Friday, April 13, 2012

Photography 101 | Depth of Field


Just joining in? Here's what you've missed:
Photography 101 | Coming Soon!
Photography 101 | Aperture
Photography 101 | Shutter Speed
Photography 101 | ISO
Photography 101 | Exposure Meter

Welcome back to my Photography 101 series! I'm so glad that you have decided to join me. Today the fun begins, as we learn about depth of field (DOF).

If all you wanted was to have properly exposed photos, you could just set your camera on automatic and most of the time your camera would do just fine. If you are taking the time to learn how to use your camera in manual mode, I’m guessing you want to have more creative control over your photos, and a large part of this control comes from understanding depth of field and how to use it in your images.

Depth of field is essentially the portion of your photograph that is in focus. You can use depth of field to isolate your subject from the background by keeping it in focus and making the background out of focus, like I’ve done in this photo…


Or you can use depth of field to keep your entire scene in focus, like I’ve done in this photo…


By learning how to control depth of field, you can choose what to focus on and decide how much of each photo will be in focus. To demonstrate this idea, the next series of photographs are photos I took during my Intro to Photography class, when I was first learning about depth of field. The instructions were to take four photographs of the same scene. In the first photo we were supposed to focus on the foreground, and blur the rest of the image. In the second photo we were supposed to focus on the middle ground, and blur the rest of the image. In the third photo we were supposed to focus on the background, and blur the rest of the image. In the fourth photo we were supposed to keep the entire scene in focus. I wasn't using a tripod, so my scene isn't exactly the same from one image to the next, but it still demonstrates the point...





So, let's talk about how how you can control the depth of field in your photos. There are 3 things that affect depth of field: lens aperture size, lens to subject distance, and lens focal length.

Lens Aperture Size
  • larger aperture size = smaller depth of field
  • smaller aperture size = larger depth of field
As you increase the size of the lens aperture, the depth of field gets smaller resulting in a smaller portion of your photo being in focus.  The flower photo above was taken with an aperture of f/2.8, and the first three photos in the DOF series were taken with an aperture of f/4.

As you decrease the size of the lens aperture, the depth of field gets larger resulting in a larger portion of your photo being in focus.  The mountain photo above was taken with an aperture of f/18, and the final photo in the DOF series was taken with an aperture of f/22.

Changing the size of the lens aperture is the only way to change the depth of field without changing the composition of your photo.  For this reason, lens aperture size is the first thing that I use to control the depth of field when I shoot.

As you can plainly see, the composition of the following two photos is roughly the same, even though what is in focus varies greatly between them.  The first image was taken at an aperture size of f/2.8 and the second at f/11.  As I decreased the size of the aperture I caused the depth of field to increase resulting in an increase in the the amount of the scene that is in focus.




You will notice in the first photo above that the yellow flower is in focus as well as the edge of the leaf down near the left corner of the image.  The reason these two things are in focus as the same time is because they are both in the same plane (or in other words they are both the same distance from the camera lens).  This becomes important when you are shooting portraits and trying to get multiple people in focus at once.

For example, take at a look at the image below, where I was trying to take a photograph of Lexi and Zoe.  I couldn't quite convince them to lay right next to each other, so Zoe was slightly farther away from my camera lens than Lexi was.  My lens aperture was set at f/2.8 which resulted in a depth of field too small for both of them to be in focus at the same time.  I was focusing on Lexi, and Zoe ended up out of focus.


While I prefer to adjust my depth of field using my aperture size, there are times when lighting conditions will not allow me to use my first choice for lens aperture size.  In those instances, one or both of the next two things will help me to achieve the depth of field results that I want for my photo.

Lens to Subject Distance
  • shorter lens to subject distance = smaller depth of field
  • longer lens to subject distance = larger depth of field
(As a side note, this also changes perspective, which I will talk about in a future post.)

Lens to subject distance is easy to change unless you are shooting in a confined space.  If you want to decrease the depth of field in your photo, move closer to your subject.  If you want to increase the depth of field in your photo, move farther away from your subject.  But keep in mind that as you move closer to or farther away from your subject you are going to be changing the composition of your photo.

I took this first photo, and then backed up about 5 ft and took the second one. You can plainly see the difference in how out of focus the back branch is between the two photos. By lengthening the distance between my camera lens and my subject, I made the depth of field get larger. I also changed the composition of my photo rather drastically.



Lens Focal Length
  • longer focal length = smaller depth of field
  • shorter focal length = larger depth of field
Lens focal length is usually the last thing I use to control the depth of field in my photos.  I don't always carry all of my lenses with me when I am out shooting, plus changing lenses can be difficult and time consuming depending on the circumstances.  So, I usually try to get the shot with the lens that is already attached to my camera body.  However, this can come in handy when I am using a zoom lens and have a range of focal lengths readily available. Again, changing the depth of field by changing the lens focal length will also change the composition of your photo.

The following three photos were taken using my 70-200 zoom lens.  The first photo was taken at a lens focal length of 170 mm, the second photo was taken using a lens focal length of 115 mm, and the third photo was taken using a lens focal length of 70 mm.  As I shortened the focal length, the depth of field increased for each photo resulting in more of the scene being in focus for each subsequent image.  Of course, the composition for each photo was changed, as well.




(Depth of field is also affected by the size of your camera's sensor.  However, you do not have control over this, with the exception of purchasing a new camera with a different sensor size, so I am opting not to discuss it in detail.)

One other thing to keep in mind is that the farther away something is from the subject you are focusing on, the more out of focus it will be. So, if you want to decrease the focus of the background behind your subject, you can simply move your subject farther away from whatever is behind it.

I would recommend spending some time with each of your lenses and practice changing the lens aperture size, the lens focal length, and the distance between your lens and your subject to get a feel for how all of these work to create depth of field in your photos.  Then join me back next week to learn about photographing motion.

Michele Whitacre is a portrait photographer serving Phoenix, Arizona and the surrounding area. Visit Michele's website at michelewhitacrephotography.com. Become a fan of Michele's work on Facebook. Follow Michele's updates on Twitter.

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