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I have a Nikon D5300 and always shoot on Auto White Balance because it can be easily fixed in Lightroom or other RAW Processing applications. However, I wanted to experiment a bit with shooting Video on the Camera, so I need a proper white Balance.

As all stores are closed now and I'm a big fan of DIY, I was starting to wander if you could use a phone screen with 18% grey or some other neutral color (white should work as well) as a grey card (the D5300 doesn't allow for fully manual WB). Could this work or is it problematic that the phone is emitting the light by itself and not reflecting the natural light? (I have a Samsung Galaxy J7 if that should matter)

I'm also aware of other methods like using a sheet of paper or printing 18% grey on it (however, my printer is not very accurate), but wondered if the phone wouldn't be more accurate.

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    \$\begingroup\$ Idea for a product: a phone case molded from 18% gray plastic. \$\endgroup\$
    – Eric S
    Apr 17, 2020 at 14:18
  • \$\begingroup\$ You can order a gray card from one of a near countless number of online sellers. I'd start with B&H or Adorama. DIY is good for many things, but for a gray card you need something that is a specific color and brightness that is hard to guarantee without some fairly sophisticated equipment. \$\endgroup\$
    – Michael C
    Apr 17, 2020 at 17:18
  • \$\begingroup\$ As far as the printing option, while it would be better than using a phone screen, it's still not guaranteed. Even if your printer was 100% accurate, the glossiness of the paper might not match a purpose-made gray card. If there's a hardware store still open near you that sells paint, you might be able to get the right color sample swatch for free. But ordering something from a photography supply place designed for the purpose is still the best option. \$\endgroup\$
    – Darrel Hoffman
    Apr 17, 2020 at 19:38
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    \$\begingroup\$ I always have a light grey (near white) cleaning cloth for eye glasses in my camera bag. It servers two purposes that way, cleaning lenses and for white balance. \$\endgroup\$
    – Gerald Schneider
    Apr 20, 2020 at 7:00
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    \$\begingroup\$ Supposedly the inside back covers of all Canon User Manuals (or maybe it was the user manuals for all Canon flashes?) can be used as a WB target, but they are not 18% grey and can't be used for exposure calibration. \$\endgroup\$
    – Michael C
    Apr 21, 2020 at 17:01

2 Answers 2

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No, it won't work. The gray card works by reflecting ambient light (here "ambient" is used to include whatever flash, gels, etc. you're lighting your scene with other than the phone).

You know it's supposed to be gray, so the difference between what you get and gray is the correction you need to make. By emitting pure gray light (assuming your phone can do that perfectly, even), you fail to get any reading on the ambient light).

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  • \$\begingroup\$ A color checker (grey card) for editing does not work by reflecting the ambient light. It works by being a constant (known color) w/in the scene; which gets shifted by the WB the camera sets for the scene. \$\endgroup\$
    – Steven Kersting
    Apr 17, 2020 at 12:25
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    \$\begingroup\$ @StevenKersting I am pretty sure your grey card is just that, a card. Then the only colour we see on it is from reflected ambient light ... \$\endgroup\$
    – Hagen von Eitzen
    Apr 17, 2020 at 17:47
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    \$\begingroup\$ @StevenKersting If the phone emitted perfect grey, how could you use this information to get information about the light temperature in the scene? Think about a room with two lights of extremely different light temperature, then you could have two different "correct white balances" depending on which light you pick to be neutral. Same with the phone and the ambient light. \$\endgroup\$
    – Jannik Pitt
    Apr 17, 2020 at 18:41
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    \$\begingroup\$ @StevenKersting Notice that the phone emits the same light no matter the scene. So you'd get the same white balance for every scene, not depending on the ambient light. How does this accomplish the goal of white balancing the scene? Why do you need the phone in the first place? \$\endgroup\$
    – Jannik Pitt
    Apr 17, 2020 at 20:15
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    \$\begingroup\$ @StevenKersting yes, the card is a known color, but the color the camera sees depends on the ambient light the card reflects. The difference between the known and what the camera is getting is what allows you to make corrections. \$\endgroup\$
    – mbrig
    Apr 17, 2020 at 21:23
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The phone is likely to be more problematic than other simple options (white paper). But it depends on the intended use I guess...

The ambient/natural light will mix with the light emitted by the phone affecting the white balance, but how much will depend on the relative light levels... it would be easy for the phone screen to significantly overpower the ambient. And it will never be the same as purely reflected natural light.

So if you want to use it to set a custom manual white balance in-camera (preset/PRE for Nikons) it won't work.

But if you want to use it for a "color checker" type function for editing in post that doesn't matter... because it doesn't matter why something should be white in the image, only that it is white. The only considerations here is that the WB is fixed, or the phone is small enough in the scene to not significantly impact the WB the camera sets, and you are correcting for white light (daylight, daylight bulbs, electronic flash).

However, there is still the issue of the color accuracy of the phone screen... the chances of the white on your phone screen actually being white are slim-to-none IMO.

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