DIY gray card?

Mr Bill · Jun 26, 2020

DREW WILEY said:
... but I still distrust most reference grays except the gray patches on clean unfaded MacBeth Color Checker charts.

Likewise, except that I mostly trust the Kodak cards. As I recall Kodak's gray cards have been using the ColorChecker colorant for some time (perhaps 10 years or so??). I don't have a good reference for this, though; could be wrong.

For most practical purposes, though, I doubt that one needs to be so critical.

If I needed a reflection reference, and no access to an official gray card, I'd probably just go for a white card and correct for the exposure. Regular white paper is not that great cuz it almost always contains "brighteners." Same as most white clothing. Under near-UV light they fluoresce with a weak bluish tinge. (If you filter out the UV light this is not a problem.)

The easiest way to get a brightener-free white board is to get a piece of museum-grade matte mounting board from a local art supply store (this tends toward a yellowish tinge, so get the whitest they have). It'll probably have a good diffuse surface, and be pretty flat spectrally (until you get near 400 nm wavelength - the falloff there is what gives the yellowish tinge). To make the exposure correction, consider the reflectivity of such a white card to be roughly 86 to 88%. So it's gonna reflect about (87%/18%) = 4.8 times as much as an 18% gray card, which is about 2.3 f-stops, that is 2 3/10, equivalent.

It just depends on exactly what you plan to do with it. If you plan to make prints, using the gray card as a neutral reference, then a white card is not very useful. It IS a reference point, but since it's so close to paper white it will barely have any color to it in a print.

AgX · Jun 27, 2020

One "card" that one always has at hand: ones own hand.

One has to establish the reflectivity if the inner-hand. Some people might even blush at the inner hand. Otherwise good enough for b&w work at least.

Bikerider · Jun 27, 2020

AgX said:
One "card" that one always has at hand: ones own hand.

One has to establish the reflectivity if the inner-hand. Some people might even blush at the inner hand. Otherwise good enough for b&w work at least.

Tried this and when compared it to a grey card and it is about one stop too bright. So long as you remember this it will work quite well

guangong · Jun 27, 2020

For emergencies, a newspaper page covered with text was the old standby.

Donald Qualls · Jun 27, 2020

(re: using one's hand as a "gray card")

Bikerider said:
Tried this and when compared it to a grey card and it is about one stop too bright. So long as you remember this it will work quite well

Caucasian skin is normally expected to photograph around Zone VI to Zone VII, so that's about as expected. Palm skin should be about the same regardless of race.

Bikerider · Jun 27, 2020

I can only write what I said from experience. I used a spot meter so there was no outside influence. I remember this clearly because that was the only slide that was under exposed in the role of film

DREW WILEY · Jun 27, 2020

Kodak card were among the worst. I suspect a lot of them had already faded or discolored long before actual sale. Discoloring can make a difference. When someone might choose to rely on a gray card reading, like for exposing color chrome film, and then have it half a stop off, would be unforgivable. Then some plastic one are simply too reflective and give false readings. Today we also have collapsible gray discs. I obtained a relatively expensive one that was spot on; but the cheapo import clones turned out to be around half a stop off - unacceptable for critical applications. All my spotmeters are precisely calibrated to the same objective standard by a specialist.

Mr Bill · Jun 27, 2020

DREW WILEY said:
Kodak card were among the worst. I suspect a lot of them had already faded or discolored long before actual sale.

Just curious, how long ago, roughly, was your testing? Are you talking about relatively recent, say the past 20 years or so, or perhaps 1970s or 1980s, etc.?

My experience has not been at all like yours. I used to be periodically involved in fairly heavy film (pro color neg) testing from roughly late 1970s into the era of digital. This was all for portrait studio work, and we always included Kodak gray card references in the photos. The company had a fresh RIT photo science grad who was a big stickler for monitoring (and recording) everything, and sorry to say, a lot of it rubbed off on me. Anyway whenever we got new gray cards we always wrote density readings (tri-color) on the back, as a way to detect when fading would set in. I just don't recall any ever showing problems. We periodically swapped them out, though, cuz they'd gradually get scuffed up, etc.

