Select the Right Aperture – Part 2


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Copyright: Robin Whalley, 2011

Continued from previous blog…

So, looking at the use of aperture to control depth of field is actually a very effective tool for doing this and depth of field is a key creative decision you need to make when capturing your images. Let’s say you want to create a portrait but the background is distracting. The solution is to open the aperture wide to throw the background out of focus and isolate the subject. If you are shooting landscapes and want to achieve the maximum depth of field so that everything from the foreground to the distant background is sharp, you would select a small aperture. Sure there are other factors at work here which can emphasise the impact of the aperture but they do just that, emphasise the effect. The priority should still be to select the right aperture for the image.

Now let’s consider some of the problems that come with the aperture control and lenses. The first occurs when we stop the lens down to its smallest aperture. This often results in diffraction and potentially distortion. I once had a wide angle Pentax lens that when stopped down to its smallest aperture would bend and distort images in the corner. Diffraction is a little different and will result in soft and slightly out of focus images. Again I remember when I was first starting in photography and I had a cheap wide angle Tokina lens that suffered badly from diffraction. Not understanding this I would stop down the lens to f/22 and the results were always poor.

At the opposite end we can open up our aperture as wide as possible. Most lenses however don’t go very wide with a maximum aperture in the range of f/3.5 to f/5.6 (particularly true of zooms). At their widest aperture most lenses are also soft, especially in the corners. Many also suffer from uneven lighting and tend to vignette due to light fall off in the corners – not always something you want. Higher quality lenses from top manufacturers such as Zeiss and Leica tend to be the exceptions and will still perform well when wide open or stopped down. Unfortunately they are also expensive.

This then leaves most of us with an area of optimum lens performance which is usually 1-2 stops above wide open for usually 1-2 stops. With a typically zoom lens on an SLR this is probably in the range of f/8.0 to f/12.0. Does that sound familiar?

To be continued.

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