Aperture controls Depth of Field

Aperture Part 2 -How it Effects the Photograph

Aperture does more than just allow a lot, or a little light to enter the lens.  Aperture controls Depth of Field.  By using depth of field appropriately, you can create the artful images you desire.
Depth of field is the amount of the picture that is in focus.

Review the mechanics of how the lens aperture works with shutter speed by reading Part 1.

Aperture also Controls Depth of Field

A shallow depth of field is created by using a wide open aperture. It has the subject in focus but a soft, blurred background. This is a nice effect for portraits and close ups like the photograph at the head of this post. It brings attention to the subject.

A long depth of field is created by using a small aperture. It has as much as possible in focus –the subject as well as the background. This is used when there are many objects at varying distances from the lens that you want to have in focus, such as landscapes where there are trees in the foreground and pretty mountains in the background.

Why a large aperture creates a shallow depth of field and a small aperture creates a long depth of field is a mystery to me –it just 200px-aperture_diagram_svgdoes. In preparing for this article, I actually went to the Wikipedia article about Depth of Field and read all about Points of Confusion. I got about a third of the way through before I nodded off and fell out of my chair. You’re welcome to give it a shot and come back with a more scientific answer than “it just does”.  It’s magic: A wide aperture gives a shallow depth of field; a small aperture gives a long depth of field –that’s my story and I’m sticking to it.

Controlling your depth of field is an essential piece of photographic creativity. Knowing when to use a long vs shallow depth of field is important.

Long Depth of Field

Use a long depth of field when you want as much of the picture as possible to be in focus: those objects in the foreground, closer to the camera, as well as objects in the background further away from the camera. Some examples:

Landscape or scenic shots:

Long Depth of Field
Long Depth of Field

In landscape shots, usually there is something in the foreground to frame the shot, or give it depth.  This picture has the fence going of into the distance and leads the eys through the shot.  Having a long Depth of Field is conducive to that.

Another example would be a sot of several people at varying distances from the camera; some standing in front of a landmark that’s a far distance away –you would want both in focus.

Again, this is done by using the smallest aperture setting as possible. Take the camera off Auto and put it on Aperture Priority. This lets you set the aperture you want and the camera adjusts the shutter speed accordingly. Remember Filling the Water Glass? If not refresh your memory by reviewing Part 1

Shallow Depth of Field
Shallow Depth of Field
Shallow Depth of Field

Close ups of flowers or insects are one of my favorite subjects. Another mystery of depth of field is that the closer the camera is to the subject, the shallower the depth of field is going to be naturally –no matter how small the aperture.  As you get closer to your subject the depth of field will diminish no matter how far down you close the aperture.

Why? It’s magic.

In controlling the depth of field by adjusting the aperture, you are controlling the amount of light entering into the camera. What’s the second part of that equation?  That’s right, you also need to adjust for it by controlling the amount of TIME the light is allowed to enter the camera –shutter speed.

Let’s say you are taking pictures on a nice sunny day. You have your significant other standing in front of the Grand Canyon or some other beautifully scenic place and want to get him/her in focus with the majesty of the Canyon behind. What do you do?  Close down the aperture as far as possible to get a nice long depth of field. Luckily the day is bright and sunny, so the camera adjusts the shutter speed to be relatively long to compensate for the small amount of light you are letting through the lens to give it more time to get that perfect exposure. Since it is sunny, even the relatively longer shutter speed will allow you to take the shot by hand-holding the camera.


Now, as you are looking through the viewfinder, you see the perfect, relaxed look on your partner’s face and decide you also want a good Portraitoutdoor portrait of just that beautiful face with that smile you fell in love with –Grand Canyon be damned. Without shifting position or moving in anyway, you can create a shallow depth of field by opening up the aperture as wide as possible. This will soften the background making it out of focus (if you are standing at a respectably close distance –if not move in some).

By opening up the aperture, you will be letting in much more light, so the camera adjusts the shutter speed to be very fast to compensate for this flood of light. Same lighting, same conditions, two vastly different shots by controlling the aperture and shutter speed.

One thing to keep in mind:  Your camera will always have the aperture wide open; you won’t see the effect through the viewfinder (unless you have a Depth of Field preview function).  You will only see the effect after you’ve taken the shot.  Another beautiful thing about digital photography is you have the opportunity to see the results immediately and try again if you aren’t satisfied!

In our next technical discussion I’ll tell you all about ISO Speed. Remember –that was the third point of the Exposure Triangle. ISO speed is the underappreciated, unsung hero of the photography world. It’s the safety net to our high-wire photographic adventures.

Stay tuned…

~Ron G.

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