18% Gray Card

<p>Hi everyone<br>

I'm not sure if I choose the proper category for my question or not..<br>

The thing I want to know is about 18% gray card<br>

Can you please inform me the defenition of 18%gray card<br>

actually I know the defenition in theory [An <em>18 Percent Gray Card</em> is a simple gray-coloured card which uniformly reflects 18% of the light which falls upon it. Gray cards can be used as a reference to set the camera exposure or to calibrate a light meter ]<br>

But my main question is that why it is called 18% gray if it is the -middle gray-and why it is not the 50% ?<br />I mean what is the 18 % ?<br /><br />thanks in advance<br>

Nima</p>

<p>A 50 per cent reflectance card would be lighter. There is no fixed definition of "middle grey". The "theoretical" definition you give of the 18 per cent card is the only one.</p>

<p>I stopped using gray cards for metering decades ago, preferring to use incident light meters that read light falling onto the subject:</p>

<p><a href="

<p>That said, you'd do well to read this article by Thom Hogan:</p>

<p><a href="http://www.bythom.com/graycards.htm">http://www.bythom.com/graycards.htm</a></p>

<p>Hard to describe, but I was told:</p>

<p>If you take a look at one proper exposed picture and mix all the collors into one then you will have 18% gray.<br>

In other words a perfectly exposed picture in average contains 18% gray.</p>

<p>Now I am confusing, sorry, but then again my mind is not fully awake.</p>

<p>Also when the exposure is set for 18% gray the camera metering wants to aim to make 18% gray. So a black cat might show gray, or a white cat might show gray. Remember to compensate your exposure setting for this.</p>

<p>Going back to bed now, still under the weather.</p>

<p>It's 18 percent because that's what reflected-light lightmeters are calibrated to. It's arbitrary, just a standard set by the industry. It does have some basis in practical use, however -- green grass in the summer is about the same brightness as an 18 percent gray card. And presumably there was some research that found the average brightness in the average scene is around 18 percent. So the average photographer pointing a camera the average scene can be assured to getting something close to a reasonable exposure. If they had set the standard at say 10 percent, the photographer would have to always be looking for something dark to meter off of, and if it was set at 70 percent they would be looking for something bright to meter from.</p>

<p>Many years ago Kodak did an extensive survey of subjects and determined that the average reflectance was 18%, so that's what averaging meters were calibrated for. Spot meters didn't exist at the time.</p>

<p>When spot meters were invented they needed something to look at that could be used to check their calibration, or directly to determine proper exposure as with flash or artificial lighting.</p>

<p>Thus the 18% gray card was invented.</p>

<p>The trick to using the gray card is that it should face midway between the light source and the meter. Many people point the meter directly at the card, perpendicular to its face, and can't figure out why their readings are inaccurate.<br /> - Leigh</p>

<p>A little history lesson on the Gray Card:<br>

In the pre-historic days of the film and camera, every film box included a data sheet complete with printed tables. Bright sunlight = f/16 @ 1/100 second or Overcast use f/8 @ 1/100 second Sports arena lighted f/4 @ 1/100 sec. The tables were helpful but ….</p>

<p>A breakthrough occurred in the late 1930’s, when electric hand-held light meters came on the market. Messrs. Jones and Condit at Kodak Labs measured and demonstrate the average scene had a reflectance that statistically averaged between 18% – 20%. They suggested, using the yellow box top as an exposure determination tool (sheet film had big boxes). A box top was temporally placed in the scene, making sure it was illuminated just like the subject, and an electric reflection light meter was used to take a reading and the camera set accordingly. It worked!</p>

<p>In 1941, Ansel Adams, a prominent landscape photographer and his friend, Fred Archer, a photo<br />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 replaced 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 the ISO is established using the 18% gray card.</p>

<p>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<br />happen).</p>

<p>The incident method yields the same reading as a reflected meter take from a gray card however it eliminates most of the pitfalls of 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 cameramen. Why? Often thousands of dollars are at stake considering the cost to produce a single cinema scene. </p>

 

