Be Like Rita - Check Your TTL Meter

The Through-The-Lens (TTL) Metering System is how a digital camera light meter measures the exposure value of the scene you’re photographing. The TTL Meter has a scale at the bottom or side of your viewfinder that tells you the ideal (mid-tone) exposure value. The scale is usually divided up in 1/3 f/stop increments going up 3-stops and down 3-stops. This is helpful in understanding how much your photograph is off from the correct exposure. You can make adjustments to the shutter, aperture and ISO to achieve the exposure value you want.

Most dSLRs have a few modes for the TTL Meter - spot, center-weighted and Matrix are the most common. The spot setting is about 2% of the area within the viewfinder at the center of the focus ring. Hash marks or a circle indicate the spot meter reading point. Use the spot meter to check the exposure value, measured in f/stops, on various individual parts of the composition. For example, you’re shooting a fairly bright scene outside with a reading of f/11, but the subject you want to photograph is standing underneath a tree in the shade. By using the spot meter, you’ll see that the subject - your boyfriend - is actually measuring an f/5.6. So to expose properly for him, you would need to stop down the aperture to f/5.6. This will overexpose the rest of the scene by 2-stops by letting in four times as much light. But, the alternative is having your boyfriend appear in shadow or as a silhouette.

The spot meter is useful for finding the exposure value of specific elements in the frame where you want to set the key exposure setting. You might want to see what the street lamps are reading in a night shot, an f/3.5 for example, and then you determine that you want them to be 3 or 4 stops over the rest of the scene and still be acceptable. In this case you set the f/stop to f/1.4 and the street lamps are 3-stops overexposed. The image sensor should be able to handle that level of latitude without introducing ugly and unwanted noise to your image.

The Center-weighted mode expands the field of light measured to the center of the viewfinder frame. This is about the area around the focus ring. You use this setting when you want to better approximate the exposure value of a larger subject, say a group of friends at dinner. You want to make sure all of their faces are properly exposed. Or, for example, you’re shooting a painting in a gallery under its own light source. You want to know the exposure value for the painting and maybe some of the wall around it. The Center-weighted mode allows you to measure a select portion of the frame that you know will be the main subject where the lighting is fairly consistent throughout. But there could be some dark or light pockets that could throw off the measurement if you were using the entire frame as the reference for the exposure setting.

The Matrix mode is the broadest means of measuring the exposure. Many cameras have 7, 9 or even 12 points within the viewfinder that the camera reads and then averages for an exposure value for the entire frame. This is ideal for landscapes, most exteriors or when you know the lighting conditions are even throughout the subject, like inside a sports’ arena or your living room.

The TTL meter is critical to master in order to get the exposure you want, whether it’s normal exposure or some artistic design, like a face peering out of the darkness in a shaft of light. Whatever your intent, understanding the TTL meter is critical to mastering digital photography.

Kurt Hansen

Find out more at: High FX Photo.net