When working with close-up photography, as we are in shooting small subjects such as watches, focus is a very
important consideration. Depth of Field (DoF) refers to the amount of an image that is recorded in acceptable focus. With close-up and macro photography DoF can often be mimimal.
There are three main factors that control DoF. These are as follows:
1) Focal length of lens: shorter focal lengths will give greater DoF
2) Distance from subject: greater distance will give greater DoF
3) Aperture: smaller apertures (higher f-numbers) will give greater DoF
A fourth factor is format or size of the film/CCD array. Smaller formats will give greater depth of field. However, as we are generally limited to the camera/format we are using this is not a factor we can readily alter.
It is also important to note that DoF extends 1/3rd in front of the point of focus and 2/3 beyond, as per the illustration below:
Thus we can place the point of focus so that the DoF falls where it is desired.
Controlling the Depth of Field.
Taking into account the 3 factors already addressed we can control the DoF in a number of ways. By using shorter focal length lenses we can increase the DoF. However, short focal lengths are not always desirable as wide angle lenses may distort the image or simply not give the view desired. Then we have subject distance. By placing the camera and subject farther apart we will increase the DoF. This often isn't overly practical with such small subjects as watches. We really need to get close either physically or via the use of longer focal lengths. Of course those longer focal lengths will decrease DoF and so any gains are effectively cancelled out. So, we come to aperture. In effect this generally proves to be the most practical way of increasing DoF. By reducing the size of the aperture, (selecting higher f-numbers), the DoF will be increased without us doing anything else. You do not get something for nothing, though. Smaller apertures permit less light to transmit to the film/CCD array, and therefore shutter speeds will lengthen in order to gain correct exposure. Shutter speeds will often be long enough to require a firm support such as a tripod to eliminate camera shake. Some form of remote shutter release can also be useful to ensure the pressing of the shutter button does not affect the image, (the self-timer delay can be used as a makeshift remote release). Exposure times may extend to many seconds. Be aware that moving watch hands will be recorded as blurs with such long exposures if they are not stopped.
Below are two example images shot to illustrate the effect of aperture on DoF. The image on the left was shot at f4, whilst that on the right was shot at f11. The point of focus was placed on the middle of the watch dial, and nothing else was altered between shots. You will note that in the left image shot with the larger aperture the watch and the rear bottle are in acceptable focus, but the bottle at the front is distinctly out of focus. In the image at right, shot with a smaller aperture, all of the subjects are rendered in acceptable focus. The smaller aperture has resulted in increased DoF.
Below is another pair of comparison images again shot with different apertures, f4 on the left and f11 on the right. This sort of close-up is a common situation in watch photography. With the subject being closer to the camera and/or a longer focal lens being used, the DoF can be quite minimal. However, increasing the aperture can still give significant increase in DoF, as illustrated.
Why should we want to increase the Depth of Field?
We don't always look to maximise the DoF, but in close-up work it will generally provide a more 'natural' looking image. The interesting thing is that it isn't natural at all. Our eyes also have limited DoF and can generally only focus on a small plane at any one time. However, our mind is able to combine the many images sent by the eye and provide us with an image that appears to portray the whole subject in focus. Therefore this is generally what we expect to see in a photograph. When much of a subject is out of focus it can look a little odd to us, and so most of the time we are looking to increase the DoF in our watch photographs. However, limiting the DoF is also a useful photographic technique called 'selective focus'. Instead of attempting to render the whole image, or as much as possible, in sharp focus, we can select which parts will be in focus. This can be a powerful method for centering the viewer's attention on the main subject. In the example above, for example, it may be that we wanted to focus attention on the crown whilst allowing the dial to fall out of focus so that it doesn't draw attention. By selecting larger apertures (smaller f-numbers) this can be achieved. In many cameras/digicams the effect of changing aperture can be viewed through the viewfinder prior to shooting. The most important issue is that we have the ability to control the DoF to give the result we desire.
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Copyright 2005 Paul Delury