18 percent grey & exposure theory needs clarifying :-)

<p>I am trying to understand the 18 percent rule and how you have to be mindfull of it when photographing<br />a black dog or a white dove for example.<br>

I understand when photographing a black dog you have to stop down and when photographing a white subject you have to open up a stop or two but why?<br>

Is this correct:<br />When photographing a subject in a dark suit you stop down because the camera will be tricked into letting<br />more light in to render it 18 percent grey. To prevent this you reduce the aperture of / stop<br />down which means less light is received by the camera and the result is a black suit looking<br />black as intended.<br>

Is my thinking correct? I really want to crack this one so if anyone could clarify why snow looks grey and why you have top open up the aperture to get it white I'd<br />be v graetefull.<br>

Thanks,<br />David</p>

<p>A reflective light meter is calibrated with a grey object; often in the 12.5 to 18 percent reflectance.</p>

<p>( to keep it simple; ignore there are two camps of folks! )</p>

<p><strong>You use the SAME exposure; say 1/500 at F16 with iso 400 on a bright sunny day; whether shooting a black or white object.</strong><br /> <strong> </strong></p>

<p>If you meter off the white dog you will underexpose and he will look grey</p>

<p>If you meter off the black dog you will overexpose and he will look grey</p>

<p>If you shoot hockey or folks in the snow ; one's meter "sees" a field of a lot of white; thus one gets grey images; ie due to underexposure. Thus one adds say 1 stop more exposure; because the meter "thinks" it is looking at grey!</p>

<p>A reflected light meter does NOT know what it is looking at.</p>

<p>Think of its goal in life is to make all objects look grey!</p>

<p>If one shoots separate say Kodachrome slides of a white; grey and black dog and uses a reflected light meter;</p>

<p>the exposure would ONLY be correct for the grey dog.</p>

<p>ie all three dogs will be grey in the slldes.</p>

<p>Thus if one is shooting the black or white dog; you use a grey card to get a better exposure</p>

<p>If all three dogs are in the same image; there is only one exposure.</p>

<p>A single slide including the black, grey and white dogs will look ok if one meters off the 18 percent grey dog</p>

The refelective light meter either hand held or in a camera, tries to average everything it sees out to a middle gray. That usually works out well but if there are too many lights or too many darks in a scene, then the meter gets "fooled" and gives a bad exposure setting.

 

Suppose you have a large brick wall that is painted with large white and black squares. You frame a shot of the wall so there are equal amounts of white and black squares. The exposure setting is, let us say, 1/125 @ f/11. You take the shot and you get a good photo of white and black squares with brick texture showing. Very good. 1/125 @ f/11 was the correct exposure for the light falling on the scene. There are equal amounts of lights and darks so the meter works fine.

 

Now you get closer so only one square at a time is showing in the frame. As you move the camera around from white to black squares the meter reading keeps changing from 1/1000 @f/11 to 1/15 @f/11 and everything in between . What is going on here? The squares were properly exposed at 1/125 @ f/11, the lighting hasn't changed so why is the meter moving all over the place? The meter is simply being "fooled" by all that white or black with nothing to average out and is giving an exposure setting to make the white square appear gray and the black square appear gray. If you take the photos at those exposure settings, you will get gray squares. Ergo, that is why you have to open up or close down to compensate.

<p>Photographic films can record a brightness range of about 10 f/stops (chips approach and maybe exceed). One f/stop change is a doubling or halving of the amount of light allowed to play on the film/chip. Thus the dynamic range is 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 1024. Ansel Adams, a prominent landscape photographer called these zones.</p>

<p>Zone I black like black velvet in shadow<br>

Zone II black construction paper<br>

Zone III black velvet in direct light<br>

Zone IV gray card in shadow<br>

Zone V gray card in direct light<br>

Zone VI light gray construction paper /human skin<br>

Zone VII human skin in bright light<br>

Zone VIII light gray construction paper in direct light<br>

Zone IX bright reflections (highlights)<br>

Zone X pure white</p>

<p> In times gone by, getting the exposure right was a matter of experience and guess work. Films came with tables to help photographers set their cameras. The tables gave exposure suggestions for conditions like overcast or cloudy bright or sunny. A breakthrough came in the 1930's with the introduction of an electric light meter. </p>

