Tip Thursday: Depth of field

While helping Beth understand her camera and all the controls we came in to a discussion about depth of field. That is the amount of the photo in or out of focus. In a DSLR it is mainly a product of aperture which is a term for the size of the opening letting light through the lens. It is an iris that can be closed down or opened up.

It is expressed in a term f, ie. f1.4 or f16. The smaller the number the larger the opening and conversely  the larger the number the smaller the opening.

A larger number and the corresponding smaller opening equates to more DOF and the converse is true.

There is also a correlation to shutter speed. If the opening is larger the shutter speed is faster and vice versa. This really effects hand holding. If you use a very fast f1.4 lens for low light situations, to raise the shutter speed, you must sacrifice something and that is DOF. There is no free ride it is a give and take situation.

Sometimes you want limited DOF such as when doing an outside portrait so as to blur the background into a smooth tone.

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In this photo I use f4 to give a pleasing background.

Then other times, such as a landscape you want maximum DOF.

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Here I used f16.

I ran a little experiment to show the effects, on DOF, of different apertures.

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Here the camera is set to f4 notice how the water can is blurred and the specular highlights of the leaves show a round shape.

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Here the camera is set to f16. Note that the water can is more defined and the specular highlights are gone from the background.

I ran another little experiment. Some people believe that focal length effects DOF. It does in a way. If you shoot and object from the same spot, at the same aperture, but different focal lengths it will appear to change the DOF, However if you keep the same relative size in the photo you will see they are virtually the same.

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F4 and 70mm

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f4 and 200 mm moving back to keep the same field of view. Due to the compression factor, background objects appear closer to the foreground, it seems the specular highlights are larger. it is an illusion and you see the same amount of blur in each.

I hope this has helped you. If you have anymore questions just leave a comment

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