snubbr.com

Why 18% Gray?
Speaking of gray cards, Am I right in believing that the 18% figure refers to the amount of light reflected by the card? If this is true, how is the gray card considered middle gray? Shouldn't a middle gray figure be 50%?.

Please tell me, how does 18% reflectance give a "middle" gray?.

I am under the impression that meters are calibrated with an arbitrary standard, but this does not explain how gray cards made for use with this standard can be considered middle gray...

Comments (17)

Http://www.bythom.com/graycards.htm.

Http://www.cambridgeincolour.com/tutorials/camera-metering.htm.

Http://www.youtube.com/watch?v=VF5B4McYPls&NR=1Rationally I have no hope, irrationally I believe in miracles.Joni Mitchell..

Comment #1

I have seen all of these and they don't answer my question.Why is only 18% reflectance considered middle gray?..

Comment #2

Joesf wrote:.

Speaking of gray cards, Am I right in believing that the 18% figurerefers to the amount of light reflected by the card? If this is true,how is the gray card considered middle gray? Shouldn't a middle grayfigure be 50%?.

Please tell me, how does 18% reflectance give a "middle" gray?.

I am under the impression that meters are calibrated with anarbitrary standard, but this does not explain how gray cards made foruse with this standard can be considered middle gray..

As always, Wikipedia to the rescuehttp://en.wikipedia.org/wiki/Middle_gray.

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window..

Comment #3

I also saw that and have to say that is probably the worst wikipedia article I have ever seen. It states that exposing a photo-sensitive material in 5 steps will give you a 5 stop progression. How could that be true? Imagine exposing each step for 1 second..

The first will have an overall exposure of 5 seconds and the next 4 and the next 3 and so on. an actual 5 stop progression would be 32 seconds, 16 seconds, 8 seconds, 4 seconds, 2 seconds, 1 second..

I want to know why reflecting half of the light (50%) isn't middle gray...

Comment #4

OK, the truth is probably that 18% gray is a rather arbitrary standard established long ago by the printing community that decided this percentage to represent what appears as a midway value between black and white on the printed page..

Remember, however, that exposure meters are calibrated to luminance rather than reflectance, and reflectance is more or less 12%, about 1/2 stop difference..

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window..

Comment #5

So what you're saying is there is a discrepancy between what we view as a middle tone and what actual 50% reflectance appears as?.

Could someone elucidate this paragraph for me..

"Naturally this number did not come as an inspiration in a dream but was caused by the typical five field sensitivity test made in the early days on films and papers. To do a five field test you place the photo-sensitive material under a controllable light source and cover up 4/5 of the material and start the light for a defined time. After that you cover 3/5 and so on. The result is that every field is exposed twice as long as the one before, or in mathematical terms : It is a 5 stop doubling progression. The geometric mean of a 5 stop doubling progression in between 0 and 100 happens to be 18.".

I am most interested in the last sentence. about the math...

Comment #6

Joesf wrote:.

... Shouldn't a middle grayfigure be 50%?.

Please tell me, how does 18% reflectance give a "middle" gray?.

The term "middle gray" refers to human perception, not the actual energy reflected off the card. When light reflected off a card increases from about 18% to 100%, it subjectively appears to have only doubled in brightness. This is because the human visual sense has a non-linear response to light intensity..

A 50% relecting card would appear to be almost as bright as a 100% reflecting card. You would judge it to be a light gray..

JerryG.

See my galleries at:http://www.pbase.com/jerryg1..

Comment #7

They weren't trying to make a middle gray but rather a representative an average scene if you "mashed" or averaged all brightnesses and colors into one. Depending on whose statistics a gray between 12% and 18%..

Comment #8

Why get "hung up" on an english phrase "middle gray," just because somebody mentioned it in print? How about "french gray" (which looks like concrete, by the way.) At some point the industry must have settled as to how to calibrate their lightmeters and white balance averages, and came up with a shade of gray that was not too bright to overpower their sensors, nor too dark to be hard to use in dim areas, and in their judgment was close to the "middle tone" in a b&w photo. Once they agreed on a shade, obviously then they determined the proportions of white paint or ink, and black paint or ink, it would take to achieve this shade. It turned out it took 18% of one, and 82% of the other. I don't know if the above description would stand up to documented facts, but I know having been involved in the creation of several technical standards, that is the basic procedure.RUcrAZ..

Comment #9

I don't know whether this is true or not, but the bog standard answer you'll find in most intro to photography books is that 18% gray was chosen because it replicates the typical amount of reflectance from things like foliage or a blue sky, which are the most common things seen in photographs..

In relation to this, most intro books will also tell you that if you want a properly exposed shot, grab your exposure off of some grass or green leaves, lock the exposure, and then compose your shot back on your intended subject. This usually doesn't work perfectly in the real world, but it's a starting point...

Comment #10

I also saw that and have to say that is probably the worst wikipediaarticle I have ever seen. It states that exposing a photo-sensitivematerial in 5 steps will give you a 5 stop progression. How couldthat be true? Imagine exposing each step for 1 second.the first will have an overall exposure of 5 seconds and the next 4and the next 3 and so on. an actual 5 stop progression would be 32seconds, 16 seconds, 8 seconds, 4 seconds, 2 seconds, 1 second..

It's not clear from the article is it? I remember doing this 5-step exposure test when I did my own B&W printing, and did it like this:.

