Depth of Field again


One of the easiest factors to control with your picture taking is the Depth Of Field, usually written as DOF. Mastery of DOF will return you much better photographs and raise your pictures right out of the amateur snapper with a compact or camera-phone.

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DOF refers to an optical characteristic and depends solely on the ratio of the lens being used and the aperture selected. Altering the shutter speed, has no effect on the Depth of Field.

DOF really refers to the zone of “sharpness” (or being in acceptable focus) from foreground items to background items in any photograph. This is different from what the eye sees, as the eye can instantly focus on near and far objects, giving the impression that everything in your field of vision is in sharp focus. The camera, however, gives you a slice of time.

The first concept to remember is “1/3rd forwards and 2/3rds back.” Again this is a law of optical physics, but means that the DOF, from foreground to background in your photograph can be measured, and from the focus point in the photo, extends towards you by one third and extends away from the focus point by two thirds.

With older manual focus SLR’s, you will even find a series of marks on the focusing ring of the lens to indicate the Depth of Field that is possible with that lens.

You see, for each focal length of lens, the DOF possible is altered by the Aperture. The rule here is simple – the higher the Aperture number, the greater the DOF and the lower the Aperture number, the shorter the DOF. In simple terms, for any given lens, you get greater front to back sharpness with f22 and you get very short front to back sharpness at f4.

Just remember the higher the Aperture number, the greater the DOF; the smaller the Aperture number the smaller the DOF; plus the longer the lens gives a shorter DOF, the shorter the lens, the longer the DOF (just remember the ‘opposites’ – the longer gives shorter).

Now to apply this formula – when shooting a landscape for example, where you want great detail from the foreground, right the way through to the mountains five kilometers away, then use a short lens (24 mm is ideal) set at f22 and focused on a point about 2 km away.

On the other hand, when shooting a portrait where you only want to have the eyes and mouth in sharp focus you would use a longer lens (and here the 135 is ideal) and a smaller. Aperture number of around f5.6 to f4 and focus directly on the eyes to give that ultra short Depth of Field required.

These optical laws hold good for all cameras, even digital. It may sound confusing at first, but just remember for short depth of field you want the lens aperture numerically as low as you can.

The other number you’ll see is focal length. We typically use this number as short hand to indicate the angle of view you’ll get when it’s attached to your camera. This gets a little complicated when you talk about cameras with different sized sensors because that can actually change the effective angle of view. Most entry-level and even mid-level DSLRs ship with a “kit lens” with a focal range of 18-55mm. That’s what’s called a “standard” zoom because it goes from wide angle up to a short telephoto on the long end.


Other buying tips

It’s no exaggeration to say we could write an article five times this size and not cover every aspect of digital cameras. But with this information, you should at least have a handle on the basics, which will let you narrow down your selection.

Don’t forget to consult professional and user reviews on the web to get a better idea of what any camera can do for you. By the way, the piece of glass at the front of your lens is the most important part as far as the sharpness of your photos is concerned. Cheap lenses are plastic and they do denature over time. Like all things in life – you get what you pay for!