<p>Work on exposure a Kodak Labs by Messrs. Jones and Condit at Kodak Labs told us that we could place a Kodak film box top in the scene and measure it using the new electric light meter. The box top reflected 18% of the light that played on it. Further, they had measured thousands of pictures and vista brightness and the average was middle gray with an 18% reflectance. </p>

<p>In 1941 Ansel Adams, and his friend, Fred Archer, a photo magazine editor, jointly published the Zone System. They advocated using a 18% gray target in place of the box top and this placard 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 and other gray test materials. These test materials predate Adams and Joes and Condit.</p>

<p>Today we use the gray card by holding it so that it receives the same light intensity as does the subject. A proper reading coincides with an average of multiple readings taken from shadow and highlight. This is a reliable method because if properly applied, a photograph of a gray card included in the scene, will be correctly rendered. If true, the gray card and most all other shades will fall in line by law.</p>

<p>The disadvantage is the need to carry a gray card and contrasty scenes with extreme values likely exceed the dynamic range of the film. Thus the gray card method, without modification, forts your ability to modify tones to best advantage. As an example, human skin will be rendered battleship gray. Now some complexions are lighter and some are darker. You might be better served to over expose and render your subjects skin lighter or under expose to render the skin darker. There is no replacement for experience. </p>

<p>Since all this, modern cameras have built-in light sensors driven by software. The idea is optimize exposure eliminating much of the guess. All this is well and good but keep in mind, there is no substitute for experience. You gain expectances by doing and by studying and by practice then more practice.</p>

<p>David,</p>

<p>You are basically right. The key is simply this: your meter is set to evaluate how much light it has to let in to render whatever you are metering 18% gray. Averaging has nothing to do with this; it is true of spot metering too. So, if you point it at something that is close to neutral gray in terms of luminance, you are fine. But if you point it at a black dog, it will do the only thing it knows how to do: figure out how much light to let in to render the dog gray. You have to compensate and let in less light than it asks for.</p>

<p>This assumes that the meter is measuring reflected light, but if you are using your camera to meter, it is.</p>

<p>Dan</p>

<p>You REALLY want to read this:<br /><a href="http://www.bythom.com/graycards.htm">Meters Don't See 18% Gray</a></p>

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

<p>The scripts as to meters seeing 18% or some other value reads like scientific research papers. The fact is, an 18% reflective surface reads on a reflection densitometer 0.75 density units. A properly exposed and developed film image of such a target reads on a reflection densitometer 0.75 density units plus base fog. A properly exposed and properly developed print of that negative/slide reads 0.75 on a reflection densitometer.</p>

<p>Thus the 18% target tone is chosen because it is the only tone that reads the same as to density - in reality - on the film - on the print. Additionally, the tone is the central axis of the photographic system. It is the center point of a film characteristic curve plot and it is the center point on a paper characteristic curve plot. These graphs are used to establish the ISO speed of film and the corresponding paper speed.</p>

<p>Now, one can argue 18% vs. 12% or some other percentage, nevertheless this value remains one of the key aim points used in film manufacturing, both color and black & white film. 18% is also the key aim point for calibration of densitometers. 18% is also the key value used to calibrate enlarging meters. 18% is the key value used to calibrate high-speed photofinishing printers. 18% is the key value used to calibrate mini-lab printers. 18% is the key value use to calibrate hand held light meters. 18% is key value used to calibrate in-camera sensor system - etc.</p>

<p>The 18% placard has been with us since the founding of the twin sciences of sensitometry (exposing photo materials with precision for test purposes) and densitometry (instrumentation to measure film and paper). The argument as to what meters see and don't see is like the chicken and the egg argument. Sorry to report it's too late. The argument has already been hashed out years ago and 18% is the winner.</p>

