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  #1  
Old 1st November 2014, 02:35 PM
Adrian Adrian is offline
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Default How to make an 18% Grey Card

Here's a tip for making an 18% grey card that represents mid-tone for a 5 stop range using a tool like Paint Shop Pro (which I use) or Photoshop.

All you need in addition to the software is a decent light meter, stable light source a printer and some paper (reflective - but diffused)

1. Create a set of squares in your photo editor, setting each with the following RGB values to:

255 (full white 0 stop)
181 (-0.5 stop)
128 (-1.0 stop)
91 (-1.5 stop)
64 (-2.0 stop)
45 (-2.5 stop) (18% grey should be this one)
32 (-3.0 stop)
23 (-3.5 stop)
16 (-4.0 stop)
11 (-4.5 stop)

If you want to work the values out it's simple: 256 * 2^(stop)
where "^" is the "to the power of" button on a scientific calculator. You can step in however fine increments of stops you like. 256 units of light is full-white. So it follows 128 units is -1 stop, 64 units is -2 stops and so on...

Now, in an ideal world, you'd end up with perfect grey scales in 1/2 stop steps. But what with "gamma" etc and printers, ink, monitors and all the other many variables involved, it's difficult to achieve, which is why it's useful to print a range.

2. Print out your test card using high quality settings on your printer.

3. Mount the test card against some white card off the paper white in strong stable light (e.g. from a white balanced photo flood light) and record the value.

5. Meter each grey square in turn until you find the one that gives you -2.5 stops down on your full white value. That is your printer's 18% grey. It may not correspond to RGB=45 (-2.5 stops) on the above scale. For my printer it was 45 so pretty much "bang on".

6. Prepare a larger area of the selected grey value in your software and print it out - make sure the ink is not running low!

7. Mount onto white card. It is helpful to leave a white border as a reference.

8. Go check out the grey card. It should give you about 18% (-2.5 stops).

Disclaimers: It's not exact. It depends upon (a) the light meter calibration, (b) the reflectance of your paper, (c) quality of ink and printing and no doubt several other factors. But it's likely to be within 1/3 stop.

Also note: 18% grey is only mid-tone for a 5 stop exposure range!!! For the landscape photographer's typical 7 stop range, mid-tone would be -3.5 stops, a 9% grey card. So if you meter, expose and process for a 7 stop range 18% grey will be too light (it would be about Zone VI). Use the same technique to give you a 9% grey card if you like - useful for comparing film tests - if you trust your home made card that is!
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Old 1st November 2014, 02:52 PM
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Argentum Argentum is offline
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What have I started

note that even a white card will only reflect around 90% of the light falling on it but that top 10% is actually very little.

ALSO

Your next challenge
I wonder if you can work out and make a card with a full 11 stop range from 0 to 10. Difficult to get the light bright enough to use it.
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Old 1st November 2014, 04:31 PM
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BUT

why meter a card when you can meter your actual subject and place it where you want on the curve.

What's important is metering the subject and having it land on the curve where you expect it to land. A grey card won't do that for you no matter how accurate your metering of it is. (except your gray card will turn out as grey)

The way to get it right is to do a zone patch photo test and actually print the test using fresh paper and see that the print values are what you expect.

A grey card just turns your spot meter into an averaging meter and negates the benefit of having a spot meter. You would be better off with an incident meter instead of using a spot meter to read a grey card IMO.

Note: for a grey card to give the correct 18% reading it has to be aligned at very specific angles to the light source and to your intended subject. Do you know what those angles are and can you accurately reproduce when out in the field?
Just hold up your grey card and tilt it. Do you see the colour change as you tilt it ? You will and that is becasue the changing angle changes the percentage of reflectance coming to your eye/meter. Grey cards are a waste of time and effort. Bin it and meter your actual subject or use an incident meter.

Last edited by Argentum; 1st November 2014 at 04:36 PM.
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Old 1st November 2014, 08:50 PM
Adrian Adrian is offline
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Hee Hee! What have we started indeed!

I completely agree with you about spot meters and grey cards. I don't use a grey card in the field with spot metering. But I have occasionally used one in a studio though where I'm shooting in a 5 stop range on reversal film.

