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by Joseph Jones Photography
Welcome back to Photo Guru!
Last installment we looked at some of
the ‘mechanics’ of exposure. The things you
do to the camera, or that the camera does
automatically, that helps set a “correct”
exposure. This time we are going to look at
what is meant by correct exposure, how it is
achieved and how to avoid the two common
pitfalls of over exposure and
under exposure.
Let’s jump right in. What exactly is a
correct exposure? Well there are actually two betting you it’s grey and not really black!
very good answers to this question. Keep the camera setting the same (automatic and no flash) and now make a picture of
One answer is objective: correct exposure a completely white object like a piece of copy paper. Go through the same observations as
is accomplished when the amount of light before. The white paper should now be grey.
striking the CCD is sufficient to properly A completely black or white object is not average and so does not have average reflectance.
render a scene witch has average reflectance. The camera’s meter does not know this! It assumes that every thing you point the camera at
We will explore this answer a bit more. has average reflectance so it gives an exposure based on this assumption. This assumption
In digital cameras, built in reflective light will work most but not all of the time. Whenever we photograph a scene that does not
meters are used to measure light and gauge have average reflectance our exposure will be off. Unless we do something to address the
exposures. All reflective light meters are situation.
calibrated to measure all light striking them as The trick is to be on the lookout for the situations when the subject you are photographing
average reflectance and suggest an exposure is not average, ie when the subject contains large areas that are either too bright or too dark.
based on this assumption. This is how correct Large dark areas will tend to give overexposed images because the camera’s light meter thinks
exposure is accomplished. Most of the time. it’s average and allows in too much light for the exposure. Large bright areas will tend to give
Now I don’t want to get too technical here underexposed images because the camera’s light meter thinks it’s average and lets in too little
so I’ll approach this on a practical level as light. The meter does the same thing to both light and dark subjects; it makes them grey. As
much as possible. The key is to have some a mater of fact ‘average reflectance’ is also known as middle grey. Download the two images
working idea of what exactly is a scene with from the experiment and open them in image software so that you have them both side by
average reflectance. Well the simple answer side. If the experiment worked flawlessly them both images should be about the same grey
is ‘most of the average scenes that we would color. More than likely they will not match as there are exposure algorhythms working with
want to photograph every day’. A more useful the light meter to help set exposure as correctly as possible (but not yet perfectly). Understand
answer is that an average scene does not have that the same thing happens with a full color image. Just as the black has been bleached out
large areas that are either too bright or too to grey the color image that is overexposed will be bleached out as well. Similarly the white
dark. was rendered dull and murky so colors will be dull and dark with under exposure.
It’s a lot easier to understand how metering Here are a few scenarios. You are photographing a lush green landscape with nice blue
and exposure works by considering the second skies. This is actually a scene with average reflectance (assuming the sun is not in the picture).
answer. However if you try to shoot the same scene with a cloudy or a overcast, hazy sky then the
Grab your camera and do this experiment: scene is no longer average. The bright sky will influence the meter. It assumes that an average
set the exposure mode to automatic if it isn’t guy is pointing his camera at an average scene with a lot of light reflecting off the scene. It
set so already. Go out in a bright daylight spot adjusts the exposure for the light it measures and underexposes the scene. The lush green will
and make a snap of a completely black object now be too dark.
like a piece of black fabric. Make sure that the Similarly photographing on the beach can present problems with the sun glinting off
black object completely takes up the whole white sand and/or the water.
picture. Do not use any flash for this. If you point your camera at tilled soil, the dark soil will cause the meter to give an
Pull up the image on the LCD display and overexposed imaged because it was fooled into thinking the exposure needed more light
compare it to the real black object. How does than necessary.
it look? Are there any obvious differences? If you tried to photograph someone standing against a bright background you may have
What is the color of the object in the LCD? gotten them underexposed because the brighter light from the background had influenced
If you said black, look again carefully. I’m the meter.
62 | BusinessFocus • August/September 2008
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