<p>Reflective lightmeters are calibrated to provide a reading that will make everything look light gray (or 18% gray.) That's because manufacturers supposedly concluded that an average scene is 18% gray -- and whether that's true or not is largely irrelevant anyway.<br />A white dove in snow is not light gray, of course. If you use the setting that your reflective meter gives when you take a photo, both the dove and the snow will come out light gray.<br />So, to make the snow and the dove look lighter, you need to give more exposure -- longer shutter speed and/or larger aperture size.<br />Similarly, a black dog in coal is not light gray. To make a photo of the black dog in coal look darker, you have to give the shot less exposure - smaller aperture and/or faster shutter speed.<br />How much more or less exposure? One solution is to read the exposure off of something that is itself light gray. And old pair of blue jeans is about that, and supposedly so is grass. Or, carry a gray card with you and use that.<br />And if that doesn't work, use a Rule of thumb: 1-2 stops.<br />BUT...<br />Even if you make the dog look black and the dove look white, the trick is to make the dove still stand out from the snow, and the dog to still stand out from the coal. You don't want to get a photo of just a blob of white or black with a pair of eyes on it, after all. You want to set the exposure and develop the film so as to capture the detai (texture) in the dark and light areas sufficiently. And that's where the Zone system comes in...</p>

<blockquote>

<p>Is this correct:<br />When photographing a subject in a dark suit you stop down because the camera will be tricked into letting<br />more light in to render it 18 percent grey. To prevent this you reduce the aperture of / stop<br />down which means less light is received by the camera and the result is a black suit looking<br />black as intended.</p>

</blockquote>

<p>Simple answer: Yes.<br>

It is the opposite when the meter is readimg snow - you open up the aperture a bit.</p>

<p>WW</p>

<p> . . . or in some situations ignore the camera's lightmeter:<br>

<a href="../photodb/folder?folder_id=972502">http://www.photo.net/photodb/folder?folder_id=972502</a></p>

<p>(read the title of the first image)</p>

<p>WW</p>

<p>Thanks WW!<br>

You get an A+ from me. I have 50+ years in the industry - Much of that was spent making the test materials and the instrumentation and the algorithms used in conjunction with color printing. </p>

<p>THE REAL MYTH; is thinking 18 percent is a myth!</p>

<p>It was used to set meters for decades!' After 1960 they got rid of the safety factor in B&W materials; the definitions of asa got changed.</p>

<p> As a practical basis; with early meters and folks still camera; slop in fstop and shutter speed swamped the differences between setting up ones camera in say 1965 to 18; 14 or 13 percent.</p>

<p>Thus saying 18 percent is a myth is absurd; it is sort of like saying folks never use dial phones; it is weird</p>

<p>The grey card itself has been gray for 100 years now; it was just an easy thing to adjust ones meters too.</p>

<p> Plain C41 iso 800 films in a one shot camera are set to overexpose about 3 full F stops; and one gets decent prints.</p>

<p>With most film images ever shot; folks did not even use a meter; nor did the camera have one.</p>

<p> </p>

<blockquote>

<p>Additionally, the tone [18% Photographic Grey] is the central axis of the photographic system. It is the [centre] point of a film characteristic curve plot and it is the [centre] point on a paper characteristic curve plot. These graphs are used to establish the ISO speed of film and the corresponding paper speed.</p>

 

</blockquote>

<p>FWIW – this is taught (almost verbatim) and as an answer would score 100% in the theory exam for First Year Degree here in AUS.</p>

<p>WW</p>

<p>(Aside to AM and for clarification) - I rearranged the order of my posts such that I addressed the OP's question first - I assume this comment here, is the one to which you are referring / thanking.)</p>

 

<p>Hi David.....I looked at your website and see that you use digital cameras.......The zone system does not work for the digital experience.....It was designed for the <strong>film </strong>and chemical development processes.<br>

The processing and capture physics are simply incompatible with Adams excellent approach to single negative, film exposure....</p>

<p>David, you've basically got it.</p>

<p>It is true that the zone system as initially described is designed for black & white film with its typically wide range of tones--but many of the concepts can be worked into digital capture, it's a matter of learning your medium. You can make a similar argument that the zone system doesn't really work for roll film either since it implied adjusting image contrast during development. </p>

<p>I think understanding the zone system is valuable but needs to be recalibrated for your equipment. In particular, digital's relatively harsh highlight clipping means that you have to be careful about where exposure clips to white...and with digital cameras you probably don't have a full nine stops of usable range from pure white to pure black. But the idea of previsualizing where in your medium's tonal range a key tone in your scene should be placed is very much a zone system idea.</p>