I made a grey card to test my computer work flow really - just to see if my rubbish little printer could do it. (The ink costs more than the printer). I checked the result with an averaging meter on both incident and reflected modes and they are identical - so that's a good sign. So when it appeared to work I thought I'd bang out a quick "Tech Note" because I've seen a lot of misleading stuff written about DIY grey cards on the 'net.

Regarding the next challenge, well that'll be the paper and ink... One can pay thousands for a high quality test chart, so I think I'll leave that to the pros.

Anyway Argentum, I hope you enjoyed the "Tip"...
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Old 1st November 2014, 08:56 PM
Adrian Adrian is offline
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Quote:
Originally Posted by Argentum View Post
What have I started

note that even a white card will only reflect around 90% of the light falling on it but that top 10% is actually very little.

ALSO

Your next challenge
I wonder if you can work out and make a card with a full 11 stop range from 0 to 10. Difficult to get the light bright enough to use it.
Difficult to get ink good enough to match printing paper - so why not just print a 10 zone test strip in the dark room? We're back to the patch test then . BTW I've done this for characterizing my processes. Useful - but laborious. These days, if I've got a couple of spare frames on a roll I do a couple of calibration shots at Zone VIII and IX from time to time, just to make sure everything is still correct...
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Old 22nd November 2014, 10:48 PM
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Maris Maris is offline
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I've made a Zone card on photographic paper by projecting a negative exposed at 1 stop intervals on to it in the darkroom. The grey scale has a mid tone that corresponds to what I get when I directly follow a reflected light meter reading.

In the field, when I'm uncertain how to tonally render a subject, I put the Zone card directly on the subject (tree trunk, rock, etc) and see which grey value rendition is the one I like. Selecting this grey value also tells me the exposure change I have to make from a direct meter reading.

Because the grey scale card is made using the actual meter, actual negative film, and actual photographic paper the system is self calibrating!
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Old 23rd November 2014, 10:22 AM
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I have never understood the use of a grey card when one can actually meter the scene.

Bests,

David.
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Old 23rd November 2014, 12:10 PM
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Gives you a target of a known 'value' that you can meter off, preferably with a spot meter - In some difficult lighting situations, it can be very useful.

It is something I have rarely used, I think the last time was in a studio situation with multiple flashes.
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Old 24th November 2014, 07:54 PM
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Quote:
Originally Posted by Adrian View Post
Also note: 18% grey is only mid-tone for a 5 stop exposure range!!! For the landscape photographer's typical 7 stop range, mid-tone would be -3.5 stops, a 9% grey card. So if you meter, expose and process for a 7 stop range 18% grey will be too light (it would be about Zone VI). Use the same technique to give you a 9% grey card if you like - useful for comparing film tests - if you trust your home made card that is!
I wonder if you can expand on this.
1. the paper you print on can only go from white to black and no many times you divide it in to sections it can only still be the same brightness range. the divisions on print paper are not really stops vis a vis exposure stops.
2. If you expand your range to a seven stop range, wouldn't that be one on the top and one on the bottom and then 18% would still be in the middle.
thanks
Bill
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Old 25th November 2014, 01:15 PM
Adrian Adrian is offline
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1. the paper you print on can only go from white to black and no many times you divide it in to sections it can only still be the same brightness range. the divisions on print paper are not really stops vis a vis exposure stops.

I agree Bill. I used a normal computer printer to make the grey card rather than in the darkroom and set the grey level linearly (as R,G,B values) after converting from stops to linear values.

2. If you expand your range to a seven stop range, wouldn't that be one on the top and one on the bottom and then 18% would still be in the middle.

If black is 0 stops and full white is 5 stops, then half-way is 2.5 stops.
If black is 0 stops and full white is 7 stops, then half-way is 3.5 stops.

If you were to fix 2.5 stops as the 18% grey, but try to use a 7 stop range, then black would have to be -1 stops (2.5 - 3.5 i.e. blocked up) and white would have to be 6 (2.5+3.5) so your range would shift to -1 to +6, so you can see that the 18% grey card representing 2.5 stops for a 5 stop range would not be mid-scale for 7 stops.

I'm not very good at verbally explaining - pencil and paper is my thing. But there's guys on this forum know far more about this stuff than I do who can probably give you a better explanation.
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