Exposure
First,keep in mind, this and focus
are the two main things that photographers constantly try to overcome,
regardless of their skill level.
Every digital camera made today has the ability to measure the amount of light
available and change the settings to expose the sensor properly. That said, the
camera is a plastic box full of limitations which usually result in unexpected
image results.
Some cameras give you the ability to manually adjust the camera settings, and
some do not, but all digital cameras contain a light meter somewhere in them;
yes, even your phone. If they didn't have a way to determine how much light is
available, the camera wouldn't know which settings to change.
If you have access to your light meter, it will look very much (sometimes identical)
to your exposure compensation menu, which makes sense since you are most likely
about to manually compensate the exposure by changing the settings yourself.
In SLR cameras, it appears in your LCD (top, and/or back panel) as a +2.+1.0.-1.-2 and
an indicator telling you where your current exposure will be.
You would think that with this ability
the camera would be able to get perfect exposure every time, but
unfortunately this isn't the case.
Exposure Problems
There are two main things that cause bad exposure:
1.) The camera sensor can only capture a limited range of light from the darkest
part to the lightest part; this is called latitude. So far digital technology
is still slightly behind film in this area.
Traditional negative film had the ability to capture about 5 stops
of light (2 under exposed and 3 over exposed)
which is how disposable cameras are able to work in such a wide range of lighting
situations.
A sensor can capture around 3 to 3.5 stops
(about 2-3 stops; 1-2.5 under-exposed, and
one over-exposed). This means that it is more difficult for the sensor to accurately
represent a scene with a large variation of light sources than film.
(A stop of light is 2X the amount of light...so 3 stops
= 2x2x2 = 8x more light and 5 stops = 2x2x2x2x2
= 32X more light.)
This link has a good run-down on the differences between film and sensors:
http://www.shutterbug.com/pointofview/0303sb_point/
2.) The camera's light meter always try's to find the average amount of light
reflecting off the subject and back to the camera. Because the camera doesn't
really know what you are pointing at, what your artistic goal is, or even what
color is, it assumes that you will always have a perfectly even amount of light,
so that when it measures it and changes the settings, everything is perfect.
But we all know that is bunk!
The problem is that not all things reflect light the same way or in the same
quantities, and the camera doesn't understand that. If a subject is highly reflective
(snow, sky, sand...) then the camera meter is being overly influenced by that
highly reflective surface. In other words, it is using the brightest area to
determine what the average or middle tone of the image is; this results in darker,
grayer, flatter images.
The opposite occurs if you use the darkest area to determine what the middle
tone is going to be; everything is blown out.
It's a difficult concept, but try thinking of it this way: Your camera is always
trying to make a middle tone (called middle gray which is
18% of the reflected light). If it uses the available light to average and determine what is the middle
tone, it can then decide what is darker and what is lighter. But if you point
it at the lightest point, there will be nothing lighter (that will be your middle
tone), and vice-a-versa with pointing at the darkest spot.
Meter Patterns
This is getting a bit advanced, but many cameras, even
point and shoot cameras, have the ability to change the area
that is used to determine exposure. There are many different
names and icons for these, but here are the most common ones:
| Evaluative (a.k.a.
multi segment, matrix, etc. |
This
uses the whole area and divides it into segments or areas
and then averages the readings together. |
| Center
Weight Average |
This does
the same, only applies more emphasis (70%) to the center
area. |
| Partial |
usually
a 9% area in a type of pattern |
| Spot |
1%-3% area |
By changing how the meter gets it's information,
you can get better meter readings by reducing inaccurate readings
from very bright areas.
Tips for Exposing Better
1.) Think about the relative amounts
of light from the brightest area to the darkest area in your scene.
Ask yourself, "What is the camera using to determine the middle
tone?". If there is a huge sky, big white wall, or large expanse
of light area, then your meter is probably being tricked. Just
knowing this and using your exposure compensation will likely fix
the problem.
2.) Change your exposure area (pattern) to give you the best reading possible.
3.) Try to bring the darkest and lightest portions closer together in range by
blocking some light or adding some (flash, bounce, reflector).
4.) Shoot in the mornings, late afternoons and on overcast days. Mornings are
good clean light and are known as the magic hours. Late afternoons gives very
dark heavy shadows, but they make sense, so loss of detail appears to be intentional.
Overcast days act like a giant diffuser and even out the amount of light without
too many heavy mid-day shadows.
5.) Try to avoid backlighting. The hardest exposures to deal with are ones that
have bright light directly behind your subject. This usually results in silhouettes
as the light behind the subject is where your camera gets its exposure information.
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