Macro Shorts #1 by Frank Peele
Depth of Field: Making the Most (or Least) of it
Depth of field in photography is the zone of acceptably sharp focus in any scene which includes subjects at differing distances from the camera. The factors which control depth of field are these: Lens focal length, lens aperture (f/stop), and the distance focused on. (There is a fourth, known to scientists as the Allowable Circle of Confusion, but that’s a set value for each film or digital sensor format, so we’re not going to let that confuse us.) As we get really close to our subject – and of course, that’s what macro shooting is all about – we quickly learn that depth of field is severely limited. There are 3 ways to deal with this limit. The first is to celebrate the limited depth of field, carefully composing and focusing so that only the most important part of our subject is sharp. Letting everything else go out of focus really draws the viewer’s eye to what we want to feature in the image, and reduces or eliminates distracting “clutter” at the same time.
There are times, however, when we really need to have greater subject depth in sharp focus, and that brings up method number two: stopping the lens down to a small aperture (expressed by a large f/number, such as f/22, f/32 or f/45).
But there is a price to pay when we use such small apertures. It’s a law of physics known as diffraction, which causes any lens’s optical performance to decrease when its aperture is stopped down too far. The law of diffraction applies to all lenses, no matter how expensive, no matter how new, and its effects are quite real. Look closely at the next two images, which are magnifications of the previous two shots:
There’s an easy rule of thumb we can use to determine the optimum aperture of a lens – the f/stop which yields the greatest sharpness – and it works for most general-purpose lenses. (There are special-purpose lenses designed at great expense to have maximum sharpness at other apertures – even wide open – but we aren’t likely to run into any such lenses in our work.) The rule of thumb is this: The optimum aperture is found by stopping the lens down between 2 and 3 f/stops from its maximum (widest) opening. Thus an f/2.8 lens should provide its greatest sharpness between f/5.6 and f/8, or f/7.1. A lens whose maximum aperture is f/4 should be stopped down to between f/8 and f/11, or f/9; an f/5.6 lens will be sharpest at between f/11 and f/16, or f/13 … and so on.
Now, using the optimum aperture doesn’t give us greatly extended depth of field. What it does give us is our lens’s greatest sharpness possible. That is a very good basis for going on to the third way of getting greater depth of field when we need it. It’s called Focus Stacking, and it’ll be the subject of the next of these “Macro Shorts” from PPC. Stay tuned!