A matter of focus

Which of these photos is in proper focus:

1. front focus




In this case, focus becomes a matter of preference.

What we’re really dealing with is another issue, Depth of Field (DoF) – that portion of a photograph which is perceived to be acceptably sharp.

Depth of Field is one of those “advanced” topics in photography… until you grasp how it works, and how to control it. (Note to the purists – this discussion is henceforth simplified)

Controlling DoF is all about controlling the three variables which directly affect it –  lens focal length,  lens aperture and camera-to-subject-distance. All three interact to produce differences in DoF. And while your camera may be largely automatic and you may have no control at all over one variable, you’ll almost always have control over one or both of the others.

Lens focal length is usually expressed in millimeters (mm).  Longer is the same as “zoomed in” – short is “wide angle.” Most point-and-shoot cameras have modest zoom lenses of the 3x variety; some cameras have “ultra-zooms” in the 10x to 15x range. Cellphone cameras often have no control at all over this variable. Note it is not necessary to use a zoom lens at least with cameras supporting interchangeable lenses; you can also use “prime” or non-variable lenses… and a prime lens will usually have a bit better control over aperture values. Wide-angle and telephoto are relative values – the size of the sensor (or film frame) determines the useful ranges. Within 35mm photography (and most dSLR systems) 18mm is wide, 200 mm is telephoto, 500 mm is serious telephoto, and 1250mm is wicked close (and the DoF is paper-thin!).

For our purposes, longer focal length (telephoto) produces shallower depth of field and shorter focal length (wide-angle) produces deeper DoF.

Lens aperture is the size of opening of the iris in the lens; i.e. how big a hole the light comes through. This is expressed as a logarithmic ratio in the form of f-stops. F-stops for camera lenses run from f/1.4 at the wide-open (think really huge) end to tiny pinpricks of light up around f/64. As you go up the scale, the amount of light is halved; thus if we take f/1.4 as “full” open, then f/2 (next increment up) allows 1/2 the light, f/2.8 is 1/4, f/4 is 1/8 and so on. Photographers usually reference low f-stop values as “open” (or “fast” – as it gathers the light in faster), and high f-stop numbers as “closed” (or “slow”).  Zoom lenses are limited as to how open they get; f/2.8 is a very fast zoom (wide open) and f/8 is rather slow (closed down).

For our limited interest in Depth of Field, the rule is this: for a given focal length, depth of field is reduced as you open the lens, increased as you close the lens.

Camera-to-subject-distance is self-explanatory… isn’t it? In the photos above, camera-to-subject-distance is the variable which has changed – I shifted the subject from the up-close gun barrel to the more distant aircraft. Focal length stayed constant at 50mm and aperture at f/10. If I’d backed off a bit (probably about 10 feet would have done it) both subjects would have been in focus… but that wasn’t the effect I wanted!

Let’s look at these two photos, shot back-to-back. The closeup is at f/5.8 and 300mm focal length; the wide-angle view at f/4.5 and 75mm.



The increased focal length more than makes up for the slight closing of the lens; note how blurred the background is compared to the second photo. They were taken back-to-back, about ten seconds apart.

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