Select the Right Aperture – Part 3


The final post in the series…

Compact mirror less cameras such as Micro 4/3 are slightly different from this. Most seem to be good performers from wide open, hit their best performance when stopped down by one stop and then gradually tail off as diffraction kicks in. The cameras do however have a huge advantage in terms of their smaller sensor size increasing the depth of field. A typical Micro 4/3 camera has a sensor with a 2x magnification. This also means my depth of field is also effectively doubled. I know that at f/7.1 on a 14mm lens (28mm equivalent) I can achieve a full depth of field. My LX5 has an even smaller sensor so by the time I have stopped down to f/3.5 at 24mm equivalent focal length I can achieve sharp focus from 1m to infinity.

So how do you use this advantage?

Firstly understand how your lenses perform at each of the apertures. When are they at their sharpest and when do they suffer from problems such as diffraction.  This gives you your ideal range which you should try to keep within.

Now select the focal length of the lens you will use. This has a big impact on depth of field with longer lenses having less depth of field than wide angles. I suggest selecting the focal length first as I see this as a more important consideration in composition than depth of field.

Once you have composed your image consider how much depth of field you need to achieve. A number of important factors come into play here:

  1. The focal length of the lens as wider lenses give a greater depth of field than telephoto lenses at the same aperture
  2. How far you are from the closest point you want in focus. The nearer this is to the camera the less the depth of field.
  3. The size of your sensor as small sensors give a greater depth of field at the same aperture than larger sensors
  4. Where your point of focus is. The depth of field at a given aperture extends roughly 1/3 in front of the point of focus and 2/3 beyond.

The reason I was able to shoot my New York Skyline image at f/2.8 is that my point of focus was at infinity and I was shooting at the wide angle end of my lens. These points alone were enough to give me the depth of field required. Once you have mastered the points above you suddenly realise the common wisdom of stopping your lens down to its smallest aperture often isn’t correct and won’t give you the optimal image.

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