Why is 18% grey considered to be in the middle for photography?

Question

I heard someone (a photographer) say recently that 18% grey is half way between black and white, not 50%. This seemed a bit illogical to me, and when I asked her why, she said she didn't know. After reading a few online articles, I found that 18% is often referred to as middle grey, and considered to be half way perceptually. Is 18% for some reason half way between black and white, and if so why (maybe these percents work on a non-linear scale for whatever reason... ). If not, why do we think 18% is half way, not 50%. Do we see color non linearly?, do our cameras capture light non-linearly, or is this just a sort of relative brightness illusion.

After reading the question this is supposedly a duplicate of, I still don't see why Ansel Adams choose 18%, was it a visual thing?, or why it was so widely adopted. Is this number arbitrary? just what someone though looked correct... or does it have some valid claim to being the middle grey, due to perception (it appears our eyes see things linearly, do cameras do likewise?) or other technical reasons.

Answer 1

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 something more like 12% gray as middle gray.

Whatever the number, the idea behind middle gray is not that is "reflects 50% of the light". Or even that "it is half way between absorbing all light (pure black) and reflecting all light (pure white)". It has to do with your perception.

Your eyes are logarithmic detectors. That is, if a source gets brighter by a factor of 4, it will only seem brighter by a factor of 2 to you. If it increases by a factor of 32, it will only seem brighter by a factor of 5. If it increases in brightness by a factor of 128, it will only seem 7 times brighter to you.

The above are not the actual numbers. As you can imagine, measuring how bright things seem to people is very tricky, and varies from person to person. The important thing is that it is this weird logarithmic nature of your eyes that keeps middle gray from being 50%.

Answer 2

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 mapping, or conversion, between the linear (camera/image) data and the logarithmic sensitivity of the human eye.

The human eye can resolve something on the order of 10-14 f-stops of dynamic range at a fixed pupil size. This is up to ~3 stops better than the best DSLRs shooting in 14-bit RAW. Our brain is also capable of using all of that data at once - it's like we have a 16-bit RAW image processor built into our visual cortex[*] and it automatically adjusts the highlight and shadow levels, etc, to get a perfect exposure in real time. ~18% grey is just an empirical value that fits the processing that our eyes will naturally apply to the scene they see.

It is empirical because it works and looks mid-grey in a typical scene. The eye is easily fooled, however, and is extremely context sensitive. The brain will mercilessly photoshop what the eyes see in order to try to make sense of it and greys are routinely imagined to be any shade that makes sense to us. The classic illusion of this is this :

where the A and B squares are identical in brightness. So, yes, the eye is extremely non-linear and, furthermore, is not even uniform in its rendering over our visual field. Darks are brightened, brights are darkened, and the entire scene is heavily compressed into a narrow perceptual range that we can extract detail from.

When shooting high dynamic range scenes this is intuitive, I think, to photographers - we really have to work in post to balance a high dynamic range scene into a form that appears similar to what the eye perceives. When we can control the light, we add LOTS of it - fill, fill, fill. Getting a balanced colour photo that doesn't need a lot of post requires that we add as much light as possible to fill in the dark areas of the scene - reducing the dynamic range as much as possible to produce a scene that is flatter and more uniformly lit (just like our brain tries to do with the scenes we see).

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To answer the comment below, this is taken from the image above to make the point :

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[*] To be more precise, for those who wish it, some of the initial image processing and compression is done by several layers of specialized cells directly behind the retina before the information is sent to the brain.

Answer 3

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 the scene would yield an average value of e.g. 5%, then things that were dimmer than average would hardly get exposed at all. If one uses the guideline that typical 35mm film has five f-stops of latitude, 18% gray will fall almost exactly in the middle of that (2.47 f-stops down from 100%), putting it right in the middle of a five f-stop range.

