Depth of Field (DOF)

Depth of field, henceforth referred to as "dof" can be a hard concept to wrap your brain around. Before we start talking about it let's define what it is...

Wikipedia states:

In optics, particularly as it relates to film and photography, depth of field (DOF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image. Although a lens can precisely focus at only one distance at a time, the decrease in sharpness is gradual on each side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions. (http://en.wikipedia.org/wiki/Depth_of_field). 

Basically what this is saying is:

1. Your lens can only focus to one distance at a time.
2. The distance between the nearest object in focus and the farthest object in focus is called depth of field.
3. everything inside these two distances will be in focus and everything outside will be out of focus
4. the amount of out of focus depends on the distance from the focus distance of your lens.

  Lets expand on each of these points.

Focus distance of the lens

Do not confuse the focus distance with focal length of the lens. Focal length is another subject all together. No matter what your focal length a lens can be focused to different distances using the focus ring. Usually lenses have a distance scale either on or near the focus ring.

Nikon 50mm 1.8D showing the focus ring and focus scale

 

The lens shown above has the focus distance in feet in yellow numbers and meters are in white. This particular lens is focused at 20ft. The small white numbers next to the line is a depth of field scale on this lens. Most newer lenses don't have this on them any more but if you're using an older lens, what this scale shows is what your depth of field is at different apertures. I don't use it.

Distance Between nearest object and farthest object

As an example of this lets say that we have something in our image that is 4 feet away from the camera and it is in focus and we have something 10 feet away from the camera and it is in focus at the same time... Our dof will be 6 feet.  Every thing in between these distances will be in focus also.

Focus Distance and Depth of Field

The next thing that I'd like to cover with dof is exactly what that field will be given our focus distance. Using our example above of 4ft and 10ft what exactly is our focus distance? There is a simple rule that we can use to determine this...
1/3rd in front of our focus distance and 2/3rds behind is what is in focus. So if our depth of field is 6 feet and our nearest point is 4 feet with our farthest point being 10 feet our focus distance is 6 feet. that gives us 2 feet in front and 4 feet behind.

How to control our Depth of Field

Ok we're done with the textbook explanation lets get into how we can control our depth of field.

There are three things that are going to affect your depth of field and they all work together in controlling depth of field.

1. Aperture
2. Focal length of your lens
3. Focus distance

Aperture

The easiest to explain and show is aperture. The aperture controls how much light is let into the camera but it also controls depth of field. Keeping it simple: the smaller the aperture the more depth of field you're going to have. The three pictures below were taken at f/1.8, f/8 and f/22 with a 50mm lens. Notice that as my aperture decreases from wide open at f/1.8 to completely stopped down at f/22 our depth of field between our objects is increased. Also notice that as we close down our aperture we must increase our shutter speed and/or ISO to balance our exposure. All of these were taken from 10ft.

f/1.8 @ 1/400th ISO 200

f/8 @ 1/30th ISO 200

f/22 @ 1/125 ISO 6400

Focal Length


With focal length the longer the focal length the less dof you'll have at the same distance... for example if you're using a 50mm lens and you take a photo of a subject at 6 feet away you will have more depth of field than if you took a photo of the same subject at 6 feet with a 300mm lens. The three images below were taken with a 14mm, a 50mm and a 70-300mm set to 300mm at 10 feet away with an aperture of f/5.6. As you can see our depth of field decreases as the focal length increases. Notice however the difference between the field of view (wide angle to telephoto) of the  three lenses.

14mm

 

50mm

 

300mm

 

 Focus Distance

The final thing that affects depth of field is focus distance.  The closer we are to our subject the less depth of field we have, the further away the more depth of field we have. This is independent of focal length and to prove it we have below three pictures using each lens that we used above. The 14mm, the 50mm, and the 70-300mm. We will use the same aperture as above as well, f/5.6. The images were taken at 10ft, 3ft, and 1ft with the 14mm, 10ft, 5ft, and 2ft with the 50mm, and 40ft, 20ft, and 10ft with the 300mm. Notice the closer we get the less depth of field we have.

14mm @ 10ft

14mm @3ft

14mm @ 1ft

50mm @ 10ft

50mm @ 5ft

50mm @ 2ft

300mm @ 40ft

300mm @ 20ft

300mm @ 10ft

These images weren't color corrected or edited in any way and were done rather quickly to demonstrate depth of field so don't knock me on focus and color please. I'd also like to thank my daughter and wife for putting up with me while I took these pictures.

I urge you to take this information and try it out yourself so you know exactly what you have to do to get the depth of field that you want for your images. Weather you like very shallow depth of field or you like everything in focus remembering these concepts will help you obtain the results that you're looking for faster.