SHUTTER AND APERTURE BASICS

DoF, Subject Distance and Focal Length

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For a given aperture, a subject that is close to you will have less DOF than for a subject that is for away.

For a given aperture, a subject at higher magnification will have less DOF than a subject at less magnification.

The first statement is simple to grasp and remember, the second statement needs some explanation.

Magnification is the ration of subject size to image size on the sensor, expressed as subject:sensor. So, if I photograph a 1.8m tall person, and their size on the sensor is 1.8cm, thats a ratio of 100:1 – 180cm to 1.8cm.

Thus for two different focal length lenses, say a 50mm and a 200mm, for the image size to be the same with both lenses you would have to be much closer to the subject with a 50mm lens. If you then made these two images at the same aperture, where the image size was the same, thus moving the camera accordingly to achieve this sameness, then the DOF would be the same.

But, from a given camera position, if you want a large DOF use a short lens and a small aperture – say 35mm at f/22.
If you want a small DOF use a long lens and a wide aperture – say 200mm at f/4.

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Lens length and DoF

Most people mistakenly believe that a long lens has less DOF than a short lens. This is incorrect. They forget to consider the difference in image size, or magnification.

If you take two images from the same camera position using two different focal lengths, then the image from the long lens will have a shallower DOF because the magnification is much greater, not because of the focal length.

If you stepped back far enough such that the magnification with the long lens was the same as that with the short lens from the original position, the DoF would be the same, for the same aperture.