Note that the process of shooting and printing negative film creates non-linear behaviors which are very different from those of digital cameras. Areas of the film which are not exposed to any light should be as transparent as possible, and should cause the resulting print to be solid black. Achieving a good solid black print will require exposing the print for long enough that areas of the film which are sufficiently close to transparent will also print as black. Thus, if one wants prints to have good solid blacks, then things which aren't supposed to be black must have a certain minimum level of exposure to stop them from vanishing into nothingness. On the flip side, it takes a lot of light turn turn film totally black; even part of a scene which is significantly overexposed may retain some detail.

When shooting digital, things are a little different: bright areas are more apt to get saturated (losing all detail), while dark areas are apt to appear "noisy". Generally, dark areas will still contain significant detail even when so badly underexposed that the noise dominates. Because different cameras have different amounts of noise (and the noise level varies depending upon various conditions), the "ideal exposure" mid-point for a digital camera may often be very different from what it would be for film.

Answer 4

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 publish a recommendation; place a Kodak film box in the scene. Seems the box reflected 18% of the ambient light. Now measure the reflected light from the box top and use this reading to set your exposure.

In 1941, Ansel Adams, a prominent landscape photographer, and his friend, Fred Archer, a photo magazine editor, jointly published the Zone System which provided photographers with a method to precisely fine-tune exposure. Their zone system revolves around the use of an 18% placard (battleship gray). This card replaces the Kodak box top. The 18% gray target became the de facto standard. Today film and paper speed as well as the digital chip are calibrated, and film and digital ISO is established using the 18% gray card.

Because of the pitfalls associated with reflected metering, a second measuring method evolved called the incident-light reading method. This method places a transparent sphere placed over the entrance of the light meter. The meter is positioned close to the subject and pointed backwards towards the camera. Thus, the meter measures the light just prior to striking the subject (incident old French word for about to happen).

The incident method yields the same reading as a reflected meter taken from a gray card however, it eliminates most of the pitfalls revolving where to hold and place the meter. In sunlit vistas the photographer can merely turn about and point the meter backwards at an imaginary camera. This method is highly accurate and was adopted by Hollywood camera operators because they are filming a scene and maybe a hundred thousand dollars rides on a correct exposure. .

Technical stuff: When negative film is correctly exposed and processed, an image of the gray card on the film will be rendered to a specific shade of gray. This shade of gay is equivalent to a neutral density filter with a factor or 5.5, it cuts light transmission 2 ½ stops. When written as percentage this value is 18%.

When the image of this gray card on the negative is printed, and if the print paper is exposed and developed to specification, the resulting image of the gray placard on the print paper will have the same 18% reflectivity as the original gray card.

Summation -- The 18% placard is the only tone that: 1. In actuality it has 18% reflectivity. 2. The resulting image of gray card on the negative has a transmission of 18% . 3. On the print the image of the gray card matches the original gray card reflecting 18%.

This 18% value is the key tone or axis of the photographic system - film – digital – and lithography. This is science -- not guess work.

More gobbledygook from Alan Marcus

Answer 5

I'd like to add that eyes and silver halide emultion naturally have a logarithmic response, unlike CCD/CMOS sensors, for the same underlying reason.

Consider a patch of molecules spread over the focal plane. An incomimg stimulus (two photons hitting the crystal in a certain time window, in the case of film) is recorded by changing the state of that molecule (organic dye molecule or AgX crystal), that unit is now used up. Consider when half the units have already been hit: another stimulus has a 50% chance of hitting one that's already used, so does not add anything. It takes twice as much incoming light to make the same darkening as it does in a pristine area.

Now the two-photon complicates things, but the overall shape of the distribution is the same kind of curve. I recall reading about a "zombie door" in a math column. Imagine a red carpet leading up to a wall with a door in one narrow spot. Zombies are regularly spaced in line walking down the carpet, and each is randomly (uniform distribution) positioned side-to-side.

The distribution of zombies coming through the door is called an inverse log. Now imagine there is a row of doors all across the wall, like a subway turnstyle bank. Each door can only be used once. Later, after the exposure, you note how many turnstyles have been used vs remain unused.

