Understanding Depth of Field
When done well, a photograph that has good control over the depth of field can add dramatically to the impact of the picture. When we are talking about depth of field, what we are referring to is a shot were the main subject of the picture is in sharp focus but other elements in the picture are blurred or out of focus. Two key terms to learn here are focal plane which is the area of the image that we want to have in sharp focus and bokeh which is the out of focus sections.
If you can master controlling the depth of field in your photographs you will be taking your photographs to a whole new level. What do we mean by controlling depth of field (DoF)? This is the effect where the subject is in sharp focus but the rest of the image is slightly out of focus. The effect is referred to as bokeh. The more bokeh, the more dramatic the blurring effect. In this article, we will discuss how to control the amount of bokeh in your images.
Being able to control the depth of field is not very easy on lower end cameras as you need to be able to shoot in either an aperture priority mode or in a full manual mode. If you have a fully automatic point and shoot then you will not be able to achieve the effect you want
The setting you need to be able to adjust is the aperture setting, the larger the aperture (the smaller the number) the shorter the focal plane will be. This is one reason that some people buy very fast (larger aperture) lenses. A less expensive lens may have an aperture range of something like f/4.0Â -Â f/5.6, while you will achieve some bokeh effect with this lens, it will not be as pronounced as it would be if the lens has a f/1.6 – f/2.8.
Without having a lens with a very large aperture, the other way to achieve a dramatic bokeh effect is to use a long zoom lens to shoot very close objects. Since a long lens won’t be able to focus on both something very close and things at a distance, you can achieve a strong bokeh if there is enough separation between the subject and the rest of the image.
Diagram of decreasing aperture sizes
(increasing f-numbers) for “full stop”
increments (factor of two aperture area per
stop) – Source: Wikipedia
The aperture setting is the size of the opening that is used to let light into the camera and onto the sensor. The size of the opening is measured is F-stops. The confusing part is that the larger the aperture setting, the smaller the hole that will be used to take the picture. The reason why lenses with a very large aperture (smaller f-stop number) are more expensive is because the lenses require more engineering and typically use better and more complicated lens elements. These higher end lenses usually have glass that is manufacturered to higher tolerences and may include special coatings on the glass elements, this helps explain the high costs of the top end lenses.
The other side effect of getting a lens with a small f-stop number is that the smaller the f-stop, the more light will come in per image thus allowing you to shoot in lower light conditions. A lens with a f/1.8 can shoot well exposed pictures in a situation that is much darker than an f/4.0 lens could shoot in.
Why Control DoF?
So now that we know the how of controlling depth of field, let’s talk about why we would want to do this. Isn’t the goal of every picture to have the ultimate tack sharp image? Why would you intentionally blur part of the image? The simple answer is mood. Quite often, the background of an image may be distracting from the subject matter, think of a bride standing in front of a wall of flowers. In this case, the bride is obviously the main subject and anything else may distract from the subject, by blurring out the flowers behind the bride, you can retain some of the color and texture without the detail, adding a much more dramatic feel to the image.
One thing to be aware of is overdoing the bokeh effect. Since the eye is drawn to the lightest parts of an image first, a common problem with a heavy bokeh is creating a large blob of very light color, this can be more distracting to the image than if the background would have been in focus. Used right, the effects can be stunning.
When choosing a new lens, once you decide once you decide the focal range you are looking for, then you go for the lens with the largest aperture (again, lowest number) that you can afford. A common misconception with new photographers is that a lens labeled with a single aperture will only do that aperture. The labeling denotes the maximum aperture at the listed focal length, So if you have a 70-300 f/4.0 – f/5,6, then your maximum aperture at 70mm will be f/4.0 but will shrink to f/5.6 at 300mm. Whereas a 70-200 f/2.8 can maintain f/2.8 throughout the entire focal range but can also go to the minimum aperture of you camera (often f/22 or lower),
Now that you have these basics down, you should be able to tell that the faster (smaller f-stop number) the better able the lens is to shoot in darker conditions and when used at the lens’ maximum aperture, the more pronounced the bokeh effect will be.
Be sure and post some images that you have taken to the Flickr user group and tag them with DoF and cameradojo.