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Depth of field is dependent on just two factors: Lens aperture (focal length of lens divided by actual diameter of lens opening), and reproduction ratio (size of object divided by size of actual image on the film or sensor). Both of these are ratios, and thus have dimensionless units. So, regardless of the focal length of the lens being used, the only way to increase depth of field is to use a smaller aperture, or decrease the reproduction ratio by moving further away from the subject. The former has a limit because lens sharpness decreases at very small apertures, and the latter would necessitate a bit of cropping if you want a tighter composition. Most people have a hard time understanding that simply changing the focal length of the lens, and then moving closer or further away to maintain the composition will not change depth of field. The only thing it will change is the field of view, which is why longer focal length lenses give more pleasing backgrounds for portraits.
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