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Tuesday, November 30, 2004

Understanding Metering

Well, I'm back after a full 3 months. No I had not given up on Photography or G5! Let's just say moving and sattling into a new house and getting broadband to start working again can be very hectic. Anyways.

Here is some explation on metering and why you need to understand it.

What difference does knowing about metering make anyways ?
You mighthave noticed symbols like below on your camera's buttons or dial.

[ o ]
[ ]

They indicate various metering modes supported by your camera. Why is metering important? Well take a look at the two pictures below.

Evaluative Metering Mode

Spot Metering Mode

As you can see metering makes a whole lot of difference. If you want to stop doing Point and Shoot and stop taking pictures with built-in flash, you must understand metering modes to make sure that you don't end up with under exposed or over exposed pictures.

The Problem
Let's say you are doing Portrait Photography, the subject is still, you want to increase the Depth of Field and you don't really care for the shutter speed. So you will put your Camera in Aperture priority mode, so that you can playaround with DOF while Camera keeps sdjusting the shutter speed adjusting such that your subject looks well lit. Now, how does the camera know what is "well lit" or "well exposed" ?

The Solution
The simple thing that your camera trys to do is to achieve exposure level that is referred to as 18% Grey. What this means is, if the amount of light coming in through each spot in the frame was same (i.e. a simple plain frame) then the resulting picture should be a certain shade of grey marked as "18% grey" (let's not go into "why 18% ?"). Such an exposure would look like the pic below.

18% Grey

Fortunately, none of the scenes are boring like the grey screen above :-). Usually you get a mix of dark and light areas in any scene. The camera "mixes-up" all the brightness and comes up with a calculation for a shutter speed or aperture opening such that total exposure is equivalent to the 18% grey pic above, no matter what the scene is. This calculation is called the Metering. In aperture priority mode, it is used to select the shutter speed; and in Shutter priority mode, it is used to select the aperture.

Going back to smart stove analogy (see "Cooking and Photography"), metering is equivalent to deciding how much heat is required to cook any particular food. Different ways of doing this are referred to as different metering modes.

In metering process, Your camera "looks" at the scene, checks how much various areas of the frame are lit and decides how much total exposure (total light) is required to create the picture such that it is equivalent to 18% grey shade.

For example, G5 supports following three Meterting Modes

Evaluative Metering [(0)]
Camera divides the scene into several zones and does calculations to come up with the right exposure. In Other words, Camera gives equal importance to the entire scene to make sure that over all best exposure is achived in the entire scene.

Evaluative Metering Mode

Take a second look at the elephant image with Evaluative Metering above. You can see that as most of the background is bright, to achive 18% grey level, the camera reduces the exposure to such an extent that we lose the fine artwork details in the elephant body as they are underexposed.

Center Weighted Metering [ ]
This is similar to the Evaluative Metering, except for the fact that instead of giving equal impiortance to all areas, Camera gives higher importance to center of the image. In other words, Camera trys to make sure that the center is properly exposed and may make compromise (underexpose or overexpose) the rest of the image.

Below is the same scene with center weighted Metering.

Center Weighted Metering Mode

This time, the camera is giving higher importance to the middle of the scene i.e. makes sure they dont get underexposed. Even after giving less importance to surrounding areas (i.e. risk over exposure as they are brighter) We are getting the artwork details on the elephant a little bit underexposed.

Spot Metering [o]
In this mode, the camera only considers a very small area of the screen to decide exposure level. In G5 this area can be selected as a small part in the Center or a small area around the adjustable Auto Focus Point. The Camera makes sure that 18% grey exposure is achived in this spot area, and doesn't care for over/under exposure in rest of the frame.

Spot Metering Mode

Take a second look at the Elephant image with Spot Metering above. As the center spot was selected, which happens to be covering mainly the back and side of the elephant body, the camera tried to achive 18% grey exposure in this area. So all the artwork details have come out very well. But the camera completely disregarded the exposure level in rest of the frame. To achive the required exposure in spot area, which has a lot of black, Camera reduced the shutter speed so much that the rest of the frame is completely overexposed ! But hey, we did get our artwork details !
Important things to remember about Metering Modes
  • Metering is camera's way of deciding how much exposure is required.
  • Evaluative metering mode trys to make the whole picture properly exposed.
  • Center Weighted metering mode trys to make sure that objects towards the center are properly exposed, objects on the edges may be under or over exposed.
  • