camera histogram is misleading

Started Apr 26, 2007 | Discussions thread
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Bill Richardson
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camera histogram is misleading
Apr 26, 2007
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A few days ago, I posted a question about how misleading the histogram is on our cameras if shooting RAW. Kent C suggested I contact Seth Resnick for an expert opinion. Mr Resnick was so gracious in replying that I thought I should share his very educational response as to why we have to judge the camera histogram very cautiously (unfortunately dpr restricts the lengths of messages so I could only post part):

Bill,

.... From a purely technical standpoint you should expose a digital
capture so that the most data falls to the right of center of the
histogram; as far to the right as possible without blowing out
highlights. Remember, digital camera sensors allot one half of their
dynamic range to the brightest stops.

In digital, since the greatest amount of detail is in the first
brightest stop, you may find that for “normal” images you are better
off slightly overexposing the image and then bringing down the
exposure during RAW processing in order to capture the best digital
file. Let’s think about film for a moment. When shooting
transparencies, we typically underexposed to increase saturation and
color. The problem here is that if we simply underexpose in camera,
we will lose valuable data in our RAW files. Essentially, digital
exposure is exactly the opposite of film. In digital we want to make
a judgment call on the overexposure side rather than the
underexposure side. It is during processing that we can achieve our
rich, saturated color, rather than during exposure where we would
sacrifice the quality of the file.

EXPOSURE TIPS
Determining exposure for digital capture is quite different than
exposing for film. Digital camera sensors have a wider dynamic range
than many films and are much more sensitive to light than many films.
You can use less created light with digital than you would with film.
Lighting digital capture the same way you would light film will
generally result in an image that looks overly lit.

Another common exposure problem is that people will base their
exposure on a gray card and as a result shoot at that ambient light
value. A light meter reading taken off of a subject of known
reflectance like an 18 % gray surface is only giving you a base
exposure. With digital it is vital to actually expose for the color
as well as the ambient light. Colors change and vibrate depending on
the exposure. Our best suggestion is to shoot at the same ISO value
so that your brain gets used to the colors produced under similar
light conditions.

WARNING: USING THE HISTOGRAM DISPLAYED ON THE BACK OF YOUR CAMERA
The problem with both the LCD preview and the histogram produced by
the digital camera is that the information and the way it is
displayed does not represent the reality of how your RAW capture will
look when processed. Most cameras do an on-the-fly conversion from
the RAW capture to the equivalent JPEG file in an sRGB color space.
The resulting histogram only displays the luminance or brightness of
the image and only of a JPEG preview file in a very small color
space. So the display has little relation to the RAW file captured,
and the information is not particularly useful except as a rough
guide to exposure.

Finally, the “blinking whites” or over-exposure warning areas that
flash on the LCD display on the back of your camera are usually at
least one stop conservative. The camera manufacturers set the
threshold for this display too low, which will lead you to
underexposure your image.

Best,

Seth Resnick
Co-Founder D-65 llc

Bill Richardson
Barrington, IL (USA)

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