06 Nov 09: Systems & processes - Lens control

Focal Length of lenses Focal length is the distance between the centre of the camera and the back element of the lens. Within the lens housing are a series of lenses – “elements”:

A common arrangement eg in a standard / 50mm lens is 5 elements in 3 groups.  Each element has a different purpose eg perspective, colour imbalance correction, tonal imbalance correction. The overall purpose is to focus the light coming into the camera on the sensor.  In a fixed focal length lens the elements in the housing are fixed. In a zoom lens the elements in the housing move.  Typical focal lengths:

A standard lens is one that gives a perspective most close to human vision.  As you move from 50mm to 15mm a subject will appear to move further away and appear smaller, and wider coverage is achieved ( you can see more of what is in front of you) ie the angle of view increases.  As you go from 50mm to telephoto the opposite effect occurs ie you have less coverage, perspective narrows and flattens and the object seems closer.

In portrait photography, you may not want to exaggerate the subject, so traditionally a telephoto lens (eg 135mm) is considered good as it will flatten perspective.   To see the effect of using different lenses I tried some full frame portrait shots with wide angle, standard and telephoto lenses:

1.  Standard -

2. Wide

3. Telephoto 135mm

4. Telephoto 200mm

The standard and telephoto shots are much more flattering.  The wide angle lens distorts perspective, elongating the face and bringing it forward towards the viewer (Ian is not pleased at me posting this shot!).  I think the telephoto shots have the edge over the standard lens shot as they are softer. (These images were taken under incandescent lighting which can have a yellow tinge making the images very warm and I adjusted the white balance using Lightroom software to cool them slightly).

Traditionally, for landscape photography a wide angle lens is used and for sports photography a telephoto is best.

Focus
Cameras can be focused manually or using auto-focus. Autofocus can be faster, as it does it for you, you can concentrate on the composition. However, auto-focus is not always accurate and using manual focus can also allow you to be more creative. Camera viewfinders can be accessorised with an insert with a split prism to assist with manual focusing.

Lenses can be adapted using extenders or converters to achieve a wide angle or telephoto effect without having to buy a full lens.

Many cameras have multiple focus systems – it is possible to set them just to use the central spot (AF point). When focusing manually, pressing the shutter half way will light the AF spot if the subject is in focus.

Depth of field (DOF)
DOF is not directly affected by focus but is controlled by aperture. A lens can only focus on one thing at once and objects in front of or behind that object may be in or out of focus depending upon the depth of field:
At minimum aperture the point you focus on is the only point in focus – this is referred to as a shallow depth of field (SDOF). At maximum aperture there is a deep depth of field (DDOF).
The effect also depends on how far away the subject is as DOF is not equally distributed – in a landscape the focus is 1/3 of the way into the scene.  Portrait shots usually use a SDOF.
Some cameras have a DOF preview – this allows you to stop the aperture down to preview the shot.
The website http://www.dofmaster.com/  has charts you can download for your make/model of camera: