Depth of field (from now on we'll call it DOF) is a technique used to make foregrounds and backgrounds to be in focus together or one or the other out of focus. I have set out below some examples so that you can see visually how it works.The best and easiest way to control DOF is to set the camera to aperture priority. Aperture priority was developed on production SLR's in the early - mid 80's and it was developed to make it easy for the user to create depth of field, this was and suppose still is what the setting was created for. If I was looking to create an image that had narrow or wide DOF I would always choose aperture priority. The aperture is essentially a diaphragm built into the lens, and if you want to see what it REALLY looks like, turn on your camera, set to AV, set aperture to f12. Now look down the lens of the camera, a torch could be handy if not in bright light, half-press the shutter button and you should see the 'aperture diaphragm' close up quite a bit, if it doesn't show press the shutter button all the way down and watch for it close nearly all the way up. It is not the one that is solid, this is the shutter itself.

While in AV mode you will see 'f-numbers' at the bottom of the screen, these go from f2.8 upto f11.3 at wide-angle and at full zoom they go from f3.1 upto f12.4.  The aperture is a funny thing, for setting the background out of focus you need to use a LARGE aperture, but the complication is a large aperture means the smallest numbers, therefore an aperture of 2.8 will give you a shallow depth of field (background out of focus). An aperture of 12.4 will give you a large DOF and most things will be in focus. The reason the aperture works like that is because the f numbers (2.8 to 12.4) are fractions of how far the aperture opens. Therefore when the camera reads f3.0 it means the aperture is 1/3rd open, likewise f12 means the aperture is 1/12th open and therefore smaller.

Setting the aperture using AV will of course have a direct effect on how long the shutter stays open, when the aperture is large (2.8) the shutter can fire at faster speeds, and therefore catch action better. An aperture of f12.4 of the same object would require a longer exposure because the aperture is a lot smaller. To demonstrate using shutter priority instead, and how this affects the aperture I explain it like this. If you imagine holding a pint glass, looking down it with your hand wrapped around it, we'll pretend your hand is the aperture diaphragm, this setting would be f2.8 (lots of light shining through). Now imagine your hand around a tall glass and this would be f8.0 and finally imagine your hand around a shot glass and this would be f12.0(hardly any light shining through). When the shutter is set at 8 seconds the camera would set a small aperture (shot glass) because the light on the CCD would be coming in for a long time and so to expose the photo properly it would close the diaphragm down to let smaller amounts of the light in, therefore this would not give an out of focus background. Setting the shutter to 1/4 second makes the camera open up the diaphragm (pint glass) to let more light in because the shutter opens so much more quickly. Hope that is understandable  The pictures below explain this a lot better than words.

F2.9 camera sets1/165sec shutter speed, the camera focuses on the object at the front but throws the other two out of focus completely, notice how wide the diaphragm, looking down the lens is.

F7.0 camera sets 1/13sec shutter speed,  The camera is still focused on the front object and while the middle object comes into focus the other still remains out of focus, notice how the diaphragm has closed up a little

F11.8 the camera sets 1/4sec shutter speed, this time all the objects are in focus and the aperture has closed to a very small hole.

As you can see when the aperture is at small hole stage (f11.8) the shutter needs to stay open that bit longer to let the light shine in longer, while at a large hole stage (f2.9) the shutter is 4 times faster because not as much light is needed to expose the image properly.

Finally the last two shots show just how much the aperture plays an important role in macro photography. Between the body of the bear and the ladybird there is an approximate measurement of 1inch (25mm), in the first photo at f3.0 the ladybird is virtually all in focus, but everything else is clearly not.  This is OK if you really only want to capture the detail of the insect.  Close the aperture to f12.0 though and look at the difference, anything in between these numbers would give very slight but noticeable changes

'Til Next Time!
Photosmarty

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