Without modern electronics, it's difficult to look at a patch and say that's xx% optical density, but coarse grained film and a microscope will let you count how many black dots (exposed crystals) are in a sample square. I don't know how he determined a linear coverage based on the test exposures: emperically, doing one-stop increments you can find the capabilities of the media, and point to the one in the middle. But how do you know that's 18% on a linear scale, without a densitometer? Maybe mixing pigments in ratios, so it came from a tradition of painting.

Answer 6

My understanding is that 18% grey is considered the average reflectance of light from the world around us - not snow (about 90%) or a black cat in a coal mine at the other end, but an average on an average day. Grass, for example, reflects about 18% grey, so if you are reading your meter you can take a reading off the grass and then calculate from there. Caucasian skin is considered around 36% grey, so you can meter off your hand and then compensate back to 18% from there by opening or closing a stop - this is for film, either negative or transparency.

Answer 7

Ok...a card with 18% reflectance appears to the human eye to be middle grey. More formally stated: an object with a relative luminance of 18%( relative to a reference white ) will have a lightness of 50%. This is not some random number. It is a consequence of our non linear perception of brightness.

https://en.m.wikipedia.org/wiki/Lightness

Answer 8

The other answers are not wrong. "18%" is also related to the video world: if the gamma of a monitor is 2.4 and you give it a 50% signal then the light output is 0.5^2.4 = 19%.

By a magical coincidence, analog video signals and digital image files (sRGB) or signals (BT.1886) are coded almost perceptually uniform. A 50% signal gives an 18-19% Luminance, but that is perceived as approx. 50% Lightness. A quantity with "-ness" in the name is always about human perception.

Later the video industry has tried to quantify Lightness perception and perceptual uniformity. Peter Barten (Philips) has laid much of the ground work, resulting in a PhD thesis and a summary paper (SPIE 2004). This work has been used by the Dolby company for standardising the "Perceptual Quantizer", this is the OECF for HDR TV as written in SMPTE standard 2084. Later Poynton, Nijland and myself have published a new formula for the same OECF curve and named it the "Barten Lightness" function (SMPTE MIJ 2015). It assumes perfect adaptation of the eyes to the average Luminance level, wherever that is.

This formula shows that human Lightness perception follows a gamma curve (1/2.07) in low light (< 0.1 nit) and a log curve in bright light (> 1 nit). With this formula the "50% Lightness = 18% Luminance" relationship is only exact for 18% vs. 100% of 0.57 nit. For example, 10.9% of 10 nit or 5.6% of 100 nit or 2.4% of 1000 nit or 0.9% of 10000 nit Luminance are also perceived as 50% Lightness. Again, this is after perfect eye adaptation to sequential stimuli, not when shown side by side.

If you want to know more then I suggest you look up Charles Poynton's PhD thesis, our Lightness formula is on page 93.

Please note that Dr. Barten has only investigated black-and-white perception, so anything derived thereform is only valid for the greyscale. Application of perceptual uniformity to color imaging is a different matter, and we have also taken a shot at that. This was all done in the context of high dynamic range and wide color gamut television.

Answer 9

Your eyes aren't logarithmic detectors. If only life were such a party; it'd sure be convenient to be able to walk into and out of my dark room without having to wait for my eyes to adapt. Instead, it's a mess of iris adaptation, Michaelis–Menten kinetics, power factors, context effects and more.

Jones and Condit were concerned with the brightness characteristics of scenes, not with reflectivity. Think about it, most of the scenes they studied were taken outside, often with visible sky in it. If they came up with the 18% figure, show me a citation.

I think it's a convention / tradition, but I don't know the ultimate source. Possibly 18% is standardised from some ad hoc object used previously.

Answer 10

It baffled me for years too. Quite simply, the average amount of light reflected off objects around us is 18%. Some things are darker, some things are brighter. But 18% is average. Our eyes will perceive this average reflectance as the midtone to the highlights and shadows around us. Some other people have used maths and graphs to explain the difference between linear and log style data. But I'm just happy to know 18% of the direct light on my subject is being reflected at me, and that will give me my midtone to which my highlights and shadows can dance.