Photography Step By Step

Metering mode refers to the way in which a camera determines the exposure. Cameras generally allow the user to select between spot, center-weighted average, or multi-zone metering modes. Various metering modes are provided to allow the user to select the most appropriate one for use in a variety of lighting conditions.

SPOT METERING: With spot metering, the camera will only measure a very small area of the scene (between 1-5% of the viewfinder area). This will typically be the very center of the scene, but some cameras allow the user to select a different off-center spot, or to recompose by moving the camera after metering. Spot metering is very accurate and is not influenced by other areas in the frame. It is commonly used to shoot very high contrast scenes. For example, if the subject’s back is being hit by the rising sun and the face is a lot darker than the bright halo around the subject’s back and hairline, spot metering allows the photographer to measure the light bouncing off the subject’s face and expose properly for that, instead of the much brighter light around the hairline. The area around the back and hairline will then become over-exposed. Spot metering is a method upon which the zone system depends. Another example of spot metering usage would be when photographing the moon. Due to the very dark nature of the scene, other metering methods tend to overexpose the moon. Spot metering will allow for more detail to be brought out in the moon while underexposing the rest of the scene.

CENTER-WEIGHTED AVERAGE METERING: In this system, the meter concentrates between 60 to 80 percent of the sensitivity towards the central part of the viewfinder. The balance is then “feathered” out towards the edges. Some cameras will allow the user to adjust the weight/balance of the central portion to the peripheral one. One advantage of this method is that it is less influenced by small areas that vary greatly in brightness at the edges of the viewfinder; as many subjects are in the central part of the frame, consistent results can be obtained.

AVERAGE METERING: In this metering mode the camera will use the light information coming from the entire scene and averages for the final exposure setting, giving no weighting to any particular portion of the metered area.

PARTIAL METERING: This mode meters a larger area than spot metering (around 10-15% of the entire frame), and is generally used when very bright or very dark areas on the edges of the frame would otherwise influence the metering unduly. Like spot metering, some cameras can use variable points to take readings from, (in general autofocus points), or have a fixed point in the center of the viewfinder. Partial metering is found mostly on Canon cameras.

MULTI-ZONE METERING: Here the camera measures the light intensity in several points in the scene, and then combines the results to find the settings for the best exposure. How they are combined/calculated deviates from camera to camera. The actual number of zones used varies wildly, from several to over a thousand. However performance should not be concluded on the number of zones alone, or the layout. In general, the most advanced metering is found on DSLR cameras.

With this lesson we conclude the technical part of this beginners tutorial. For anything else you want to know about please let me know. In the next lessons I will start talking about composition and other important aspects that you need to know to be able to take photos that will be appreciated. Stay tuned!

I’ve decided to talk about Exposure Time and Aperture in the same lesson because it is a strong relation between them, which I’ll explain later. First I will discuss about each of them to help you understand them better.

EXPOSURE TIME: The Exposure Time is the period of time in which the sensor will be exposed to light. This is done in the moment push the button to take the picture. A mechanism inside the camera will open for the precise amount of time that has been manually set or automatically calculated by the camera. The exposer time on a digital camera can vary in a wide range. For example, on my Canon 450D DSLR the exposure time interval is from 1/4000 to 30. That means that I can expose the sensor from the tiny amount of time that is 1/4000 of a second up to 30 seconds. This is set accordingly to light conditions where the picture is taken.

APERTURE: This is a little more complicated to understand than exposer time, but I’ll try to explain it as simple as I can. The aperture is a characteristic of lenses and it is usually specified on these as a value like 1:3.5-5.6 (on lenses with zoom capability) or 1:1.8 (on lenses with no zoom). To make an analogy with the human eye, aperture is similar to the pupil. It gives us the ability to set the amount  of light that enters through the lens by closing or opening a mechanism similar to the eye pupil. A smaller number means a wider opening so more light will come in. The numbers specified on lenses means the maximum opening of the aperture that can be achieved. For example a value like 1:3.5-5.6 (or f/3.5-f/5.6) on some zoom lenses means that we can get a maximum aperture opening of 1:3.5 (or f/3.5) on no zoom and a 1:5.6  (or f/5.6) one on maximum zoom.
Another very important aspect about aperture is the Depth Of Field (or DOF). Depth of field is a term which refers to the areas of the photograph both in front and behind the main focus point which remain “sharp” (in focus). Depth of field is affected by the aperture. A larger aperture (smaller f-number, e.g. f/2) has a shallow depth of field. Anything behind or in front of the main focus point (the main point that you want to be sharp) will appear blurred. A smaller aperture (larger f-number, e.g. f/11) has a greater depth of field. Objects within a certain range behind or in front of the main focus point will also appear sharp. Here is an example:

The relationship between aperture and exposure time is that a smaller aperture will result in less light that will enter through lenses, so a longer exposure time will be required and vice versa. So you can achieve the same luminosity in a picture using different aperture/exposure time combination. The main reason for playing with these two is again the DOF (Depth of Field). There are many situations when you want a shallow DOF (so a blurred background), like in portraits, macros, etc, and there are situations when you’ll prefer a greater DOF, for example in landscape photography. To master this two in combination will require time and a lot of exercises, but I assure you that it worth the effort.

Digital cameras have an ISO rating indicating their level of sensitivity to light. The normal setting for most cameras is ISO 100, although some go as low as ISO 50. The sensitivities can be increased to 200, 400, 800, or even 3,200. High-end digital SLRs also allow some intermediary values. When increasing the sensitivity, the output of the sensor is amplified, so less light is needed. Unfortunately the undesired noise is also amplified. This creates more grainy pictures, just like in conventional photography, but because of different reasons. It is like you will turn up the volume of a radio with bad reception. This will not only amplify the desired transmission but also the undesired noises. Lately a lot of improvements has been done in sensor technology to reduce the noise level at higher ISOs, especially on higher-end cameras.

Here is an example of how ISO level influence the final image:

So your question may be: why should I use higher ISO level if it adds so much noise in the picture? The answer is very simple. A photography with some noise in it is preferred to a blurry one. When you press the button to take the picture, a mechanism in the camera opens and expose the sensor to light for an amount of time (it is called exposure time). The lower the light level is, the greater the exposure time will be. If you are taking the picture from your hand, a longer exposure (how long? this will discuss in further lessons) will result in a blurry picture because of the uncontrollable shake of hands or movement of subject. In this case, if you can’t control the level of light, the only thing you can do is to increase the sensibility of the sensor (higher ISO level) which will require less light for the picture so a lower exposure time.

In conclusion, the different ISO levels makes the cameras more versatile, allowing us to use them in different light conditions, but with the disadvantage of adding some noise into the final picture.