<p>When values fall over a very large range, statisticians recommend using a geometric mean instead of an arithmetic mean. That means instead of adding the numbers together and dividing by the number of values, you multiply the values together and take the Nth root of the result. (e.g. the average of 1, 10, and 100 would be the cube root of 1000).</p>

<p>Testing showed that the most reflective surfaces like pure snow reflected essentially one hundred percent of the light falling on them. A coating of black soot gave the least reflective surface they could find, reflecting around about three and a quarter percent of the light falling on it. Therefore, they took the geometric mean of those two: sqrt(3.25*100) = ~18.</p>

<p>The values used as to the gray card, stems from the work of Ferdinand Hurter (Swiss/English Chemist 1844-1898) and V.C. Driffield (English 1848-1915). These men founded the science of Photographic Sensitometry and Densitometry.<br /> They published in 1890. Today, their methods are used in film manufacturing, process control and calibration of photo instrumentation.<br /> Now a target with a density of 0.75 reflects some light and absorbs some light. The value 0.75 is just an abbreviation in logarithmic notation and is written as 10 ^0.75. This number can be transformed to its anti-logarithmic value which is 5.5 (rounded). This value can be handled just like any other Filter Factor. If it were a filter with an FF or 5.5, we can calculate its effect on a 100 watt lamp by division. Thus 100 ÷ 5.5 = 18 watts. Stated another way, a 100 watt lamp filtered by a 0.75 density filter has a FF or 5.5 and the light energy after filtration is 18 watts. If it were a piece of film it passes 18 watts for a 100 watt source. If it is a placard target and 100 watt lamp shines on it, it reflects back 18% of the light.</p>

<p>Thank you so much all of you for your decent and useful comments on my question.</p>

<p> </p>

I thought it was actually a 13% grey card?

<p>My gray card (made by Kodak) is very close to 18% reflectance. I measured it.</p>

<p>Great background, Alan. And all this time, I thought it was firmly rooted in the average sky color here in Rochester.</p>

<blockquote>

<p>It's 18 percent because that's what reflected-light lightmeters are calibrated to.</p>

</blockquote>

<p>A common misconception. Read <a href="http://www.bythom.com/graycards.htm">Meters Don't See 18% Gray</a>.</p>

<p>It's been widely reported that Ansel Adams convinced Kodak to make the cards 18% because that corresponded to Zone V in the <a href="http://en.wikipedia.org/wiki/Zone_System">zone system</a>, the middle zone. Hence "middle grey."</p>

<p>Henry Posner<br /><strong>B&H Photo-Video</strong></p>

 

<p>If you don't have an incident meter a reflection one, such as the camera has, will work for you if you can get close to the subject* and take a reading off the back of your hand filling the frame. This is usually one stop brighter than the average, assuming the hand is caucasian.<br>

* or even if you are far away but lit by the same light. Assuming you are caucasian.</p>

<blockquote>

<p>If you don't have an incident meter a reflection one, such as the camera has, will work for you if you can get close to the subject* and take a reading off the back of your hand filling the frame.</p>

</blockquote>

<p>Healthy green grass (not the dreck in MY lawn) is also very close to 18% gray.</p>

<p>Henry Posner<br /><strong>B&H Photo-Video</strong></p>

<p>¿Why do you think 50% is medium gray? You are think in "arithmetical average" but this is "geometrical average".<br>

Think in this way: black is 4%,white is 80%.<br>

The ratio beetween G (medium gray) and B (black) must be the same as the ratio beetween W (white) and G (medium gray) to see the same "jump" in luminosity. It is:<br>

G / B = W / G<br>

So:<br>

G^2 = B * W<br>

So:<br>

G = SQRT (B*W)<br>

If you calculate the square root of 80 and 4 it is 17.88%<br>

You know, almost 18%.</p>

 

<p>Now that I know the math, per Paco, I will stick to 18%, vs 12% or whatevef, although I am not going to place a sizeable cash bet on this business anytime. i have a Lastolite target that looks gray enough compared to Kodak card. This guy Charles Campbell (Backpacker's Photo Handbook) writes that medium gray as 18% relates to film's ability to record light. Or midway in film's 5 stop exposure latitude . Between pure white and detailess black. If that does not 'compute,' we yell at Charles Campbell, nature photographer and teacher. I typically get best results using an incident meter with multiple lights and even then I have to finesse with the reflectance of really dark subjects to get them to look really black or really white. <br>

