Photography Tutorials
How the camera works
Types of camera
Exposure
Aperture and depth of field
Focus sharpness shake and blur |
Aperture and Depth of Field
As mentioned in previous pages, the Aperture is the hole through which light passes on its way from the lens to the sensor and is adjustable to control the quantity of light that reaches the sensor and aiding in perfect exposure.
Adjusting the Aperture also however changes the Depth of Field in the image being created, this is how much of the image seems sharp or in focus. The larger the aperture(lower f-number), the less of the image will be in focus but more light will be collected by the sensor in a shorter space of time reducing exposure length.
The smaller sensor sizes in modern digital cameras mean that a lack of focus (or blur) in an image is very hard to achieve without a very wide aperture, in fact with a compact camera the sensor is so small it is almost impossible to get blur in an image. However with the larger sensors in mid to high end DSLR's it is possible to create sharp and unfocused areas of an image. |
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At f-1.8 only a small amount of the wicker back is in focus the foreground and background are completely blurred our attention is drawn to the area of focus. |
At f-22 we now see virtually all of the foreground and background in the image. |
Still at f-1.8, as we move back (effectively zooming out), more of the chair seems to be in focus in fact the same amount of the image is in focus, there is just more of the chair in the focused area. |
The same shot at f-22 to complete the comparison. |
Why would you want to do that? you may ask, portrait photography is a good example of why! The central theme of a portrait photograph is the person in it, other things in the scene will distract the viewer from looking at the person. It is not always possible to completely isolate an object, be it a person, flower, vintage car, or anything else for that matter from its surroundings, but a low depth of field allows us to lower the impact of other items which we do not want the viewer to focus on.
So what about the other side of the coin? What if I want everything to be in focus?
Say for example you we want to take a picture of a large garden in a stately home, a low f-number (wide aperture) will be of no help as you will want to take in every last part of the garden to show it to its full potential, here you will need to close the aperture right up, which unfortunately increases exposure time to the point where a tripod becomes essential to avoid blur caused by hand shake and body movement. where as with a portrait style photograph in which you may use f-2 or lower, in this situation you will be aiming for f-22 and higher.
Using a wide aperture allows us to create an area of focus and an unfocussed area so this is a "portrait" of the chair. |
A smaller aperture gives us more definition of the context the chair is in, the dead tree in the background now competes with the chair for our attention and we see more of the whole picture. |
The effect of zoom on depth of field:
Simply put the more you zoom in the lower the depth of field becomes (it gets flattened) so if you want a really low depth of field zooming in can actually help, but don't zoom if you want to take a high depth photograph!
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