Our in-house gray cards were mostly kept in the original envelopes; controlled office temperature and humidity (~50% RH). Whatever was specifically used in testing might spend a couple weeks at a time on a test stand, under typical ceiling fluorescent lamps. And again, I don't recall ever seeing a "change" in the tri-color density readings. For sure not a good spectral test, but it IS three narrow spectral range snapshots.

Perhaps the Kodak cards might suffer badly under extended exposure to sunlight, or perhaps certain humidity conditions - I dunno cuz we never put them under those conditions.

DREW WILEY · Jun 27, 2020

My instrumentation was far more expensive and accurate. I didn't personally own any of it - earlier models were more expensive than a new Mercedes Benz. These plotted a virtually continuous spectrum as well as reflective values all along that. Last time was less than 20 yrs ago. But I was deliberately testing the variation of gray cards, including Kodak, that were avail from photo stores, and there were still some big pro stores in this area then. Not a single Kodak card was true gray under a standard illuminant. If you want a reliable standard, go McBeth and take good care of the chart. Carefully compare any supplementary cards to the appropriate gray patch on that before trusting it. The spectrophotometers themselves were daily calibrated using an almost indelible ceramic tile. Most portrait studios relied on wide latitude color neg films where one can get away with all kinds of things provided the overall workflow is largely standardized. I'm just speaking from the technical angle. I rarely even bothered with a gray card in my own occasional portrait sessions because if it was anything serious, I printed it myself anyway, and knew what to expect. I ordinarily shot chrome film instead, and even did mainly Cibachrome portraiture instead of C-prints - a lot more fussy, but that's what they'd pay the big bucks for. I charged per print, not per project.

mohmad khatab · Jun 27, 2020

MattKing said:
It will be available in Chile tomorrow?
Maybe not.

Are you from the state of Chile?
Greetings, Chile
I love that country without logical reason.
There is some secret, I don’t know.

RalphLambrecht · Jun 27, 2020

Rafael Saffirio said:
Hi
Is there a way to do a 18% gray card in the darkroom? I need a 8x10 gray card quickly and I thought it could be made with the enlarger and then checked with the light meter of a digital camera.
Has anyone done something like that?

You need a refelction densitometer to do it correctly;the gray should measure 0.75. BTW it can also be done with an inkjet printer and a densitometer; again, the gray should measure 0.75! In my case, a rectangle filled with 6% black in Illustrator or PS gave the right value!

tomkatf · Jun 27, 2020

Rafael Saffirio said:
Hi
Is there a way to do a 18% gray card in the darkroom? I need a 8x10 gray card quickly and I thought it could be made with the enlarger and then checked with the light meter of a digital camera.
Has anyone done something like that?

Rafael, If you like I will send you a new old stock, unopened Kodak Gray card, gratis. If you want it, send your address to my inbox here. I have no idea what your mail system is like in terms of how long it will take to get to you. Hope this might help!
Tom

DREW WILEY · Jun 28, 2020

That's a great a idea. Homemade gray cards every bit as bad as ones you can instantly buy from a camera store really cheap anyway.

alanrockwood · Jun 28, 2020

Rafael Saffirio said:
Hi
Is there a way to do a 18% gray card in the darkroom? I need a 8x10 gray card quickly and I thought it could be made with the enlarger and then checked with the light meter of a digital camera.
Has anyone done something like that?

Do you need it for black and white photography or color photography? If is is for black and white you could get away with a bit of color cast in the card. If you need it for color then the problem becomes much more difficult.

DREW WILEY · Jun 28, 2020

Then there's the adage of taking a reading from the palm of your hand for a stop above middle gray, already mentioned. First, double check your ethnicity if in doubt. Then never go outdoors and get a tan. Tan cards just get tanner in such cases.

Rafael Saffirio · Jun 28, 2020

alanrockwood said:
Do you need it for black and white photography or color photography?

