How to Manually Set Exposure on Your Digital SLR
One of the most powerful exposure tools in your dSLR arsenal is you. Given an understanding of what you want to accomplish creatively, combined with the limitations of your equipment and the lighting on hand, you can effectively troubleshoot exposure by bypassing the camera's autoexposure modes and handling it yourself.
Each photographic stop has the same effect on exposure, whether it comes from shutter speed, aperture, or ISO. You can swap one for another to get to the exposure you need.
Here are some suggested steps to setting exposure manually:
Choose Manual mode.
Decide what exposure control you want to set first.
Let your creative goals guide you to limit one of the three exposure controls:
Aperture: Smaller is better for landscapes. Larger is better for portraits.
Shutter speed: Set a fast minimum (this is the point you are not willing to shoot slower than) for action and dim light when going hand-held. If you try another mode first and the shutter speed is too low, limit shutter speed. Set the shutter speed high enough to avoid blurring.
ISO: For shooting still subjects from a tripod, set to lowest ISO and slow shutter speed. If your camera shoots relatively noise-free photos up to ISO 800, use anything from ISO 100 to ISO 800.
Set the first value.
After you’ve decided which side effect is most important (limiting depth of field, limiting blur, or limiting noise), lock down that control by choosing a value.
Set the second exposure control.
Adjust the other exposure controls to get the right exposure.
The exposure scale tells you whether the camera thinks you're under- or overexposing the photo. When you’re troubleshooting, you may need to ignore the camera. If you've already taken a shot and it was too bright, tone down the exposure by a third, a half, or a whole stop. Raise the exposure if the photo was too dark.
Don't make wild changes unless the other photos were significantly off. Keep changes small and try to be methodical about it.
Take a photo.
This is the most important step. Don’t rush this step.
Look at the photo on your monitor and decide if it’s too dark or too light.
Check the color histograms to see if any colors are clipping.
Zoom in, if necessary, to check details.
If the exposure looks good, you're basically done. You may be able to use the settings for more photos, provided the scene and lighting don’t change much.
Continue adjustments, if necessary.
If the exposure is off, return to Step 5 and work with your floating exposure control. You may also return to Step 4 and revise the second control. If your exposure solution won't work for more drastic reasons ― you can’t set the right shutter speed to avoid blurring or you’re concerned about too high an ISO, go back to Step 2 and reset your priorities.
Shutter speed is generally the least forgiving exposure element if you set it too slow. It's easier to accept different depths of field or noise levels, but camera shake and motion blur provide little artistic leeway.