Focusing and Metering Modes

Other than Manual focus, there are three focussing modes on my camera: Auto, Single Servo and Continuous.

Single Servo is the most common mode, and the one I use the most. With this the camera uses one focus point in the middle of the frame, allowing the photographer to decide on one single focus point that he/she deems more important than all other objects/people in the frame. The good thing about this, is it allows one to select said focus point, then recompose the shot. This is done by placing the spot onto our desired focus point, pressing the shutter half way, then moving to recompose before pressing it.

Continuous is largely used for moving subjects and/or when the camera needs to be 'panned' to follow the subject, as- once the shutter is pressed half-way and held- the focus will lock until said subject has left the frame.

With Auto-Focussing, the camera will choose its own focus point based on the scene, whether the subject(s) are stationery or moving. The problem with this, however, is-  much like the problem of choosing Auto as the overall exposure mode- you are basically allowing the camera to make its own decisions. If I was shooting a group of people scattered throughout the frame at an event , can I really trust that the camera will choose the one I want in focus?

Metering Modes

The metering mode refers to the system the camera uses to determine the 'correct' exposure for a given scene. A good exposure is one that contains an equal amount of tones across the whole tonal range, from perfect black through to perfect white. Ideally, we should be able to know whether an image has been exposed correctly by the the light metre provided on the view finder, a picture of which can be found below.

The numbers on the scale denote how under or over exposed an image is, as the centre represents a 'perfect' exposure according to the camera's metering system. This will read differently for each metering mode, for the camera is taking information from differing parts of the same scene.

There are normally three main metering modes on most of today's dSLRs : Spot, Centre-Weighted and Matrix.

Spot metering is the one I use the most. This is when the camera uses a very small area in the scene to determine the exposure (on my Nikon D3100, this is said to be "about 3.5mm in diameter, or about 2.5 percent of the frame") (1). This is most useful for situations where the subject is unlikely to move (e.g. still life). So why do I use this for near enough everything then? I suppose its because I deem the subject the most important part of the scene regardless of where it is set. I suppose, when using a shallow depth of field, you've already decided on a key focal point anyway. My camera is set to this mode all the time because I like to metre for the subject and not the scene. Not that I take much notice of the above scale. The main drawback of this, however, is- when we have a scene containing a mixture of varying tones (darks and lights)- the camera will struggle to interpret the subtle changes across the whole scene.

The symbol for Spot Metering.

The Matrix metering mode measures exposure by calculating an average of all the tones from the whole scene based on the information contained in the different areas of said scene. So if a scene has a mixture of lights and darks in it, it might be an idea to choose this mode. For this reason, this is often known as the default mode (for beginners?) due to the fact it places most of the decision-making in the hands of the camera.

Matrix symbol represents how the scene is divided into separate segments. After all, there has to be some science behind it.

Centre-Weighted is similar to Spot, in that it is measuring the exposure from one part of the scene. However, in this case the area is bigger and always in the centre ("Nikon D3100's Center-Weighted metering mode gives a weight of 75% to an 8mm diameter circle in center of frame") (2). With more sophisticated Nikon models users can widen the area the camera uses to metre, taking care of any subtle changes in tone outside of the centre circle.

Great representations, these symbols. The space metred is just like the Spot only larger.

1)  http://www.dummies.com/how-to/content/how-to-select-an-exposure-metering-mode-with-a-nik.html

2)  http://www.imaging-resource.com/PRODS/D3100/D3100A5.HTM