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Depth of Field

March 13, 2012

You may have heard or read the term “depth of field” and wondered what it means.  Simply put, it means the distance between the closest and farthest objects in your picture that are in sharp focus.  If you have a large depth of field, a lot of your picture will be in focus.  If you have a small depth of field, only part of the image will be in sharp focus.  In general, landscape photos will have a large depth of field, where portraits will often have a small depth of field so that only the person is in focus, and the background and foreground are out of focus.

You can use your knowledge about depth of field to make creative choices in your photography.  You can decide that you want the image of your child to pop against a blurry background, so you choose to have a shallow depth of field.  In order to do that, you will choose a wide aperture- which is a small f/number (for example, f/1.8-f/5.6).

Or, you might want to take a picture of your child’s entire outdoor birthday party, and include as many details in sharp focus as possible.  To do this, you would be choosing to have a large depth of field (amount in focus) and choose a closed down aperture- which is a large f/number (f/8-f/22).

Some things to consider when choosing your aperture and how it affects the depth of field:

*If you are photographing more than one person, you will need to take that into account when choosing your aperture.  Your subjects’ eyes will not be on the exact same focal plane, therefore you will need to have a larger depth field in order to get them all in focus.  There is a rule of thumb that the number of people in your photo should be the same as the aperture.  So, if there are 4 people in a grouping, you wouldn’t have a number smaller than f/4.  Of course, if you take the picture at f/4 and some of the people are blurry, you will need to choose a bigger f/number.

*Your distance from your focal point affects the depth of field.  If you are very close to your subject, your depth of field is smaller.  If you are far away, your depth of field is larger, even if you don’t change your aperture number!

*Your focal length affects the depth of field.  Say you’re using your kit lens and it zooms from 18-55 mm.  At the 18 mm lens it is a wide-angle (meaning you can see a lot in the scene) and at 55 mm it is a zoom (so you see less in the scene, even without moving your feet).  When you are at a wide angle, your depth of field will be deeper, but as you zoom in, your depth of field gets narrower, even without changing settings or your camera’s physical distance from your subject!

*Generally it’s ideal to have both eyes in focus.  If you are close to your subject, he/she is not facing you straight-on, and you choose a very small aperture number, one of the eyes may be out of focus.  In that case, it is better to have the eye closest to you in focus, therefore, place your focus point on the nearest eye.

See here,  the eyes are not on the same focal plane (distance from the camera).  I focused on the nearest eye.  But, because I chose a wide aperture (f/1.8) and I’m really close to him, the back eye is out of focus!

If both of eyes were on the same focal plane, I would get better focus on both eyes.  Here in this photo, I’m still at f/1.8, but you can see that both eyes are in focus because my subject is facing me straight on, so both eyes are on the same focal plane.

But, if you want your subject to be facing a different angle and still have both eyes in focus you can just adjust to a larger f number (smaller aperture). By doing so, you make your focal plane larger, which means more will be in focus. Here I chose f/4, and it was enough to get both eyes in focus!

Alternatively, you could just back up, and by doing so the focal plane would get larger, so you could get both eyes in focus without changing your aperture.

In review, three factors control your depth of field: aperture, focal length, and distance between camera and subject.

Do you want a large depth of field (lots in focus from near to far)?  Use a wide angle and/or step back from your subject and/or use a closed down aperture (f/8-f/22)

Do you want a shallow depth of field (little in focus, with a blurry foreground and background)?  Use your zoom and/or get close to your subject and/or use a wide aperture (for example, f/1.8- f/5.6)

I hope that helped!

~Sonja (MWAC host)

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