over it and leave the grey card alone from now on ? Lastly, should I use the grey card as-is but realise it is 2/3rds of a stop out of 'calibration' with my meter ?

Measurements were within 10 seconds of one another if you're thinking there may be a time factor in it.

This is not the first time I've queried the grey card mind you. It was out with my other meter too - an L358.

Rgds,

Steve

[Leigh]

Disregarding the fact that the sun is upside down for you folks...

You're not using the gray card properly.

They're designed and calibrated for use with off-axis light, i.e. not coming from directly behind/above the camera.
The card should be oriented such that a line perpendicular to its surface bisects the angle between the light source and the camera.

The Kodak cards that I've purchased over the years included those instructions.
Perhaps they were missing from yours, or maybe Kodak no longer includes them (to save money).

Also, forgive the observation, but the quality control used to produce those cards exceeds the accuracy of your method by many orders of magnitude.

- Leigh

[swmcl]

OK. Instructions are long gone I suspect. I don't recall seeing any actually.

The card is now probably towards 10 years of age ... does this make a difference ? The direction of the light wasn't completely behind me as such and adjustments on the angle of the card made diddly squat difference really. Not anywhere near 2/3rds of a stop. The difference is also backed up by another lightmeter.

I know this isn't the most scientific test but hey ! Cut some slack. The results are too far out to ignore and too far out to be because of an alignment problem. I just wonder if others have seen similar results or if there is a glaring problem with what I'm doing.

Its night time now but I could repeat at different incident angles to the card tomorrow but I don't think it'll make much of a difference.

The QC issue is between Kodak painted surface and Sekonic ? Hmmm. I guess I must trust the Sekonic as it will be the piece of equipment that comes with me.

Rgds,

[hoffner]

Excuse me the sincerity but your experiment as well as its description borders on technical lunacy. A ball, measured in its centre - one must wonder about the weird idea. Sorry for the sincerity.

[hoffner]

Hello all.

I mixed up what I thought might be a good shade of grey and painted the ball with it. As it turns out it darkened a little upon drying.



What do you think ? Could the grey card be out ? Should I now paint the grey card and the ball ?!?!
Steve

On second thought - most definitely. Take as many balls as possible and paint them up to the moment when it starts to be tiring. It could help.

[Bruce Barlow]

It is a well-known, totally unscientific fact that meters are often off, and 2/3 of a stop is not unusual. My 2 Pentax Digital meters are a stop apart at the moment, or would be if I had batteries... They were a stop off when they were both working. That's 2 meters aiming at the same thing from the same angle, with little room for error. I just changed the ISO on one so that the indicated exposures would match. From experience, I actually knew which one was correct.

If meters didn't go out of accuracy, Richard Ritter wouldn't make pocket change calibrating them. He routinely calibrates cinematographers' meters to have them match - I think one guy sends him 4 at once. 3 is not uncommon.

So just to make life more strange, I'll go with a meter that's off. If it's off so that you are overexposing by 2/3 of a stop, I'd even bet that they set it that way on purpose. Better shadow detail.

Wait a minute, just reread your original post. You said you took an INCIDENT reading? An incident reading reads the light source, not what the light is reflected from, using, typically, a little white hemisphere over the sensor. If that's so, you can't blame the grey card. Blame the sun, and now you have a real mystery. Cool!

I would also caution you that 2/3 of a stop is a lot less than you think, and your ball and the card may not be a match. Go into the darkroom and make 2 prints that are a stop apart. You'll be surprised, I think, at how similar they still seem to be. Then make 2 that are 2 stops apart. See what I mean? This exercise is fabulous eye training.

BTW, you said the ball paint darkened as it dried. Voila! You experienced dry-down in real life. Remember that when you make prints.

[Ken Lee]

I am no expert but have read that an 18% gray card will only represent middle gray when the scene has a 5-stop range of contrast. An 18% gray card is perfect for copying flat documents for that reason because there's around 5 stops difference between black ink and white paper. The same card may be less than ideal as a "middle gray" reference for outdoor portraits, landscapes, etc.

You might find this article interesting: http://btzs.org/Articles/Sensitometry%20Part%205.pdf.

[vinny]

It is a well-known, totally unscientific fact that meters are often off, and 2/3 of a stop is not unusual. My 2 Pentax Digital meters are a stop apart at the moment, or would be if I had batteries... They were a stop off when they were both working. That's 2 meters aiming at the same thing from the same angle, with little room for error. I just changed the ISO on one so that the indicated exposures would match. From experience, I actually knew which one was correct.



If meters didn't go out of accuracy, Richard Ritter wouldn't make pocket change calibrating them. He routinely calibrates cinematographers' meters to have them match - I think one guy sends him 4 at once. 3 is not uncommon.

So just to make life more strange, I'll go with a meter that's off. If it's off so that you are overexposing by 2/3 of a stop, I'd even bet that they set it that way on purpose. Better shadow detail.

Wait a minute, just reread your original post. You said you took an INCIDENT reading? An incident reading reads the light source, not what the light is reflected from, using, typically, a little white hemisphere over the sensor. If that's so, you can't blame the grey card. Blame the sun, and now you have a real mystery. Cool!

I would also caution you that 2/3 of a stop is a lot less than you think, and your ball and the card may not be a match. Go into the darkroom and make 2 prints that are a stop apart. You'll be surprised, I think, at how similar they still seem to be. Then make 2 that are 2 stops apart. See what I mean? This exercise is fabulous eye training.

BTW, you said the ball paint darkened as it dried. Voila! You experienced dry-down in real life. Remember that when you make prints.

2/3 of a stop isn't much unless you're shooting chromes. Then it's a LOT and unnacceptable

[ic-racer]

http://dpanswers.com/content/tech_kfactor.php

[Bill Burk]

I'm using a very new Sekonic L758DR.

There is plenty of Internet information that explains 18% gray card is not the equivalent of incident mode. Meters are not calibrated to 18%.

I use the same meter and have the Sekonic gray card which includes patches in 1/3 stop intervals so I can find the patch that does give the same spotmeter reading as incident mode in the same light. I find the reading is closer to 12%, which agrees with correspondence I've had here and on APUG, and it agrees with Internet lore*.

Instead of confirming exposure with a gray card, I satisfy my "sanity" by taking a spotmeter reading of the palm of my hand. When I "place the reading on Zone VI," my readings agree well with the incident mode, and then I feel more reassured.

*But of course that doesn't prove the Internet lore is correct.