As the sky was very bright, the only way to get a satisfactory exposure of the bird was to switch to manual.Getting a 'correct' exposure means recording as much of the relevant information in the scene as possible. In the shot on the left, the important information is the bird. The sky has gone completely white as it was much, much brighter than the bird. If I had let the meter expose for the sky, the bird would have been rendered as a black silhouette.
Even when the exposure is 'correct' the problem with all cameras is that they cannot record the entire range of contrast (black to white) that the eye can see. Especially when you take into account that the eye is constantly adjusting to cope with high contrast. On a sunny day if you look into the shadows of a scene then into the bright areas, the iris in your eye will quickly adjust so you can see detail in both.
Faced with the task of recording as much information as possible, the camera will try to average out all the light levels and expose accordingly. As burnt out highlights are normally considered uglier than black shadows, the camera, left to it's own devices, will normally err on the dark side. Which is no good if you are shooting against a bright background. It's the subject you want to see, and you don't really care if the background is white.
Auto Exposure Modes
The camera manufacturers have come up with all sorts of
ingenious metering systems to try to help, there are now
multi mode metering systems, which give you a choice of
'center weighting', 'spot metering' or 'multi spot metering'
on many of the better cameras, but none can guarantee to
give you what you want every time.
Tip - using auto exposure to your advantage.
If you have a modern camera, the chances are
that the default metering system is 'center weighted average',
which means that, although it takes an average reading of
the whole scene, it takes more notice of what is in the
middle of the frame. Which is good news for us. The other
good news is that it takes this reading at the time when
you take 'first pressure' on the button to take your picture.
When you push it halfway down and it beeps at you, not only
is the focus now set (on an auto-focus camera) but the exposure
reading is taken and the aperture and shutter speed are
set. So, if your main point of interest is not in the center
of the frame, it's a good idea to put it there temporarily
while you focus and take your light reading, then move the
camera whilst still holding the button halfway down and
compose the picture the way you want it to be.
A common
use for this technique is when you are taking a close up
shot of two people and there is space between their heads,
if you're not careful the camera will focus on the wall
or trees behind them. If the background is very dark or
very light this can alter the exposure significantly and
result in faces that are too dark or too light.Skin tones are what most meters are set up to consider an 'average tone', they are also usually the part of the picture that we most want to get right. If I am photographing a group of people in difficult circumstances, like bright sunlight for instance, I will often move close in to the group and take a light reading from someone's face or, if we are all standing in the same type of light, I will take a reading from the back of my hand. This is no good, of course, if the subject is in bright sunlight and I am in the shade.
Manual Exposure
Now it's time to turn that dial away from 'program' mode
and have a look at the dreaded 'manual' mode. There are
also a bewildering array of other choices such as 'aperture
priority', 'shutter priority', 'exposure compensation' etc.,
but once you understand the basics you will be able to select
the most suitable mode.The amount of light falling on the film is governed by four things.
- The amount of light reflected from the scene which, if you are outdoors, you can do very little about.
- The 'shutter speed' which is the amount of time the shutter is open, measured in fractions of a second.
- The 'aperture setting' which is the size of the hole through which the light enters. If you look at the lens of your camera you will see a diaphragm in the middle of the glass which the camera adjusts according to the light. This does exactly the same job as the iris in your eye. Aperture settings are measured in 'f stops'. For an explanation of 'f stops' click here.
- The ISO setting. This is the 'sensitivity' of the sensor. (see ISO Speed)
Why use manual exposure?
The advantage of manual exposure is that the settings
do not keep changing as your scene changes. Let's suppose
that you are taking close up photos of cars passing by.
Some of the cars will be black or dark colors and some
will be light colors or white. If you are filling the frame
with almost nothing but car, the meter will be trying to
render each car as mid gray. Although it will probably not
succeed, what you will notice is that the background is
a different shade in each photo.I often have to take portraits of people, some are wearing very dark clothes and some are wearing white. If I am not careful with my exposure settings will be affected by the clothes.
Although it is by no means always necessary to use manual exposure, an understanding of how it all works will save a lot of disappointment. Below are some links to other articles covering various aspects of exposure.
| Tutorials on Exposure | ||
| Exposure | Getting away from the auto settings. |  |
| Shutter Speed & Apertures | An explanation of the mechanics of exposure and the side effects of choosing different aperture/shutter speed combinations. | |
| Exposure Compensation | Overriding the automatic metering system. | |
| Histograms | What they are trying to tell you. | |
| Bracketing Exposures | Setting up your camera to take a series of pictures at different exposures. | |
| ISO Speed | Another piece of the exposure puzzle. | |
| Reciprocity Failure | An explanation, strictly for the jargon heads. | |
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