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Lens aperture controls something referred to in photography as ‘depth of field’. Depth of field is a phenomenon caused by the limitations of the human eye. When a lens is focused, light is formed into a sharp point. However, light reflecting from other parts of the scene, closer to or further from the point of focus (ie, areas that are out of focus) instead forms blurry circles. The further from the point of focus, the blurrier these circles – and the relative part of the scene – become.
The resolving power of the average human eye enables humans to detect in a print roughly A4 in size, from practically any current DSLR camera, the presence of circles that are larger than 0.03mm in diameter. Anything smaller than that will appear as a point rather than a circle, and therefore appear sharp (in focus) to the naked eye.
Because of the way the human mind works, people are drawn to objects in a photograph that are sharp and ignore blurred objects. The blurrier the object, the more it is ignored. So, using lens aperture to alter the extent of depth of field enables a photographer to emphasise some objects (by making them appear sharp) and de-emphasise or hide completely other objects by blurring them.
Technically speaking, lens aperture is the hole in the diaphragm in the lens that allows light to enter the camera when the shutter is activated. This hole can be made smaller or larger by adjusting the aperture value using the Command dial on the camera (or, on
some older lenses, using an aperture ring on the lens). The smaller the aperture set (eg, f32), the greater the extent of depth of field, resulting in more of the scene appearing to be in focus.
In practical terms, a landscape scene often commands foreground to background sharpness, because everything included in the scene is important. Therefore, depth of field will need to extend over a wide distance, which dictates using a small aperture (eg, f16 or f22). However, when photographing a portrait, it is typically more important for the subject of the portrait to stand out against the background. In this example, a large aperture value (eg, f2 or f2.8) will help to isolate the subject by blurring background detail and will draw greater focus onto the subject in question.
Another reason to adjust lens aperture is to gain a reciprocal change in shutter speed. Aperture and shutter speed are inextricably linked, and any change in one setting requires an equal and opposite change in the other for exposure parity to be maintained (a photographic law known as the Law of Reciprocity). For example, if the exposure settings are 1/30sec at f22, then an increase in aperture to f2.8 (resulting in more light entering through the lens) would equate to a faster shutter speed of 1/2,000sec (to reduce the duration of the exposure), since more light equals less time (and vice versa).
This can be advantageous, for example, when the subject requires a faster shutter speed and light levels are relatively low – this might be a wildlife action shot taken in the evening when wildlife is more active, or a sports shot taken on a heavily cloudy day.
Conversely, it may be desirable to set a slow shutter speed on a bright sunny day, for example, to blur the movement of water over a waterfall. Setting a very small aperture will result in a longer shutter speed, achieving the desired effect.
Aperture values are far more than a set of numbers in an LCD panel or on a lens barrel. Understanding the effects of different apertures will lead to greatly improved photography and pictures that appear closer in print to how they were originally imagined in the mind.
Minimal depth of field, keeping just a few squares on the Rubik’s cube in sharp focus
Depth of field now extends further to bring more of the cube into focus
At the mid-range aperture, where optical quality is best, depth of field is larger and more of the image is clear
Depth of field is increasing greatly, now stretching toward the outer edges
Depth of field now stretches so that almost the whole cube has sharp focus
At the smallest aperture, depth of field extends across the entire Rubik’s cube
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Debbis passionate about all things photographic: from the latest digital kit to the greatest techniques to capture a scene. Shes been at the helm of the photography portfolio of magazines, websites and more for three years.
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