There have been a few questions recently by Noobie photographers, in fact the same old question, gets asked regularly. Displaying the ignorance or the inability of noobs to do a quick search of the Internet, to find an explanation of Depth of Field.

Firstly Depth of Field, affects the area of sharpness in an image, depth of focus affects the ability to record an image sharply on film or imaging sensor, for this tutorial we will ignore depth of focus, as that is largely the problem of lens and camera manufacturers.

What is Depth of Field?

If one considers that there is a definite point of focus in an image, where the image is absolutely pin sharp, then there is an area in front and behind that point of focus that also acceptably sharp. There is a formulaic approach involving circles of confusion, which I shall not go into here.

Depth of Field as a rule of thumb extends one third in front and two thirds behind the point of focus.

So what affects Depth of Field? Depth of Field is a variable, which is influenced by the focal length of the lens that you are using, the aperture that you are using and the shooting distance.

For example Depth of Field on a wide angle lens, that is anything that has a shorter focal length than a 50mm standard lens is proportionately greater for any given aperture at any given focus distance, Telephoto lenses, typically one with a focal length of greater than 50mm, have a lot less depth of field for the same given criteria

	For example here are a couple of graphic illustrations that show the depth of field for three different lenses, focussed on the same subject at 5 metres and at three different apertures. The first image is based upon using a 24mm lens focussed on a subject at 5 metres, with 3 different apertures, from F1.4 through to F22. As you can see the depth of field at F1.4 extends from just under 4 metres to 7.2 metres. Whilst at F22 it extends from 0.8 metres to infinity. Which will mean everything that is in the photograph between those two extremes will be sharp.
	For a 50mm lens, we can see that for the same settings that the depth of field is very different, with the available depth of field considerably less than for the 24mm.
	At the telephoto end we can see that the depth of field wide open at F1.4 is near none existent. Many people new to photography can see a reason for wanting images sharp from front to back, but have difficulty in recognising why they would possibly want to deliberately shoot with less than the maximum depth of field. I shall show you a few examples in due course.
	Here is an example of a 135mm lens being used wide open, at a distance of something like 10 metres in which the depth of field is approximately 9.7 metres to 10.2 metres, basically meaning that the Mile Marker itself is sharp and very little else, a few centimetres of grass in front and behind, you can also see that the background pleasantly blurs to make the Mile Marker stand out a little from the background
	At F11 the depth of field is increased noticeably from 8.6 metres to nearly 12 metres, meaning that almost all of the grass in front of the Mile Marker is now sharp. Also the background is far sharper, more clearly defined, even that which is not within the 3 metres of depth of field
The ability to use this depth of field to your advantage can make for highly satisfying images, especially for portraits, to be able to separate the subject from the background, there will also be occasions when you will wish to have the background of a portrait clearly defined, depending upon the subject matter and what the background contains. But many in many portraits, it is the subject matter that is important and the background a distraction. The image at the top of this article was shot on an EOS1D mkII N with a Sigma 120-300 F2.8 at around 120mm, 1/60 sec at F5.6, with fill-in flash, set to minus 1 stop. This enabled the depth of field to cover the whole of the body, and yet allow the background to be nicely thrown out of focus. Using an aperture wider, for example F2.8 would have left some of the face, shoulders and arms out of focus. Whilst many will tell you that the eyes are the most important part of a portrait, to have the tip of the nose and the ears out of focus can be distracting.There are other tricks such as differential focusing, i.e. deliberately focussing on something to draw attention to it, or away from other parts.
	This particular picture was shot for a story on street gangs It was shot indoors with a Canon EOS1DmkII N and a 70-200mm F2.8 with a flashgun and balancing the exposure with the available tungsten light to blow out the background but leave enough detail in the hooded top.
	This image was taken for an article on street gangs that were using replica guns, that had been converted to fire live 0.22 rounds, these weapons are freely available over the counter and relatively easily modified. However, most are not constructed to the same standards as the real McCoy and as a result could blow up in the face of the criminal (bloody good job too) This image was shot using a DCS520, 200mm F1.8 and 540EZ flash balanced with the available daylight Slightly more graphic view of depth of field.

50mm Lens at F1.4.

50mm Lens at F4.

50mm Lens at F8.

50mm Lens at F22.

I shall at a later date, i.e. when I can be arsed, probably add a page about Hypefocal Distances as well, but don't hold your breath.

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