1. Uncover one-fifth of the paper, expose for 8 seconds.

2. Uncover another one-fifth of the paper, expose for 4 seconds (so the first strip has had 12 secs, the second has had 4 secs).3. Uncover another one-fifth, expose for two seconds4. Uncover another one-fifth, expose for one second5. Uncover the whole sheet, expose for another one second..

The five strips will then have been exposed for 16, 8, 4, 2, 1 second. This what the article is trying to get at..

Best wishesMike..

Comment #11

You getting close, at this point. Practicality and the evolution of technologies (and the understanding of the general use of meters) made this the standard, but not only way to measure light..

It is (and was) about matching the volume of light to the limits of the recording materials..

100% (white with no distinguishable tonal separation)50% is 1EV or 1 F/Stop25% is 2EV or 2/Stops18% is 2.5 EV12,5% is 3 EV6.25% is 4 EVEssentially black was 5 EV (depending on the craft skills and mfg variables).

The dynamic range of the recording and printing materials used to range in the 5 EV range from black to white. Hence, 18% gray was in the "middle" of the range..

18% puts the recorded values in the middle of that range and allows for some adjustment through processing (push and pulling, in film, curves in digital).

Today, the dynamic range has been extended to from 6 to 12 EVs. But, 18% still puts your exposure right in the middle and allows for highlight tonal separation and shadow tone separation (and more adjustability).

John.

Van..

Comment #12

Very informative, was just what I was looking for. I often tried to reason the 18% by the fact that it was close to 12.5% which is half of half of half of the whole. But I never "got it". This illustrates it properly, basically reminding everyone that exposure, as well as sight, is logarithmic. So while 50% reflectance may give you half the light used for illumination, 12.5% is closer to logarithmic middle. Would you say this is an accurate understanding?..

Comment #13

Or you could forget about reflected light meters and think about incident light meters. Incident light meters measure how much light is falling on a scene instead of how much light is reflected by a scene. In an ideal world both would yield the same answer. In reality no. Both methods have their issues. Learn to use one method and learn when it's results will lead you astray, then compensate accordingly.



Arrested for laughing and waving a light meter...

Comment #14

Joesf wrote:.

The geometric mean of a 5 stopdoubling progression in between 0 and 100 happens to be 18.".

I am most interested in the last sentence. about the math..

The last sentence that you quote is a nonsense. "The geometric mean of a 5 stop doubling progression" is just the middle number in that progression. On the other hand, there is no "doubling progression in between 0 and 100" (assuming that 0 is the first, and 100 is the last number). If one of the numbers is zero, then all must be zero..

The explanation of "n p vansteenberg" seems reasonable to me...

Comment #15

Yes, but only an engineer could get the log scale, not mortals:~).

Everybody wants absolutes. In imaging there are mostly relative tonal values and approximations seasoned to taste..

John.

Van..

Comment #16

How much light that a material reflects is not the same as the luminosity value; they measure different things..

If a material or subject reflects even 1% of light, you can still make it's luminosity (output) value very bright or very dark, depending on the exposure of the medium (e.g. film)..

Http://www.bythom.com/graycards.htm.

Joesf wrote:.

Speaking of gray cards, Am I right in believing that the 18% figurerefers to the amount of light reflected by the card? If this is true,how is the gray card considered middle gray? Shouldn't a middle grayfigure be 50%?.

Please tell me, how does 18% reflectance give a "middle" gray?.

I am under the impression that meters are calibrated with anarbitrary standard, but this does not explain how gray cards made foruse with this standard can be considered middle gray..

Galleries: http://www.dheller.net.

Many folks on dpreview.com list their equipment here, but don't list any links to their images. Do they collect equipment? Or take pictures?..

Comment #17

Click Here to View All...

Sponsored Amazon Deals:

1. Get big savings on Amazon warehouse deals.
2. Save up to 70% on Amazon Products.


This question was taken from a support group/message board and re-posted here so others can learn from it.

 

Categories: Home | Diet & Weight Management | Vitamins & Supplements | Herbs & Cleansing |

Sexual Health | Medifast Support | Nutrisystem Support | Medifast Questions |

Web Hosting | Web Hosts | Website Hosting | Hosting |

Web Hosting | GoDaddy | Digital Cameras | Best WebHosts |

Web Hosting FAQ | Web Hosts FAQ | Hosting FAQ | Hosting Group |

Hosting Questions | Camera Tips | Best Cameras To Buy | Best Cameras This Year |

Camera Q-A | Digital Cameras Q-A | Camera Forum | Nov 2010 - Cameras |

Oct 2010 - Cameras | Oct 2010 - DSLRs | Oct 2010 - Camera Tips | Sep 2010 - Cameras |

Sep 2010 - DSLRS | Sep 2010 - Camera Tips | Aug 2010 - Cameras | Aug 2010 - DSLR Tips |

Aug 2010 - Camera Tips | July 2010 - Cameras | July 2010 - Nikon Cameras | July 2010 - Canon Cameras |

July 2010 - Pentax Cameras | Medifast Recipes | Medifast Recipes Tips | Medifast Recipes Strategies |

Medifast Recipes Experiences | Medifast Recipes Group | Medifast Recipes Forum | Medifast Support Strategies |

Medifast Support Experiences |

 

(C) Copyright 2010 All rights reserved.