Love the capability to see the result and look at the histogram to confirm and tweak the spread of luminance. Anyway, a meter has to have a numerical constant, so whatever they use, I live with. </p>

 

<p>Lot of lore being tossed about:</p>

<p>Photo scientists have designed instruments that precisely expose film. This instrument is called a sensitometer. Typically the film to be tested is exposed in steps, each step a doubling of the exposing energy. The opening exposure is below the films threshold i.e.an exposure is too weak to achieve any response. The second exposure in the series is twice the first. Each subsequent exposure doubles the exposing energy. After development a series of patches results. We call this a gray scale (sometime known as a step wedge). One end is clear film (D min.) the other maximum back (D max.). For the typical film used in pictorial photography there will be 11 steps. We call each incremental step a stop. The word stop is used loosely as most photographers associate exposure changes as being made using the camera aperture which typically operates in f/stop increments based on a doubling or halving of the exposing energy.</p>

<p>When this test is carried out maximum black (D Max) step is labeled 0 (zero) and the clear film step is labeled 10. The center step, 5 is a medium gray (battleship gray). Step 5, is considered the center of the gray scale because film does not blacken linearly. If it did, a graph of the blackening would be a straight line. Instead, film is sluggish when it comes to starting the blackening action. Instead of a straight line the early part of the graph is quite gentle. As the exposure builds, the graph becomes a straight line. Higher on the graph, the film now getting quit dark begins to slow as to its reaction to light, the graph flattens. We divide this graph into three regions; the early stage is called the toe, the middle the region of the straight line, the latter part is called shoulder. The straight-line center region is the region<br />of good exposure. The toe is the region of shadows, the shoulder records highlights.</p>

<p>The center of the graph, zone 5, is a useful point as this value can be used to calibrate photographic instrumentation such as light meters. This is true because Films do not blacken linearly, instead, they blacken via a logarithmic progression. This is OK because human vision also loosely follows this same scheme. As an example star brightness, labeled in magnitude, is based on a logarithmic progression.</p>

<p>Why is this a key value?<br /> Step 5, the center of the gray scale measures on the average transmission 0.75 transmission log density. It is no coincidence that the gray card also has a reflection density of 0.75.</p>

<p>Bottom line: When a gray card with 18% reflective (0.75 reflective density) is correctly exposed, and the film correctly developed, the film area containing the image of the gray card will read with a transmission density of 0.75. A print made from this negative, correctly exposed and processes, will yield an image of the gray card that will read 0.75 reflective density (18%). In other words, this is the only tone that when photographed, produces the same tone on the negative and when printed, produces the same tone on the print i.e. it is the axis of the photo system. Thus, it is the key calibration tone.</p>

<p>The 18% was arrived at empirically by Kodak's statistical analysis of the "average" scene that the "average" snapshooter took. There is <strong>no</strong> mathematical basis or formula for it whatsoever.</p>

<blockquote>

<p>"When a gray card with 18% reflective (0.75 reflective density) is correctly exposed, and the film correctly developed, the film area containing the image of the gray card will read with a transmission density of 0.75."</p>

</blockquote>

<p>Errm, no! Film is never developed to a gamma or C.I. of 1, which is what would be needed for this "theory" to be correct. As stated by Leigh and as I just repeated; the 18% standard is simply an experimentally arrived at figure. In fact a B&W negative developed to a normal Gamma of approximately 0.6 should show a density of around 0.45D or slightly less when exposed to an 18% grey target. While an sRGB colourspace digital image should show 18% subject reflectance as a pixel level of 117 - give or take a level or two.</p><p>Anyhow. The OP's question still hasn't really been answered; namely the real definition of 18% grey reflectance. First we have to define 100% reflectance, which is a plane surface that diffuses light equally in all directions while reflecting all of the incident light energy. In short, a pure white <a href="http://en.wikipedia.org/wiki/Lambertian_reflectance">Lambertian reflector</a>. Although the Lambertian surface is a theoretical model there are real materials that approximate to it; white chalk, white emulsion paint (dried) and thick white copier paper all come pretty close. For all practical purposes you can take a light reading off any of these and subtract 2.5 stops to get an accurate enough 18% reading. Personally I'd rather use a white card and a bit of maths like this. At least you can see whether your white card has yellowed or got dirty fingerprints all over it, and it's not going to fade! It's also a darn sight cheaper than a proper grey card to replace.</p>

 

BTW, Zone V should be 12.5 % reflectance according to Adam's visual scale.

