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Similar to this question; however, I'd like to know more information about using a gray card in a dark room.

I know that I am supposed to balance off of it; however, after I make a test strip, how do I use that to determine what my times should be? What exactly will my eyes see that tells me this is the correct exposure time?

I have a couple of gray cards that I've used in digital B&W. They are a different actual color (one appears very warm); however, they show up as the same shade in the file. So I don't think I can just hold my test strip to the gray card and see which matches.

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    A warm grey card??!! Well thats different!!!! – Digital Lightcraft Sep 16 '15 at 23:01
  • @DigitalLightcraft I think a friend of mine might still have a picture he took of them. It is weird...I have three gray cards and none of them are the same color. – SailorCire Sep 17 '15 at 14:20
  • odd... as long as you are always using the same one and not for white-balance porposes you would generally be OK, but for colour correction a set card is essential. – Digital Lightcraft Sep 17 '15 at 14:28
  • @DigitalLightcraft I've seen those warm/cool cards before. They are basically for intentionally lying about the white balance to get a desired effect. Want your video to look warm? use a card that over-represents blue so the camera will will think the light is cooler than it actually is and will over-correct it the other way attempting to achieve neutral. – cabbey Sep 17 '15 at 15:25
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In the absence of anything else to judge the initial exposure, yes you match the gray card up with the print and use a value that appears closest.

But there is nothing to say that that is a "correct" exposure. The extreme case would be high key (or low key) shots, where you absolutely don't want middle gray to be in the middle. But for less extreme cases, it's a good starting point, but you would adjust from there to achieve the effect you are after.

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So first off, you have to have the gray card in the frame on at least one exposure. Say you took 5 images with the same lighting, same exposure... one of those 5 needs to have a grey card (or similar neutral density material) in the exposure. It's important that that gray card is what you make your test strip of, and that that is the one you have in the dark room to compare to.

Say your test strip has 10 sections, 1/2 second apart, starting at 2 seconds, and going to 7 seconds. Develop it, fix it, rinse it etc, just like you would a print. (though to be honest, you don't need to go the full fix/rinse and there's no point drying it, because you're only going to look at it once then toss it.)

Take the test strip and your grey card outside the dark room into normal room light, or sunlight. Hold the strip next to your grey card and figure out which strip on it most closely matches the grey card. Which ever that strip is, that's your starting exposure for those 5 frames, on this enlarger, on this paper, with this batch of chemistry. That's a lot of variables you're testing for... here's how they can impact you:

  • Different enlarger? Different bulb strength, or clarity of optics makes for a different exposure. Heck the dust layer on the optics can impact it too.
  • Different paper? Different papers react differently, require different amounts of exposure.
  • Different chemistry? the last print through the trays of old chemistry about to be replaced and the first print through the fresh trays... WAY different. Especially if they are from different batches or mixed by different people.
  • Different roll of film? even if they were the same exposure and lighting, if they weren't processed at the same time, or even if they were if they were hand processed, the negatives can come out slightly different density.

All those things (and more) are why you make the test strip in the first place.

Note that you may need to eyeball it between two frames... if 5s is too light but 5.5s is too dark, then maybe 5.25s is what you want to start with. Once you're more comfortable with the process, you can probably make fewer stripes on the strip and eyeball between them more.

Important note, this is your initial exposure time. Do a work print with that time and look at how it comes out, make your artistic decisions if you need more time or less, need to dodge/burn to compensate for exposure issues in the film, etc. Work your way through several work prints if you need to before doing your final full size print.

Now, I implied above that the grey card was only good for those 5 frames. But the reality is that as long as you are consistent in your exposure on the roll of film, you can reuse those settings from shoot to shoot too. If the first 5 frames of one shoot and the next 10 frames of another shoot are equally exposed, then you can re-use the time you got from the first test strip across both sets of images. One way to compare the exposures before you get into the dark room is to put two negatives next to each other on a light box and look at the grey cards in them... do they look to be the same brightness? (tip: lay a piece of black card stock with two small holes punched in it over the negatives and line the grey cards up with the holes.)

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A gray card, by itself will be of little value in the darkroom. If you couple a gray card with a densitometer, now you’re talking. First the gray card is a neutral gray shade that reflects 18% of the ambient light playing on its surface. In the mid 1930’s the Weston Electric Corp. marketed the first electric light meter. Kodak engineers devised a method to accurately meter a scene. This initial idea was to place a yellow box of Kodak sheet film in the view illuminated as the principal subject. A meter reading was taken of the box. The exposure as indicated by the meter rendered an image of the box as middle gray. This proved to be a marvelous way to set aperture and shutter speed.

This was followed by Ansel Adams and his friend Fred Archer a photo magazine editor, they jointly published an exposure method based on the 18% gray card. This system known as the zone system proved to be an outstanding way to pre-visualize what shades of gray colored objects would obtain on the final black & white print. Ansel Adams finalized ways to manipulate exposure, developing and printing to obtain the shades as pre-visualized.

Now the gray card together with a densitometer is a wonderful tool that can predict the enlarger exposure and paper contrast grade needed when printing black & white negative. Moreover, a gray card image on color negative and a densitometer can predict the color filter settings and aperture as well as exposure time for the color enlarger. All too vase for this short explanation however this is a tidbit:

We include an image of the gray card on the negative or on a companion negative exposed the same as the principal negative. On the film, the image of the gray card should read 0.75 density via a transmission densitometer. We expose this negative on photo paper and develop. We seek to achieve the 0.75 density reading on the finished print. If a color print we seek 0.75 red – 0.75 green – 0.75 blue as measured by a reflection densitometer.

As to contrast grade: Using the densitometer, we measure the highlight density of a black and white negative and the gray scale density and the shadow density.

Example: Highlight density 1.18 Gray card density 0.75 Shadow density 0.13 Density scale 1.18 – 0.13 = 1.05 The Gray card reading revels the negative is properly exposed and processed. The Density Scale tells us what grade of paper is needed. 1.4 or higher grade 0 1.2 thru 1.4 grade 1 1.0 thru 1.2 grade 2 0.8 thru 1.0 grade 3 0.6 thru 0.8 grade 4 0.6 or lower grade 5 I think this approach which in my youth was significant, has been lost. Alan Marcus alanmaxinemarcus@att.net

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