Selecting the Correct Aperture
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The lower the value of the aperture you select, the wider open your lens becomes and the less depth of field you will get in the resulting photograph.
Depth of field
Most successful wildlife shots have some degree of background blur to allow the subject to stand out nicely from its background.Background blur is achieved by having a wide open aperture of between 5.6 and 2.8, and being fairly close to your subject. This is something you should strive for, but unfortunately the only thing you can really do to achieve great background blur is to get a really expensive lens. Okay... Now you have your excuse, you just have to get past your spouse!
Lenses with wide open minimum aperture (like 2.8) are very expensive. Especially the long super telephoto lenses!
If you are photographing a group of animals, it might be better to have a higher aperture to increase the depth of field. This helps ensuring that more than one subject is in focus, and that is something I strive for when photographing groups of animals. The resulting photograph will be more pleasing.
This shot of a Cape Sparrow illustrates how the subject can be made to stand out from its background by employing background blur.
Copyright © Dries Cronje
Shutter speed
I try to leave my lens open at the lowest aperture for most wildlife photography applications. This helps in action photography since you can have a fast shutter speed to allow you to capture the action.A sufficiently high shutter speed is also important for sharpness. Hey, but haven’t we dealt with sharpness already? Yes, yes… But separating these issues is unfortunately not possible, and only in consideration of all these elements together will you learn to find something that works for you!
When the light is failing or you are inside a forest with low light, I recommend using your lens’ minimum aperture to obtain the highest possible shutter speed. This will improve your chances of obtaining a sharp shot. Setting your ISO speed to 400 or 800 might also help in such situations. ISO speed! See what I mean…
A final consideration for shutter speed is those special effect shots when you want a running lion’s body to blur or a flying bird's wings to blur to emphasize the movement. Here you will have to set a higher aperture to slow down your shutter speed to something in the region of 1/30.
Lens considerations
Some of the more affordable lenses do not behave too well when used wide open (minimum aperture). If you are not yet using a professional lens, my advice would be to experiment with this a bit. Take a couple of shots at your minimum aperture and then take the same shots at an aperture of one stop higher. Now compare the photographs and, while being very honest, decide if there is a noticeable quality difference. Then base your future aperture settings on what you learn here…If you are using a prime from Canon or Nikon you should be able to use minimum aperture every time without any quality sacrifice. The beauty of an expensive lens...
Conclusion
When in the field and applying your ‘ISACA’ acronym, try to set your aperture to the lowest value (at which your lens performs well) for most wildlife photography, but when photographing more than one subject remember to set it lower. Obviously, when you are trying to achieve a special effect like blurred movement you will have to select a higher aperture to induce a sufficiently slow shutter speed to achieve the blur.When you are confused (like most of us are, most of the time), just experiment and have fun! You will probably not be too disgusted with your results...
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