<p>Andrew....you can't recalibrate it, because some of the tools used simply have no analog in the digital world........also, various manufacturers have various numbers of "zones" and do not all place middle grey in the same place. The roll film point is true enough too.......you really can only process for <strong>one </strong>neg on the roll.<br>

Best is to let Ansel rest and concentrate on learning the exposure intricasies of whatever digital system you are using......</p>

<p>Re the zone system . . .</p>

<p>I found (already knowing and having used) the zone system useful for when I had to <strong>LEARN</strong> the digital medium and RE-locate my INTUITION using digital cameras - it IS different.<br />I concur with AG. The most relevant re-gigs of the zone system to adapt to digital, for me were:<br />> squeezing 9 into "five for digital" (my phrase & my concept of MY answer for it)<br />> working out how much I could push, before the highlights were totally whacked and lost forever.</p>

<p>WW<br>

Hi, Robert:<br>

I did not interpret AG's "recalibrate" in a technical sense - in any case, what ever AG meant - I did not "recalibrate" . . . I re-gigged the CONCEPT to adapt it to how I intuatively approach and use the digital medium.</p>

<p>Thank you everyone for quality answers. I'm on a mission to see scenes as my camera sees them so I can confidently get the correct exposure whatever the situation.<br>

Thanks,<br />Again<br>

http://i216.photobucket.com/albums/cc53/zymurgy_bucket/case_closed.jpg</p>

 

Even if one no longer, or never did, develop one's own film, photographers should be familiar with the zone system nomenclature. It is a lot easier to understand and explain exposure that way:

 

"Your subject's face in shadow is falling on zone 4. You should open up two stops to place it correctly on zone 6. However, the sunlit beach in the background is exposed correctly on zone 7. If you were to open up two stops that would place it on zone 9 which is beyond the dynamic range of your digital camera. You can either move the subject out of the shadows and give the same exposure as was used for the background or use flash or a reflector to raise the subject's face to zone 6 while maintaining the same exposure for the background."

And the zone system is not just for explanations. When determining the correct exposure using a spotmeter, the zone system understanding is needed.

 

"I spotmeter the dark shadow of the rock that I want to show as pure black and place it as zone 1. A meter reading of my subject's face shows only a three stop difference meaning it is falling of zone 4. Therefor ..."

Alternately, you can leave the aperture alone and adjust the shutter speed.

<p>David,<br>

RE: mission to see what your camera sees in any situation><br>

I have gotten the best results using Fred Parker's Ultimate Exposure Computer: http://www.fredparker.com/ultexp1.htm<br>

I then use my camera's histogram to calibrate and make my own exposure value list. Now I can visualize just about anything accurately without even using a meter. This is essentially like always having an incidental meter reading without the meter. I posted on my blog explaining how I do this: http://danielfulton.com/2010/10/mastering-photographic-exposure-for-digital-cameras/<br>

Hope this helps,<br>

Dan</p>

<p>Using black and white film you use a reflected light meter the scene and photograph a black horse nose-to-nose with a white horse. What do you get? A black horse and a white horse. The meter averaged both horses and produced an accurate exposure rendering both horses correctly. Now meter and photography each horse separately. What do you get? Two middle grey horses. The meter just wants to turn everything middle grey. So when metering each horse you have to turn the "grey horse" back into the correct shade of horse by either lowering or increasing exposure.</p>

<p>Grey is made black by less exposure. Therefore, after metering the black horse, reduce exposure to get the black that you feel meets your artistic depiction of the horse. Use either or both aperture and shutter speed changes as needed.</p>

<p>Grey is made white by more exposure. Therefore, after metering the white horse, increase exposure to get the white that you feel meets your artistic depiction of the horse. Use either or both aperture and shutter speed changes as needed.<br>

If the above is too complicated, buy an incident light meter as it will completely ignore the color of either horse. :)</p>

<blockquote>

<p>If the above is too complicated, buy an incident light meter as it will completely ignore the color of either horse. :)</p>

</blockquote>

<p>Works with cats too.</p>