Simply put depth of field (DOF) is how much of the photograph in front of and behind the main subject that is in focus. This however is not an even distribution. The range is approximately 1/3 in front of the main subject and 2/3 behind it.
To maximize your DOF do not focus right on your main subject. Focus 1/3 of the distance into the scene with the lens aperture set to the smallest possible. The part most people have difficulty with is how do you change the depth of field and why would you want to?
So let us find out.
The DOF can be shallow, with the main subject in sharp focus and the foreground and background out of focus. It can also be deep, with the foreground, the main subject and the background all in focus.
The three main factors to determine DOF are:
The focal length of the lens
The distance from the camera to the subject
The aperture or f-stop of the lens
How to change the DOF.
1. By changing the focal length.
A lens with a long focal length, such as a telephoto lens has an angle of view that is narrower than a lens with a shorter focal length. This results in a shallower DOF.
A lens with a short focal length, such as a wide-angle lens means a greater DOF. These shorter lenses have a greater angle of view, which results in the increase to the DOF.
2. Change the distance from the camera to the subject.
You can achieve the same result by moving the camera. The closer to the subject it is the shallower the DOF and the farther away the deeper the DOF.
3. By changing the aperture.
Changing the aperture (also referred to as the f-stop) will also change the DOF. A larger f-stop means a smaller aperture, which results in a larger DOF and a smaller f-stop will result in a larger aperture and shallower DOF.
Keep in mind that when you change the aperture you will also need to change the shutter speed.
When the f-stop is increased, decreasing the amount of light reaching the sensor, you will need to increase the time the sensor stays energized to compensate.
If you do not slow the shutter speed down the image may be too dark.
This may seem a little involved if you are just getting started but do not worry. Check your cameras manual for the semi-automatic settings.
If you chose the aperture priority (AP) setting, sometimes referred to as aperture value (AV) and select the f-stop you wish to use, the camera will automatically set the shutter speed for you.
When a slower shutter speed is selected be sure to use a tripod in order to prevent camera shake.
Why change the DOF.
There are times when you want the whole frame to be in focus such as a landscape shot. In this case set a large f-stop, for example f-22. This will close down the aperture and result in a greater DOF.
At other times you will want to keep only the main subject in focus. For example when photographing a small bird you want to keep all the attention on the main subject and not the surroundings. For this example set a small f-stop, f-2.8 which will open up the aperture and result in a shallow DOF.
To summarize. You should now have a pretty good understanding of depth of field.
For a greater DOF use a wider-angle lens and larger f-stop, or you could position the camera farther away from the subject. You can also use a combination of these two methods.
If you want a shallower DOF, then use a longer focal length lens and smaller f-stop, or position the camera closer to the subject.
Once again you could also use both of these methods in combination.
Play with the settings on your camera to see these results with your own photography. Just keep in mind, no matter what type of lens you are using, the larger the f-stop, the smaller the aperture, which results in a greater depth of field.
The inverse is also true. The smaller the f-stop, the larger the aperture is, which results in a shallower DOF.
There is no better way to understand what to do than by actually doing it.
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