I need it for black and white. what I finally did was find de minimum black and maximum white and then find the middle, I checked with a digital camera and it was almost exactly 2.5 stops darker than white.

Mr Bill · Jun 28, 2020

Rafael Saffirio said:
I checked with a digital camera and it was almost exactly 2.5 stops darker than white.

Cool. White paper reflects probably about 90%, maybe a bit less. 2.5 stops down is a factor of about 5.6 times, so your card ought to be about 90%/5.6 = 16% more or less. Plenty close for metering, etc. Watch out for specular reflections, though, if it's photo paper.

Individual One · Jun 29, 2020

I have recently tried a homemade 18% card. It is a 500x400 grid of cells with a random 82% of them filled black. I print it with an injet printer on some random white paper. It comes out looking darker than I think it should (but not by all that much). I think the problem may be the black ink soaking out into the clear spaces. It might work better with a laser printer, but I don't have access to one.
I would upload it here, but it seems to not be permitted due to it being a 6mb pdf.
B.

RalphLambrecht · Jun 29, 2020

Donald Qualls said:
(re: using one's hand as a "gray card")

Caucasian skin is normally expected to photograph around Zone VI to Zone VII, so that's about as expected. Palm skin should be about the same regardless of race.

makes sense; the back of the hand is around Zone VI.

koraks · Jun 29, 2020

Individual One said:
I have recently tried a homemade 18% card. It is a 500x400 grid of cells with a random 82% of them filled black.

Logically this approach makes an 82% grey card. In reality much less since inkjet black on actual paper will still reflect some light. So I think there are 2 flaws to your approach: a logical flaw, and the problems associated with real world reflectance of materials.

Individual One · Jun 29, 2020

koraks said:
Logically this approach makes an 82% grey card. In reality much less since inkjet black on actual paper will still reflect some light. So I think there are 2 flaws to your approach: a logical flaw, and the problems associated with real world reflectance of materials.

Thank you for your insights, but I must disagree. I was trying to produce a card that reflected 18%. As a first approximation I assumed that the white stock reflects 100% and the black ink reflects 0%. This cannot be actually true but is maybe not all that far off. So if 82% of the cells are black that leaves 18% of them white which ideally would reflect 18% of the light. And the result looks fairly close to a cheap grey card (that is probably not traceable to any quality standard).
B.

koraks · Jun 29, 2020

Sorry, you're right. 18% refers to 18% reflectance, my bad!

Donald Qualls · Jun 29, 2020

koraks said:
Sorry, you're right. 18% refers to 18% reflectance, my bad!

Furthermore, the sub-100% reflectance of the paper (most copier/inkjet paper is about 90%) and the not-quite black of the black ink (probably under 5%, certainly below 10%) virtually cancels; the end result should be within much less than 1/3 stop of 18% reflectance. And that's the nub: you don't need your gray card to be perfect, just better than the overall tolerance of the combination of meter accuracy, shutter precision, and actual film sensitivity (vs. your rating). Really ought to be pretty easy.

DREW WILEY · Jun 29, 2020

A third of a stop off - that would render it useless for anything like color chrome work. Let's say your card is off a third, the lens speed another third, the f-stop markings a little off - things add up. I might as well just use the exposure hint on the inside of an old amateur film box. But I have dropped my light meter into frozen streams a couple times, yet come back from the mtns with perfectly exposed chromes based on sheer memory of many analogous lighting situations. That doesn't mean my memory is a totally reliable instrument either. If I was that smart, I wouldn't have dropped the meter to begin with. Now I use a tight clasp on the tripod if carrying it over my shoulder when hopping rocks, with the meter loop securely attached.

Donald Qualls · Jun 29, 2020

I said "much less than 1/3 stop" -- print a page solid black and measure it against the fresh white paper (a cinch to be 90%, and if you still have the ream wrapper it'll sometimes have the actual value on the label). A little arithmetic will give you the actual value for the black, and you can adjust your coverage to get within a percent or so of your desired 18%. And still finish the job in twenty minutes, assuming you don't have to replace an ink cartridge or install a driver.

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