DIY gray card?

Rafael Saffirio

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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?
 

MattKing

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Do you need calibrated reflection, or do you need a mid-gray standard?
If the latter, make a few different grey prints and then see which one, when held at a slight angle from the perpendicular, gives you a meter reading that matches Sunny 16 under Sunny 16 conditions.
 

AgX

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markjwyatt said:
Probably would want a refection densitometer.
Click to expand...

No, as one does not need to know the density, but the reflection luminosity ratio against the lighting is right enough. As one uses the card this way. A kind of reverse engineering.
 

reddesert

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If you make some grey prints as MattKing suggested and also have a pure white piece of paper (like the back of one of the prints), place them in the same lighting and light meter them. An 18% grey should read 2.5 stops darker (more exposure) than a pure white piece of paper.

Your white piece of paper isn't 100% reflective, of course, but even if it's 90-95% it's close.
 

AgX

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Better reference, as I indicated above, is the light hitting the card. Thus one compares the incident metering against the light source to the reflected metering from the card, until by exchanging cards of differet refelective density the right ratio has been established.
 

Donald Qualls

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A simpler method: Make a grid that covers 80% of the area, and use it to mask a print exposed for Dmax. The masked, unexposed area will be 90%, and the unmasked, exposed will be close to 1-2%, giving a good approximation of 18% gray over the whole area.
 
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Rafael Saffirio

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I don't understand this idea, a grid? of what kind?

I was thinking in doing something like this http://www.darkroomautomation.com/support/zonestrp.htm and the finding the segment that is 2.5 stops darker than white.
 

Donald Qualls

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Rafael Saffirio said:
I don't understand this idea, a grid? of what kind?
Click to expand...

Essentially, you want to expose 80% of the area of the paper enough to produce Dmax, while leaving the other 20% full paper white (this may require a little adjustment depending on the actual reflectance values of your paper white and Dmax). I said grid, because that way you can be reasonably sure anything that meters off the resulting print will get a representative slice of the "gray card". One way to do this would be to lay out a literal grid on a piece of clear acetate with 20% coverage (strips of narrow tape, for instance), then contact print that to the nascent gray card sheet. Develop, fix, wash, dry and ready to use. And if you keep the acetate, you can make more gray cards any time you want.
 

MattKing

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Bikerider

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If you need a physical grey card now and very cheaply, the inside of a breakfast cereal packet usually is as near as makes no difference. I use one from a Kellogs Cornflakes packet

I have been using these for years and every C41 film the first frame is always one of the grey card so I can easily assess the colour balance when printing. If the weather is sunny when I start the film, but don't finish it and the next time if the weather is different I take a picture I take another. I fill the frame with the card and it doesn't matter if it is in or out of focus. Once I get a correct grey I know the print will be perfect,
 

Ozxplorer

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And, I use a sheet of Peterboro mat board... they have a range of “neutral grey” shades. Find one that meets the test suggested by Matt King - msg #3 above. Have fun... Fred
 

Horatio

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If you have access to a photo editing app (Gimp is free) you can make a gray image with layers and print on paper. I made one like this that works well. Or, if no app available, download one already made and print. I’m going to try the cereal box suggestion.
 

koraks

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Horatio said:
If you have access to a photo editing app (Gimp is free) you can make a gray image with layers and print on paper.
Click to expand...
Sounds nice in theory and you'll get away with it for B&W and non-critical color work, but it's not a failsafe approach. Most inkjet printers struggle with rendering a truly neutral grey.
 

Horatio

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koraks said:
Sounds nice in theory and you'll get away with it for B&W and non-critical color work, but it's not a failsafe approach. Most inkjet printers struggle with rendering a truly neutral grey.
Click to expand...

I have only used it for B&W film, but I agree about the printers. But nothing is ever absolutely perfect.
 

VesaL

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I have used a matt grey spray paint on large plastic playing cards with two coats. Gives a nice, semi-rugged expendable grey target with some leeway.
 
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Rafael Saffirio

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Bikerider said:
If you need a physical grey card now and very cheaply, the inside of a breakfast cereal packet usually is as near as makes no difference. I use one from a Kellogs Cornflakes packet
Click to expand...
This sounds like a great idea, I suppose I doesn't need to be excatly gray in color, but reflex 18% of the light. or I'm wrong?? I will use it for B&W
 

Donald Qualls

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In theory, in highly colored light (golden hour, blue hour) it might make a difference, but by that point you're starting to get into the difference in response curve between your meter and film, regardless of any color cast on your "gray" card.
 

DREW WILEY

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I made a real one, in fact, a real big one, and that took quite a bit of effort with a very expensive industrial spectrophotometer. First of all, I took spectral readings of a huge stack of just about every kind of gray card you can think of that's sold in typical photo stores. Not a single one was reasonably close to 18% gray, nor were any truly neutral gray, and they were rarely consistent even within the same brand or specific product number. Things have improved somewhat since then, but I still distrust most reference grays except the gray patches on clean unfaded MacBeth Color Checker charts. So back then I batched up a quart of paint almost perfectly 18% reflective, truly neutral gray over the entire visible spectrum plus one step into both UV and IR. The process was too intricate to publish a formula; and it was basically a "what if" rainy day fun project. It could be mass-produced in a standard industrial batch, which is 144 gallon minimum, but nobody is going to do that because the quality control step afterwards would be too fussy. No can of paint one can buy in a store is even going to be close. Who ya kidding? You don't even begin to understand the manufacturing variables. I won't go into all the reasons why, but can start by stating that no black pigment is neutral. Every one of them has some kind of distinct bias; and it take a lot of work to fine tune that. If you take deep mineral black versus lampblack, for example, then lighten them, you'll discover a greenish bias in one, and a violet bias in the other. Commercial grays vary somewhat batch to batch, sometimes badly. And for an accurate reading, the specific color of the card does matter. Otherwise it will absorb rather than reflect certain wavelengths that you meter might be responsive to. You need a predictable reference, even when scene lighting itself varies. The less variables, the better. And gray cards are "supposed to be" a reliable standard, just as the receptor in you light meter is supposed to be. But I only use a true reference gray for critical color film tests at ideal color temperature, or for things like copystand lighting calibration. In the field, I'm comfortable aiming the spotmeter at a granite rock or road asphalt, or even a green lawn, and realizing from sheer experience what to expect.... Like I said, it was a project just for sake of a project.
 
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Bill Burk

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Paint is a good idea. But you have to be careful. One day I got pigment and white and mixed it myself to make a two-tone gray target (Minor White), recently I just wanted to make a backdrop so I got a quart and told them to make it black, and another quart white. Mixed together it turned murky green.
 
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