What is Focus modes of Digital SLRs

Another major area of difference between a compact digicam and a digital SLR is in the ability to select focus modes. Whereas a compact camera focuses on a subject and then captures the image, a dSLR can easily shoot in Continuous Focus mode, tracking a moving subject and keeping it in focus until, and even after, you press the shutter button. So, there are two types of Focus mode.

1. Continuous Focus
2. Single Shot Focus

Continuous Focus:

Continuous Focus is designed for sports and wildlife photography where the subject is moving and the photographer is panning the camera along with the subject prior to capturing the image. This mode works on most cameras by pressing the shutter button halfway to activate the metering and focus tracking. When you're ready to capture the image, press the shutter the rest of the way down.

Single Shot Focus:

Single Shot Focus mode locks onto a focus point as the shutter button is pressed and then stays there. Single Shot Focus mode doesn't release the shutter until the camera has focused on the subject. As long as the shutter remains pressed halfway, and the subject hasn't moved, it will remain in focus.

What is Metering Mode of DSLR cameras

Before getting into the different metering modes, it's helpful to understand how a camera light meter works. When you press the shutter button part way, the camera activates an internal light meter, which reads the light values coming into the camera to determine what the proper exposure is. It works this bit of magic by considering the subject as middle gray. How much of the subject is included in this measurement is determined by the mode you select. All dSLRs have some form of evaluative metering and center-weighted metering, and some dSLRs also include a spot-metering mode. So, there are three kind of metering modes.

• Evaluative metering
• Center-weighted metering
• Spot metering

Each of them will be discussing in the next posts.

Focus Modes in DSLR cameras

Metering Mode in DSLR

How Evaluative / Matrix metering works in DSLR

Evaluative metering is called matrix metering in the Nikon world, honeycomb by Sony, ESP by Olympus, and segment by Pentax. By whatever name, evaluative metering works by dividing the image area into sections like those in Figure. The number of segments varies widely from one camera to the next [from 14 to 1005].

The meter reads the light value from each of these segments and averages them out to obtain the best overall exposure, which is the one that keeps the most information within the range the sensor is capable of recording, from dark to light. This is one area in which dSLR cameras really excel over their compact digicams. Because the dSLR sensor is much larger, it's able to record a wider range of light values

How Center-weighted Metering works in DSLR Cameras

Center-weighted metering concentrates the measurement of light on the central area of the image area. This allows you to get a more accurate reading for a critical portion of the scene and works well when there is a fairly even range of light values in the scene.

A derivative of center-weighted metering is center-weighted evaluative metering, where the entire image area is considered when determining exposure, but more emphasis is placed on the central area of the image.

How Spot Metering works in DSLR

Spot metering, typically available on mid-level and pro-level cameras, measures only a specific area of the image, often as little as 3 percent of the total scene, to determine exposure. On some cameras, the spot meter is fixed on the center of the viewfinder; on others, such as the Canon, you can set the meter to read from any of the auto-focus points.

Spot metering is the best choice when you have a specific area of the image that is critical for exposure. An example of this would be photographing a person against a very bright or very dark background, a situation which causes evaluative metering to over- or underexpose the main subject. If your camera doesn't include a spot-metering mode, you would choose center-weighted in such an example.