Controlling Depth of Field

 

The nearest distance that something is in focus to the farthest distance that it is in focus is called the depth of field.   Sometimes you want everything in your photo in focus and sometimes you don't.  If you are taking a landscape photo and you want to draw people into the depths of the image, you may want everything in focus for a lot of depth of field.  If your background is busy, however, your photo may look better if your background was blurred or out of focus creating a short depth of field.  So how you can control depth of field to get the look you want?

 

Since depth of field is affected primarily by the focal distance and your aperture setting it’s fairly easy to control.  But getting it right requires a bit of experimenting because focal distance and the distance of your background are not that easy to calculate.

 

The closer an object is to your camera, or appears to be based on magnifying the object, the shallower your depth of field will be at all apertures.  Similarly, the further an object is away from your camera, or appears to be away, the greater the depth of field will be at all apertures.  This is due to the affect of the focal distance.  For example, using a 50mm lens, an item 10 feet away will have a depth of field ranging from 8 inches to 36 feet but an item 20 feet away will have a depth of field ranging from 2 ½ feet to infinity.

 

Like focal distance, apertures have a certain effect on depth of field.  Higher apertures (lower F stops) always produce lower depth of field and lower apertures (higher F stops) always produce greater depths of field but the changes are not always noticeable.  For example, using that same 50mm lens, a high aperture of f2.8 produces a depth of field ranging from 4 inches to infinity while a low aperture of F22 produces a depth of field ranging from 1 foot to infinity.

 

Putting these two factors together gives us a few useful rules of thumb:

  1. It is easier to blur the background (due to narrower depth of field) when we are focused on an object that is close or we are zoomed in to make it appear close.
  2. It is not possible to blur the background (due to a very wide depth of field) when we are focused on an object that appears to be hundreds of feet away.
  3. Your aperture has a much larger effect on what is in focus at close distances than it does at far distances.
  4. Selecting the highest aperture that will keep your entire subject in focus is more challenging the closer the subject appears to be.
  5. High depths of field require higher F stops which, in turn, require slower shutter speeds and longer exposures.

So, if you want to blur the background in your picture, you must get close to your subject or use a zoom lens to appear getting close to your subject.  But, as you get closer to your subject you also run the risk of putting part of your subject out of focus.  The easiest way to make sure you get it right is to take the shot with several different aperture settings and choose the one that keeps your subject in focus but has the background blurred.  This will also help you learn which apertures produce the best results at a given distance.

 

Given the relationship between aperture, focal distance and depth of field there are some limitations that you need to be aware of too. You cannot use all apertures at any distance and expect your subject to be in focus.  The lowest F stop you can use, and still expect your subject to be in focus, increases as you get closer to your subject (or appear to get closer using a zoom lens).  For example, F2.8 has a depth of field of only 4 inches at a distance of 5 feet so if you’re taking a picture of a flower you can probably pull it off but if you want a subject that is 2 feet deep to be sharp from front to back you need to use F11.

 

There are limitations on blurring the background too.  Low F stops (4.0 and lower) are generally used to blur the background when you want a very shallow depth of field so you must use a fast lens that supports these apertures.  My recommendation is to keep at least one fast lens in your bag.  An affordable fixed 50 mm lens will generally support F stops as low as 1.4 and gives you the ability to blur a background that a slower lens can’t.  But if you need to blur the background on wildlife shots, you usually need a fast zoom lens or a fast super-telephoto lens.

 

Now lets look at the other extreme.  If you want the highest depth of field possible, use the highest F stop your camera supports using a wide angle lens so you are not too close to your subject.   A lower shutter speed will be required in order to expose the shot correctly.  This option works great for landscape photos taken using a tripod but if anything is moving, it will be blurred.  Sometimes you want the blur to show motion or blur a waterfall, but if you want everything sharp, avoid subjects that are moving.

 

So, for the best landscape photos, use a tripod, a wide-angle lens and a high F stop for high depth of field.  For sharp wildlife and action photos with blurred backgrounds use a telephoto lens with a low F stop and a high shutter speed.  There are many other creative uses to controlling depth of field but these are the two main uses for travel photography.

 

May your travel and your photography both be rewarding!

 

     Roger Nelson