Depth Of Field

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Depth of Field

What Is Depth of Field (DOF)?

Depth of field (DOF) is the distance within which objects appear to be in focus.

So, DOF is the zone of acceptable sharpness, the area in front of, and behind, a focused subject that also appears to be sharply focused.

In reality only the subject focused is really sharp together with anything else at the same distance from the camera. Everything else in front and behind the lens is out of focus to some extent.

What Affects DOF

In practice the criteria that we photographers use to control depth of field are:

  • lens aperture
  • distance from subject
  • focal length

Lets take a quick look at each in turn.

Lens Aperture

The aperture is the size of the opening that allows light to pass through the camera lens. It is expressed in f/stops, f/values or aperture values. A typical range might be f/2.8 – f/22, giving a range from maximum of f2.8, largest aperture allowing most light through the lens to a minimum of f22, the smallest aperture with least light passing through the lens.

A small f/value (in this case f2.8) indicates a large aperture and the lens is wide open using the whole diameter of the lens. This large aperture lets the maximum light through that particular lens. This will give the minimum depth of field for this lens.

A large f/value (in this case. f22) indicates the smallest aperture using only the centre of the lens and allows the minimum amount of light through the lens. This will provide the maximum depth of field for this particular lens.

This is the bit that takes a while to come to terms with:

The smaller no (f2.8 here) is the larger aperture because it uses the bigger lens diameter, allows more light through the lens and results in the smallest DOF. The lens is opened up to it’s widest aperture here.

The larger no (f22 here) is the smaller aperture because it uses a smaller lens diameter, allows the least light through the lens and results in the largest DOF. The lens is stopped down to its minimum aperture in our example lens aperture range.

A large aperture gives a smaller DOF, and a small aperture gives a larger DOF.

f/2.8 f/22
large aperture small aperture
less DOF more DOF
great for portraits great for landscapes

 

Putting Aperture into practice:

If you want the subject focused on to be crisp, and everything else  out of focus — such as a portrait with the background nicely blurred — then open up the lens aperture and use a small f/ no, ie. a large aperture.

If you need most of your picture to be in sharp focus — such as a landscape, macro or close up — then you would stop down the lens and use a small aperture.

 Distance From Subject.

If you focus on a subject close to the camera, the DOF is less than when you focus on a more distant subject using the same lens and aperture.

Putting Distance From Subject into practice:

Move in closer to decrease DOF or further away to increase DOF.

Move in really close (macro) and DOF is consequently really small too!

Moving In Close:
In close up work DOF is always relatively small and usually a small aperture (large number, eg f22) is needed to provide reasonable depth. The subject is almost always isolated from a very unsharp background.

Focal Length

Set your subject in front (say, about 3m or more) of a bush (or a tree with lots of leaves, or some other kind of busy background). Use wide-angle and take a picture. Both your subject and the bush would most probably be in sharp focus.

The background may distract from your main subject — unless you want both subject and background to be in sharp focus.

Now use a longer focul length (telephoto lens) and fill the screen with your subject’s face and shoulders. Look at the resulting image and you’ll notice that, though your subject is still in focus, the bush now appears out of focus, giving a nice blurred background that does not steal attention from your subject (‘bokeh’)

Photographers use this technique to achieve the depth of field they want for their picture.

A wide-angle lens apparently has a much greater DOF than a telephoto lens, provides a wide angle of view and forces the photographer into the midst of the subject.Great for landscapes.

A telephoto will dramatically change the perspective, allow the photographer get nearer to the subject in the picture whilst remaining at a distance and seems to create a much smaller depth of field to isolate the subject with the picture. Great for portraits.

Summary

Even though the subject you focus on is the only thing technically in focus, some objects in front and behind your focused subject also appear to our eyes to be acceptably in focus.

This zone of acceptable sharpness is the DOF.

We can increase the DOF (or the appearance of it) by using a smaller aperture (larger f no), moving away from our subject, or using a wide-angle lens.

We can decrease the DOF (or the appearance of it) by using a larger aperture (smaller f no), moving closer to our subject, or using a long focal length.

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