Controlling out-of-focus blur, in photos

I recently experimented a little with my Canon DSLR. I wanted to see if there was a quick and easy way of remembers, even understanding, the rules behind background blur.

You’ve seen the shots – a flower or face sharply in focus. The background is a blur. Creates a single focal point (in most cases).

Basically, the wider your aperture, the more it will blur. So, F5.6 will blur more than F16.

There is a side effect. By increasing aperture you increase the light into the film/sensor, and potentially overexpose your shot. Of course, the upside is you can then lower the ISO possibly down to 100, increasing resolution/quality.

There is another matter to consider. The length of the lens used – a telephoto will also increase blur compared to a wide-angle. So 105mm will have more blur than the same F-stop on a 50mm.

The actual “blur” is more often referred to as the areas outside of depth-of-field. For depth-of-field is the distance over which things are in focus. A photo of a garden with plants and hedges will for instance have a must high depth of field (no background blur) versus a bee within a plant. There are calculations to work all this out prior to your shot being made, personally I prefer a little live experimentation.


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