Depending on the chosen aperture (f-stop), the focal length and the focus distance you have a certain depth-of-field (DOF). The DOF defines a zone around the chosen focus point where objects appear to be sharp. Beyond the (floating) b order of this zone objects get blurry. Typical beginners tend to think that a good picture is a picture where everything is sharp so often small aperture values are preferred in order to maximize the depth-of-field. This assumption works fine in some photographics scenes but it's certainly wrong as a general shooting philosophy.
The following picture (large aperture) isolates a small zone around the focus plane.
The next picture (same focus plane) was shot with minimal aperture to achieve max. depth-of-field.
The next shot (large aperture) isolates the foreground ...
... whereas this one prefers a different focus plane (same aperture).
As you can see we have lots of potential creativity just by selecting the aperture and focus plane.
Focal-Length and Perspective
There's a bit more about the focal length than just the coverage of a certain angle of view - there's always a distinct effect on the relationship between the objects within a scene.
Let's have a look at a relatively wide focal length first: 28mm. The following image samples show 4 trees with an equal distance between neighbor trees. At the wide setting it seems that this distance actually increases dramatically towards the foreground (exponential behavior of the distance). In other words: the tree to the left seems to be totally separated from the rest of the gang. The background seems to be far in the distance.
The next picture has a more natural view at about 50mm. The perspective is obviously much less extreme. Due to our real life experience we can guess that the distance between the trees is roughly the same though the separation is still visible (d^2 behavior).
Now we have a 100mm lens. The trees seem to group here with a seemingly small distance between the trees. Compared to the previous sample the now enlarged background suddenly moved towards the main object. The scene is compressed now.
At 200mm the effect increases even more. The group of trees seems to be virtually on the same distance plane. The background may be blurry (due to the small depth-of-field) but it seems to be just a few meters away. We speak of a "flat" perspective in this case.
An object can be separated from its environment by various methods. For example, you can use a very wide lens to sort the scene into distinctive layers. However, while you separate the object the environment is still visible which may be disturbing because -say- the background is very ugly. Sometimes there's a workaround for this problem: we choose a very small depth-of-field so only the main subject is in focus while everything in front or behind the focus plane gets blurry and therefore virtually unimportant. Have a look at the 1st sample below. The blue marble to the right sucks the view from the first look. This is a natural reaction because the brain scans for the most contrasty subject first. The isolation of the object due to its "outstanding" sharpness is very significant here.
by Michael Wagner
Wanna see a perverse example. Imagine being in the Himalayas at a nice sunset and all you shoot is a beautiful rose. The result is not all too bad I think!
A small DOF is also a common technique for portrait photography. Usually it is quite difficult to find the right balance between people that are chosen to be the main subject, and their environment. A sharp background is often distracting here so a large aperture should be used to focus the attention on the point of interest.
by Randhir Amoganathan
A small depth-of-field can separate a subject quite easily from the surrounding but sometimes this is no option because we either operate at very long focus distances or we simply want to include a sharp focused environment in the scene. Nonetheless we often still have the option to stress the importance of a certain object.In the first picture we wouldn't gain anything by choosing a large aperture because the fore- and background doesn't contain any disturbing objects. The bath-tubs are naturally isolated by the difference in brightness and the interruption of the uniform environment. A similar scene is e.g. a boat on the wide open sea.
by Horst Schneider
The scene itself may be quite dramatic and in the first moments your attention may be drawn to the impressive mountains in the background. However sooner or later your view will get focused on the fisherman in the middle of the river because the contrast difference between him and his surroundings is the highest in the otherwise low contrast environment.
by Horst Schneider
A major problem with many presentations is an overwhelming usage of wide-angle lenses. Monotonous perspectives are poison so just add some dynamic here and go for a medium tele lens. This may be an usual thought but once you get used to the idea that typical portrait lenses like a 85mm or 135mm are good for more, much more. In contrast to the depth provided by a wide-angle lens it'll compress a scene or isolate or special object.The first picture illustrates how a tele lens can compress the several layers of the scene to a relatively flat perspective with a seemingly small distance between the layers.
The next picture shows that this also works pretty good for landscape shots as well.
Very often you've a major landscape feature like a spectacular mountain which will degenerate to just another rock in a picture. After the first look the viewer has already sucked all the information of the scene so there's not much left than the raving comments of the photographer.Have a look at 1st picture below. The volcano in the background is still the dominating feature of the landscape but there's more. We've two additional layers - the foreground and the center - both not too impressive but it adds a certain kick to the scene. It is quite the same with the 2nd picture to the right. None of the objects is very special and probably not worth a picture. However, the layered scene as a whole is quite beautiful with the fuzzy mountains at the horizon, the rocks in the blue sea and the foreground with its fine structures.
by Horst Schneider
The two pictures below show the effect of a single additional layer. The left picture may show an impressive mountain but nonetheless it looks a bit empty - the right picture looks more interesting.