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Exposure in your hands This section will help you understand what you have to do to judge when to snap the shutter and get the exposure right. In ancient religious scripture, truth is sometimes prefixed with the words, “Let him who has ears to hear and eyes to see….”. The same is true with electronic media. Unless you know how the microphone or image capture device works, you cannot very well control what you record. Our emphasis in this chapter is to learn to See with a capital “S”. Many of the photographic concepts we deal with now are based on antiquated constraints such as black and white film, negative color film, Kodachrome transparencies, ASA exposure indexes and so forth. The first exposure standards for film were established in Washington, DC on the lawn south of the Lincoln Memorial on the day summer begins, June 21. That number is based on the shutter speed required to take an image in that vicinity. The ISO standard evolved from the ASA standard and because of more advanced instrumentation is shared throughout the world and manufacturers set their design goals to be compatible with this index. Fifty years ago film manufactures named their film after the speed index with names like X, Plus X and Tri-X. Tri-X was three stops above the base of 25 which was what the shutters of early box cameras could accomplish. Enough. Before light meters, photographers were advised on a little slip of paper packed with the film, how to use the film in each environment and everybody guessed and took their chances. Some bright person made up a rhyme to help remember what was on the exposure recommendations. It is still useful today to help judge whether a shutter/lens combination makes sense. All you have to do is “See”. Sunny Sixteen There was also Shady Eight, and f11 rule of thumb (perfect day). You have to consider that photographers also thought that everybody lived at about 45 degrees latitude and had no air pollution. If you are going to use this bit of folk wisdom, let us have eyes to See and prove it to be valuable. I assume you have a modern digital camera that can be set to shutter priority. If you have the letters MASP on your control dial, set it to S and set the speed to 100 or 200 depending on the ISO setting of your camera. Now walk around in the daylight hour and point you camera at different objects to see what f stops the camera wants to use. If you have spot metering or center weighted use those settings to isolate which part of the image gives you the proper reading. As I write this in late afternoon on June 24th my blue sky registers f11, my lush lawn gives me f8 or f11 depending on whether I am facing north or South, and a small white cloud gives me f16. I have a light colored tool shed that gives me f16 on the sunny side as well. My personal non standard grey card gives me f4.5 in the shade and f20 in the direct sun. So what does this tell us? The meter does not tell us what color it sees and we know that sunlight contains almost all the colors. If the sky was blue and I had a lot of grass and a few trees, I might find f11 would get the picture. If it looked too bright in the viewfinder, then maybe f16 would be correct. I can tell you that if the scene had a large area that was red, the matrix metering would be completely unable to provide the proper exposure. But just knowing the general intensity of daylight in the middle of the day for cloudless skies would allow us to guess within one stop of an acceptable setting. The dirty secret of modern digital cameras is that the color red is not weighted very high in computing the total exposure for a picture. Even though we have histograms to show us the relative tone values the composite histogram on the back of the camera only shows the Green or Luminance and fails to warn us of exceeding the numerical capacity to record the color red. Early Nikon DSLR cameras such as the D70, D50 and
D100 do not have color histograms, but you can still use a raw editor
to display the color histograms to make this digital gray card work. D40 and D60 cameras have a color histogram which is
buried in the retouch menu under Filters.
It is convenient to know that the default menu selection while viewing
an image takes you directly to the color histogram. This is done by
pressing the OK/Enter button three times when viewing a picture. All DSLRs from D80, D300 and D2X can show the color
histogram as one of the choices of the View button. Some cameras purport to show the luminance channel
as white, but in fact only show the green channel as if it were the
luminance channel. This tricks photographers into making really bad
mistakes in over exposing the red and blue channels. Also the blinking
highlight feature to warn against over exposure is also only based on
the green channel. The wise photographer would be advised to only use
the three RGB colors to evaluate exposure. Since the fundamental tools of black and white photography have been the basis for the tools built into recent digital cameras, we must re evaluate how to use these tools as new color features become available. The “Auto” features built into the cameras work most of the time but a few of you have become aware that you can only take Auto anything very far. You are beginning to experience some of the failures of these systems. In this chapter we will examine Auto White Balance and Matrix metering, which depends on an automatic evaluation of color as it pertains to exposure. The way the digital camera deals with color is the
measure
the values of red, green, and blue (RGB) and deliver the value of each
color to
a recording device. The pattern we use at this time is called a Bayer
matrix.
It has two green sensors and a red and blue sensor adjacent to form a
square.
The combination of the four locations is called a pixel. If all are
turned on
the result is white. If only one is turned on the value is that color.
If there
is none turned on, the value is black. Green is used for the basic
color for
adjusting the balance of all the colors to produce grey. The values for
red and
blue are listed as the balance colors to produce black, grey and white.
This
balance is called “White Balance” and is critical to produce the range
of
colors for Video and Photography. If you see a picture in which the
colors seem
to have an off color cast, it is because the white balance is off. The color of light in a scene is dependent on the source which could be the sun, moon, flash, incandescent, fluorescent or gas vapor tube. The color is measured in degrees Kelvin mostly along a range from amber to blue with slight variations in the green to magenta axis. The Kelvin temperatures for the following light sources are: Incandescent........................... 3000 Indoor warm bulbs Sodium-vapor lamps.............. 2700 Big box stores and warehouses Fluorescent............................. 3000-6500 Varies widely, long and coiled, cool bulbs High temp Mercury vapor........ 7200 Sports, street lamps Direct Sunlight......................... 5200 Open sun and some window lights Flash........................................ 5400 Photographic electronic strobe, short duration, bright Cloudy..................................... 6000 Daytime, overcast skies Shade...................................... 8000 Color will vary with degree of exposure to sky It should be noted that Auto White Balance falls short of covering the incandescent range and that you can expect incandescent light to be a little amber without some post processing. Automatic White Balance is broke!It is a really obvious fact that Automatic White Balance is computed from all the light that comes through the lens and that light is a mixture of what light is available to illuminate the scene minus the light absorbed by the scene. The faulty assumption is that the scene is 18% grey. If the scene is a rose that fills the viewfinder the camera computes the exposure wrong and overexposes the image. The Auto White Balance will try to lower the sensitivity of the red channel and shift the color and because red was not weighted the same as the rest of the colors it is still not enough protection. I can’t tell you how many times I have seen complaints about improper green colors. When you look at the pictures they are talking about, they all have more than 50% of the image area as a green lawn. They knew when they took that picture that the grass was very green and impressed them. When it is the duty of a camera to adjust the color indexes of the red and blue channel for the correct K value and it sees all that green it will not do anything to the green channel because that is the base channel but it will raise the value of the red channel which makes the grass look lighter and more yellow. Now the grass looks like it has straw in it. The rules of additive color are complex and not many people can wrap their heads around it. When we try to do color correction we try to remember couplets such as Red is not Green and Yellow is not Blue which helps us to correct a faulty picture but the line of colors we see in the White Balance situation is a line from Amber to Blue and we call it warm to cool. We are asked to accept the fact that Automatic White Balance works, but in reality there are some pictures that break the computed result because they do not have a standard mix of colors. We are left with the problem of trying to separate the color of the actual light independently from the color of all the light reflected by the scene. The old light meters from the pre-digital eras used two methods to compute exposure. The two types are incident and reflective spot. When you use the reflective spot with a grey card you get a reading that is equivalent to incident. http://www.luminous-landscape.com/tutorials/meters.shtml Mystical ways to measure Light temperature.
That Pringles picture
in its entirety is a very good way to test a
screen
calibration or a printer and versions of it can be downloaded from
Smugmug.com. Click on the image or links below and transfer to your
image editor. http://www.smugmug.com/help/calibration-1400.mg http://www.smugmug.com/img/help/calibration-print-1400-2.jpg I use a copy of this image as my screen wallpaper and I am familiar with every detail in it. It is evidence to me that my screen calibration is acceptable for the day. For screen color calibration, I use a Spyder II from Colorvision. They are quite inexpensive and work well for me. For more on monitor calibration, see: http://www.smugmug.com/help/display-color Chapters: 1 How
white balance works, Eyes to See © Leon Goodman 2009 |
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