mini photography lesson…depth of field

For those who are interested, I like to share the photography knowledge I have learned to help others use their DSLR camera a little better.  (If this bores you, please skip ahead to other posts!)

As a recap: the way a camera works is that an aperture + shutter speed combination lets in a certain amount of light. If you move the aperture one stop upwards (f/4 to f/3.5 -let more light in), then you must move the shutter one stop downwards (1/125 of a second to 1/250 -less light) – to let in the same amount of light. The opposite is also true – move aperture down, move shutter speed up.  This is a balancing act. The balance is between depth of field (how in-focus the background is) and the ability to “Freeze Action.”  On a landscape, where the subject is not moving, then depth of field is important – you want more of the the scene in focus, which means a bigger aperture: f/16 or f/22, usually on a tripod to prevent hand-hold camera shake.  Where stopping the action is important – like in a horse race – you want to use a faster shutter speed.  But, to get a faster shutter speed, you must have a larger aperture (like f/1.4), which lets in more light, but limits the depth of field.

So, lets discuss Depth of Field for a moment.  The smaller the f-number (f1.4), the shallower the depth of field (background ‘blurry’ – which is called bokeh). The larger the f-number (f16), the greater the depth of field.  If you are in a low light condition and need a wide aperture (f/2.8 with a small depth of field), and you are taking a photo of 2 people – wanting them both to be in focus – you must put them on the same field plane.  Imagine they are sitting on a bench – both people would be on the same plane and in focus, but if one was standing behind the other, one person would be crisp and the other slightly out of focus due to the narrow depth of field.  You can use this information artistically as well, and purposely use narrow depth of field to tell a story.  Like showing a bride at her wedding reception talking to the guests – have the bride in sharp focus while the guests are slightly blurred to make her stand out in the photo!  Depth of field draws you into the picture and tells your eye where to look.

And: The closer you are to a subject, the shorter the depth of field will be. Likewise, the further you are from a subject, the greater the depth of field will be because of the proportion.  If you are inches from something, like a bride holding a bouquet, an f/2.8 lens may have just the bouquet in focus; but if you are five feet from the bride, the depth of field will grow proportionally to include the bride and bouquet in focus. So, if you are taking a subject’s portrait and the earrings are not in focus, take a few steps backwards – the depth of field will grow to include the earrings!  You can always crop in later…

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