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Depth of Field, Part 1
By sirsnapalot | August 15, 2009
As a photographer, you can controls many aspects of the composition of a scene. Not only the physical composition, dictated by placement of the camera, but also, what is in focus.
If the placement of objects on a 2D picture comprise the X- & Y-axes, then Depth of Field is along the Z-axis. It is the part of the image that is in focus. Anything in front is out of focus, as is anything behind it.
Depth of Field, or DOF, is related to the distance of your primary subject to the lens (where prime focus occurs), as well as f-stop. The f-stop is probably the #1 variable you will use to control DOF when composing a scene. The smaller the f-stop, the more shallow the DOF. Conversely, the higher the f-stop, the sharper the focus is across the image.
When to use different DOF’s depends on what you want to achieve with the image. For instance, with a landscape, you will usually want a broad depth of focus, meaning a high f-stop (high aperture setting). This will render a majority of the scene in sharp focus, which is what you usually want for a large scene.
Torres del Paine, Chile
In this landscape scene, you want everything to be in focus, including the grass in the foreground. In a typical landscape, f8-f11 is probably adequate- this one was shots at f8. But if you have objects very close to you, you will need to increase the aperture to f16 or even f22.
With close-up shots, DOF becomes more of a factor. The closer your subject is to the lens, the more noticeable the effect of the aperture setting on the focus depth. Below is a close-up shot at a lower aperture setting that shows how DOF can be used to isloate a subject so the background doesn’t distract your attention.
Hornet on a Thistle
In the following macro shot, I shot the same picture from f2.8 all the way to f45 (macro lenses tend to have higher f-stop values). I’ve assembled it in an animation below, so watch it a couple of times through, looking at the f-stop setting and then the increasing detail visible in each shot as the f-stop number increases (and the aperture decreases in diameter).
Depth of Field Animation
The smaller the f-stop, the less light that enters the camera, and correspondingly, the exposure time goes up. In lower light cases, you might need a tripod to stabilize the camera for the longer exposures. Also, there is a physical limit to this practice- f45 won’t necessarily get me the details I want, as there is a diminishing return due to something called the diffraction limit- or pinhole effect. That’s a whole other physics lesson, however. Suffice it to say that every lens has a sweet spot, and if you’re going past f22, you are probably going to hit the point of diminishing returns.
And finally, if you are not shooting with an SLR, you have limited options in the DOF arena. My Nikon Coolpix 8800 high-end point-and-shoot could only muster f2.8-8.0, neither fast (2.8) or sharp (at the 8.0 end). You do have some latitude and certainly will notice some effect, but you have many more options with an SLR and a decent lens. More articles and examples will be forthcoming.
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