27

The story goes that Ansel Adams came up with the "18% gray" figure. Back in the hay day of film photography he was developing the zone system and needed to define a "middle gray". It was a judgment call. Eventually, the idea caught on, but film and camera companies picked their own middle gray. It is a fun fact that your digital camera probably uses ...


18

It's worth looking at a gamma chart for additional perspective as you think about this. Standard display gamma, for example, is 2.2. The curve looks like this: 50% grey, in an 8-bit space, is 127 (horizontal axis). This lines up with ~20% luminance output of the display. Both for display and print the concept of gamma is important as it provides the ...


16

Ideally, you're shooting in an environment with controlled lighting (a single light source, or several tuned to same color temperature), your subject and black or white surfaces only. In this case, the angle does not matter - just take care that its exposure falls somewhere in the middle in your test shot (so you're not accidentally clipping a channel). In ...


16

When you have already set your exposure parameters, white could be clipped in some single color channel (but not all, so your camera won't show it as blown), therefore not being very good basis for color balance adjustment. Also, paper will turn yellow during time. And just looking at the sheets currently on my desk, there's three different tones of white ...


11

Grey and white are the same color to the camera. You can consider grey as dark white or white as light grey. So it is very much the same and you can take a white-balance measurement from any shade of white as long as it reflects light uniformly. As you read from the answers you already got, white paper is not necesarily white. Depending on your camera, the ...


10

Different lenses have different transmission curves regarding to different wavelengths. This will be more visible with older lenses, which can have a significant yellow cast. In macro/close-up photography, a lens of different color or size can affect the light that reaches the subject. In most cases though, the differences are negligible. Different light ...


9

Gray card is used to make your colors neutral from the tone of a light source. When you have multiple light sources with different color temperatures (such as sunshine, blue sky lighting the shadow areas, and reflections colored by surrounding objects) you can only pick one of them to be the "neutral" light. So if you have multiple light tones, it's your ...


7

It's all about intent and artistic vision. In studio lighting, for example, getting a perfectly white-balanced shot might be very important because the colors of your client's product need to be exactly right. You would never want to warm up (or cool down) their logo color, for example, in a product shot. Use the gray card to achieve correct white balance, ...


7

A little history will help you understand the purpose of the gray card: In the mid 1930's, Messrs Jones and Condit at the Kodak Laboratory determined that statistically, a typical sunlit scene integrated to a reflectance value of about 18%. About this time, the Western Electric Company brought to market the first light meter. Kodak Labs published a ...


6

I usually find, especially when dealing with artificial light, that an image that is white balanced that accurately is often cool and I tend to find it a little harsh as a result, so I've been known to warm up the image a little either with the white balance tool or a filter in Photoshop afterwards. I would note that it wasn't all that rare with film, ...


6

Metamerism is an effect that's usually (in photography) associated with viewing prints, especially ink jet prints. Dye based inks, and to a lesser extent pigment inks can appear to be color shifted depending on the type of light they are viewed under. This effect varies with the ink formulation. I know of no way to control or minimize this at the 'taking' ...


6

The standard gray card is 18% gray. Take a sample gray card with you to the paint store and have them custom mix. The reflection readings are: 0.75 red 0.75 green 0.75 blue 0.75 via the yellow visual filter. These values are the reflection densities of the gray card.


5

the only relevant thing is that the same light which falls on your main subject falls on your card. The field of view, distance and so on don't enter in the equation. but you have to be sure that by standing near the subject you don't influence in any way the light, which is not always easy since our eyes are very quick to adapt to variation and they ...


5

The cards are designed to reflect about 18% of the incoming light, which to a human appears half way between max white and darkest black and happens to be a fairly good guess at the average reflectance of typical natural scenes - L*50 as has been mentioned correctly above. The next question, and where the 12% comes from, is this: If we meter off the 18% ...