<p>Modern pictorial films are manufactured and developed to realize a gamma of approximately 0.8. This produces a reasonable fit for grade 2 papers.</p>

<p>If a film has a gamma of 1 (some do), the angle of the straight line will be 45°. Such a film will react to a doubling of exposure with a density delta of 0.30. A gamma of 1 has proven to be too contrasty for most applications so most pictorial films have a gamma of approximately 0.8. The angle of the straight line now becomes about 38°. The tan of this angle is the<br />measure of the gamma and this works out to be 0.80. The density delta is 0.30 x .8 = 0.24.<br />Meaning for each f/stop change the film darkens by 0.24 density units in the region of the straight line.</p>

<p>I stand on " Step 5 of the typical step wedge is approximately 0.75 plus base fog".</p>

<p>Hang your highlights you don't want blown on the 200RGB histogram shown on your camera's LCD.</p>

<p>It's worked for me on over 1000 Raw images. Now that newer cameras include all three color channels in the histogram saturation can now be accounted for especially shooting sunsets and other scenes where R>G>B.</p>

<p>LOL. Let's see you print this film developed to a gamma of 0.8 straight onto grade 2 paper then Alan. That would mean a typical Dmax of something >2.0D - far too dense to sensibly print while retaining shadow detail. Suggest you read Adam's books "The negative" and "The Print" to see what sort of gamma and density range are recommended for top-quality B&W printing.</p>

<p>Agreed that a density wedge will <em>sometimes</em> have an 18% reflectance or transmission step (strictly 0.745D, but usually rounded to 0.7D or 0.75D in reality) somewhere on it. However a reference density tablet shouldn't have any fog level, and 0.75D isn't always step 5. For example a half-stop transmissive tablet will run 0, 0.15, 0.3, 0.45, 0.6, 0.75 - which is step 6 with reference to the base density; while a reflective wedge might run in 0.1D steps like the one shown below from a certain company that's recently gone into liquidation. The step marked "M" is the mid-grey reference, and is the eighth step from the left. Other reflective targets I've seen run 0.1, 0.4, 0.7D, etc.</p><div></div>

<p>Step tablets are made of film, they have base fog. The toe of the film is sluggish so the slope angle is not uniform. I have seen some laboratory step tablets made of carbon imbedded in gelatin but these are laboratory grade, of the reach of most.</p>

<p>I stand on step 5 of a film 11 step gray scale will read approximately 0.75. This anti log of this value is 5.62 and called the opacity. The inverse 1/5.62 = 0.1778 which is the transmission. Transmissions are by convention stated as a percentage to avoid confusion. The % Transmission of this patch is = 17.78% rounded it's 18%.</p>

<p>This value is universally used as the calibration point for light meters and densitometers. <br /> <br />I am done with this discussion after adding that I fall in this category:<br /> <br />“And how many hours a day did you do lessons?' said Alice,<br />in a hurry to change the subject.<br /><br />Ten hours the first day,' said the Mock Turtle: 'nine the next, and so on.'<br /><br />What a curious plan!' exclaimed Alice.<br /><br />That's the reason they're called lessons,' the Gryphon remarked: 'because they<br />lessen from day to day.” <br /><br />― <a href="http://www.goodreads.com/author/show/8164.Lewis_Carroll">Lewis<br />Carroll</a>, <em><a href="http://www.goodreads.com/work/quotes/2375385">Alice's<br />Adventures in Wonderland & Through the Looking-Glass</a></em></p>

<p>The best step tablets are made of glass and are coated with an etched translucent metal film, so there's no fog. Only cheap repro steps are made using silver-based film.</p>