Shallow Depth of Field

I am a member of the Pix Magazine photography forum (have a look here: http://pixmag.ning.com) and from time to time members ask advice. Here’s a recent one and I’ve posted it here because I think that it could be useful to you. Here’s the question:  “Hi. I hope you are well. Roger can you please help me. I obviously must be doing something wrong on the aperture priority. I have noticed that when I try to blur the backgrounds of some of the photos it doesn’t always work. I adjust the aperture to minimum (3-5) the minimum that the camera allow”.

The depth of field is hugely influenced by your choice of lens (also by the aperture of course) and indirectly by your choice of camera. The reason I say by the camera is that if you are

This image was shot with Nikon's 200-400mm f4 lens at 400mm and at f4 resulting in a nice smooth background.

using a small sensor camera like the Nikon D300 or a Canon 7D then your “standard lens” (50mm on a full frame camera and somewhere around 35mm for the so called DX format cameras) is longer on my full frame D3x, for example, than on my wife’s D300. What this means is that in any given situation, you will be using a longer focal length lens with a full frame sensor than with a DX camera to achieve the same framing. And wide angle lenses, as we know, show more depth of field than telephoto lenses.

If you want out of focus backgrounds use the longest lens you can with the largest aperture you can and in your case it seems to f4.5. This aperture will limit things a little especially if the lens that you are using is fairly short. All those wonderful animal portraits with completely blown out backgrounds that you see are usually shot with lenses like the 300mm f2.8, 400mm f2.8 and the 600mm f4 – expensive glass to be sure.

There are 2 other things to consider:

The first is that the closer you are to your subject the shallower the depth of field – just try shooting macro. Number 2 is that the further the background is from your subject the more out of focus it will be. So the recipe for out of focus backgrounds is to use a long lens with a very large aperture (f2.8 or f4), get close to your subject (obviously within the limits of the lens that you are using) and make sure that the background is as far away as possible.

Hope this helps