5

Even beyond perceptual issues, film exposure latitude is another reason to favor 18% gray. If one tried to expose a scene so that the average gray tone on the scene would yield an exposure value of 50%, then anything which was even twice as bright as the average would get totally blown out. If one tried to expose a scene so that the average gray tone on ...


5

18% reflectivity means it should reflect 18% of whatever linear light is actually on it. Gamma should compute 117 at 46%, but your picture shows the card to be in some shadows, so then it can't be 18% of the brightest 255 (expecting gamma to 46%). But the histogram won't be the exact computation, because the camera is busy doing other things too, like your ...


4

Maybe because you already have the 18% gray card with you for setting exposure so why don't you use the same card for white balance? And as a bonus, because it's in the exact middle of the exposure it's the least likely color to clip.


4

I assumed the reason for 18% grey as a white balance target was because the incident light meter in cameras is measuring for 18% grey. Therefore, by presenting a target that the light meter is most sensitive to, and one that exposure is being calibrated against, gives the best result. I also think this is related to consistency: use a grey card in all your ...


4

What Stan said (of course :-) ). The grey card is likely to be "close enough" to spectrally neutral to be useful. The exact reflectance level is uncertain but also not crucially important - see below. Many people sell this card set with a range of descriptions. Some say The Zeikos ZE-DGC Digital Grey Card Set features a digital grey color reference card, ...


4

Expanding upon BobT's answer: Metamerism in the gray card is a non-issue with a high quality card. In fact, I'd argue that a gray card is not useful or acceptable if it's exhibiting metamerism. The basICColor Gray Card, for example, specifically addresses this, but it should be true of any gray card.


4

unless it's extremely off, which probably isn't since you have been using it, and assuming that you're not doing product photography where accuracy in colour reproduction is more important and getting it right from start is a time saver, I'd say that you can continue to use it. this is especially true if shooting in raw where you'll be able to fine tune if ...


4

Short answer, black is too dark to not clip reliably, white is too bright to not clip reliably. You need something that is pleasantly in the middle to give you an idea of proper curves and balance. A medium grey card is ideal for the purpose of determining the curves and the color of the lighting in the scene. The percentage is chosen for exposure, the ...


4

For normal lighting, yes. For bizzare or novel light, no. "Normal" means sunlight or incandescent lighting, or other lights that try to immitate that (since it's what our eyes work with). Poor "color rendering index" lamps will be lacking but that's how it looked in person too! For odd colored lights, you need more data points to know just what is ...


4

The more samples / swatches you have the more accurate your device characterisation will be, as a matter of fact X-Rite has the ColorChecker DC for that: http://www.rmimaging.com/information/colorchecker_dc.html X-Rite samples / swatches pigments are fairly stable although their lifespan is usually 2 years, it can be shorter or longer depending how heavily ...


4

To use a photo of a gray card to adjust white balance you need to adjust white balance (color temperature along the blue←→amber axis and tint along the perpendicular magenta←→green axis) until the values of the red, green and blue components of the gray card are all the same. When you hover the cursor over the gray part of the photo the values should be ...


3

As Stan points out, you can use a grey card and your histogram to judge exposure. The problem with flash is that it is a relatively small and directional light source, so the angle you hold the grey card may affect the reading. Good grey cards are designed to diffuse the light as much as possible, but none are perfect, and as you vary the angle you hold ...


3

I can't see a good reason why it would be a problem to cut it up. So long as the grey is clearly visible on the test image you take then that's all you need. I'd tend to do the colour correction on post but that's a matter of workflow. Size wise even your smallest size is a larger grey area than the likes of the Datacolor SpyderCube.


3

Most white paper containers "brighteners" -- basically a fluorescent dye that glows (slightly) when exposed to the small amount of UV in normal daylight. As a result, the spectrum you get from the paper does not exactly match the spectrum of the light falling on it. Worse, the spectral deviation is somewhat unpredictable, often differing between one brand ...


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