How to get a photo correctly exposed by adjusting aperture and shutter speed
Many people let their cameras determine exposure for them. They leave the camera on its manual setting, and hope that the camera gets everything right. There is only one problem.
If you let your camera determine exposure for you, it won't always get it right. The camera thinks that everything in a photograph should show up as a medium tone. Take a photo of a white wall and it will wind up looking grey — the camera has intentionally underexposed the image. Take a photo of a black wall and it will also look grey — the camera has overexposed the image.
How Aperture and Shutter Speed Determine Exposure
Aperture is also a measure of how wide the lens opens when you take a photo. Shutter speed is how long the shutter stays open. Both of these settings affect the amount of light hitting the camera's image sensor, and this is what determines exposure.
When the correct amount of light hits the image sensor of your camera, you get a photograph that is correctly exposed. When there is not enough light, the image is underexposed and when there is too much light, the image is overexposed.
The relationship between aperture and the amount of light that gets into the camera is a little backwards:
SMALL F-STOP NUMBER = LOTS OF LIGHT
LARGE F-STOP NUMBER = MINIMAL LIGHT
Here is an example of how this works with actual f-stop numbers:
f4.5 (wide open) = LARGE AMOUNT OF LIGHT
f8.0 (somewhat open) = MEDIUM AMOUNT OF LIGHT
f22 (almost closed) = SMALL AMOUNT OF LIGHT
The important point here is that if you have your aperture set to a small number (ignoring shutter speed for now) a lot of light is going to get into the camera. When you are shooting in low-light conditions (at dusk, for example) you will have no choice but to set the aperture wide open to capture as much light as possible.
When you are shooting in bright conditions, you have more flexiblity to change the aperture (since you can let in less light and still get a good photograph). You can set the aperture to a high number and let in less light as a result.
We've covered aperture and the amount of light let into the camera. Here is the relationship between shutter speed and amount of light:
SLOW SHUTTER SPEED = LARGE AMOUNT OF LIGHT
FAST SHUTTER SPEED = SMALL AMOUNT OF LIGHT
Again, let's say that you are only adjusting your shutter speed and ignoring aperture. In low-light conditions, you will have to use a slower shutter speed to capture enough light for a good exposure. In bright conditions, you can use a range of shutter speeds, but may not be able to use slow shutter speeds since this will let in too much light.
So far, we have just gone over changing either the aperture or the shutter speed and not worrying about the other. Well, the truth is that you can manipulate both to achieve a correct exposure.
The following table shows this relationship (assuming that you are taking a standard photo during the day).
|Slow (1/30 sec)||Fast (1/1000 sec)|
|Small f-stop (f4.5)||Overexposed — a slow shutter speed and small f-stop will both let in more light||Correct — a fast shutter speed lets in less light, but a small f-stop lets in more|
|Large f-stop (f22)||Correct — a slow shutter speed lets in more light, but a large f-stop lets in less||Underexposed — a fast shutter speed and large f-stop both let in less light|
Light Sensors and The Middle of the Road
Film and digital camera light sensors are always trying to get a correct exposure. If you don't change anything, point the camera at a scene and take the shot, chances are the camera will get the exposure correct automatically. So why ever fuss with it?
Your camera's light sensor can be fooled.
Say that you are taking a photo of some mountains with a bright blue sky in the background. You take the photograph on automatic. When you look at the photo later, the mountain looks just fine, but the sky is pure white, not blue. Why?
Your camera's light sensor wanted to make sure that everything in the photograph was correctly exposed. In order to correctly expose the mountain, it overexposed the sky.
Here is another example: you take a photo of your friend standing in front of a bright background. When the photo turns out, the background looks just fine, but your friend's face is completely dark. In order to correctly expose the background, the camera underexposed your friend.
If you want to prevent this from happening, you have to override your camera's automatic features and adjust the aperture and shutter speed yourself.
Don't Trust Your Eyes
Another problem with exposure is that cameras cannot capture the same range of light and shadow that our eyes can. You may be standing in the middle of the forest on a path with beams of light breaking through the trees and think that you can get a beautiful photograph.
The trick here is that your eyes see all of the detail in the shadows, despite the rays of light beaming through. If you try to take a photograph of this scene, one of two things is going to happen: you will either expose correctly for the shadows (which makes the beams of light pure white with no detail) or you will expose for the light (making the trees completely black with no detail).
Sometimes you have to realize that something that looks remarkable to your eyes cannot be captured with your camera no matter how much you fuss with the exposure.