What is Depth of Field (DoF) and how is it controlled?
First, Aperture or f Stop – is the size of the opening in the lens that allows light to hit the camera’s sensor. f stop is expressed as a fraction as in f/3.5 which lets in a larger amount of light than f/16. Aperture also effects the Depth of Field (DoF).
Depth of Field is area in front of the subject and behind the subject that is in focus. A large aperture like f/3.5 gives a smaller/narrower DoF than a smaller aperture such as f/16. A shallow DoF creates a less focused background while a wider DoF keeps more of your photo in focus.
Look at the two photos below. Both were taken at a distance of 4 feet from the red bottle, the subject. As can be seen in Photo 1 using a f/5.6 the green bottle is out of focus when compared to Photo 2 using f/22 where the green bottle is sharper. Why? The smaller aperture of f/22 gives a wider DoF.
The focal length of the lens also effects the DoF. Photo 3 below was taken with a 50 mm focal length aperture of f/22 as in Photo 2 above. In Photo 3, we see two changes occurring. First, the longer focal length makes the bottles appear closer and DoF is smaller/narrower , i.e. the green bottle is less sharp.
With an even longer focal length like 100 mm, we see the result in Photo 4. The bottles now appear even closer and the green bottle is more blurred than in Photo 3.
By changing the aperture to f/5.6, in Photo 5 below, we see the blue, yellow and green bottles are more blurred than in Photo 4 .
Why? The larger f stop gives a smaller/narrower DoF. So far, we see that aperture and focal length of the lens has an effect of the DoF of the photo. There is one more factor effecting the DoF: the distance to the subject, in our case the red bottle. Here is Photo 6 with an aperture of f/5.6 and focal length of 25 mm at a distance of 4 feet from the red bottle. What happens if I move closer? Will the DoF get smaller or larger?
In Photo 7 the camera was moved to about 2 ft from the red bottle.
In Photo 7, we see the blue, yellow and green bottles are blurred compared to the red bottle. When you move closer to the subject the DoF gets smaller/narrower.
Three factors controll DoF: aperture, focal length of the lens and distance to the subject.
A Quick Review:
– The larger aperture, the smaller the DOF for any given focal length lens and subject distance.
– The longer focal length the smaller the DOF for any given aperture and subject distance.
– The closer you are the the subject the smaller the DoF for any given aperture and focal length lens.
Quick One Question Quiz:
How would the DoF change in Photo 7 if the focal length is changed to 50 mm and the distance is 6 feet?
Note the yellow and green bottles are sharper in Photo 8 than in Photo 7.
The DoF is wider. Here we see changing the lens to a longer focal length and further distance to the subject resulted in a wider DoF. This brings me to old adage I learned years ago when I first started with a film camera and a fix focal length lens. Compose with your feet. Take the time to more closer or further away for your subject, change the aperture not the focal length and see how that effects the DoF of your photo. It takes time to learn to use DoF in your photography.
Note: Most cameras have DoF Preview Function located on the front of the camera body. This can provide assistance in getting the DoF you want to achieve. See your camera’s manual for details for your model of camera.
Below I provide a couple of examples where I controlled DoF from my own photo collection.
The settings and distance in Photo 9 gave a narrow DoF so the background was blurred making the subject stand-out. In Photo 10, using a shorter focal length lens, small aperture and long distance to the subject allowed for both foreground and background to be in focus.
This link, DoF Calculator provides a handy way of calculating the depth of field for your camera. An example of the information supplied is provided below. One further piece of information I should mention is the size of the chip in your camera also effects the depth of field. If your camera has an APC or smaller chip the DoF will be slightly larger than a full-frame DSLR such as Nikon D800. See example below. In addition, you should note most of the DoF is behind the subject. If you want to blur the foreground ,keep this in mind.
For those who are more detailed and are wondering about the term hyperfocal distance: Hyperfocal distance is the distance between a camera lens and the closest object that is in focus when the lens is focused at infinity. More about hyperfocal distance can be found at this link.
Here are a few links to articles about Depth of Field and the use of Aperture:
Please contact me if you have any questions.
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