Photography Lessons: Understanding Lens Blur

28 Feb 2009

We have all seen these wonderful photos. There is a subject in very sharp focus, and everything else fades to this soft blur. In photography, this blur is called Bokeh, and the cause is something called Depth of Field. This article is targeted towards those of you with point and shoot cameras or perhaps a fancy new digital SLR that you haven’t quite learned to use yet.

Depth of Field is simply defined as the portion of the scene that appears in focus in the image. Two factors determine the size of depth of field. The distance between the camera and the subject, and the speed of the lens taking the photo. I’ll break down these separately so hopefully it all comes into focus (excuse the pun!)

Lens speed is determined by the maximum aperture that the lens will allow you to use. Although it’s somewhat confusing, in lens ratings, the maximum aperture is actually the smaller number. The lenses are rated in what is called an f-stop. The f-stop is a rating that determines how much light the lens let’s through it’s aperture. The smaller the number, the larger the hole. The larger the number the smaller the whole. The subject of f-stops goes much deeper, but for the purpose of this topic, the important thing to know is that the larger the whole (smaller the f-number) the shallower the depth of field becomes. It’s pretty common for consumer point and shoot zoom cameras to have a maximum aperture of f2.8, which is actually really fast. These cameras also decrease the maximum available aperture as you zoom in, so you may find that when you zoom all of the way in on a subject, your maximum aperture could decrease to say f5.6 which in terms of relative speed, lets in dramatically less light and also has a much wider distance of what would be considered in focus.

That’s enough confusing information about f-stops and apertures, just remember that the more light your lens is letting in, the short the slice of distance that will appear in focus is. This is the effect creates these striking images that really isolate the subject in focus from the background around it.

The second factor that determines the portion of the image in focus is the distance you are from the subject itself. The closer you are to a subject, the less of the image will appear in focus. Try this example. Go outside in bright day light. Find a flower, and start about 8’ from the flower. Take a photo of the flower. Without touching the zoom on your camera, take a step forward, and take another photo, take another step, another photo. Keep moving forward until your camera is unable to focus. (all cameras have a minimum focus distance)

Open these photos up on your PC and you will find that the closer you get to the flower in question, the less detailed the image around the flower becomes. This is the effect that distance has on the image. If you want that fancy lens blur, get as close as possible.

For a more in-depth technical lesson, I recommend http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

Check out some of my photography on Flickr.

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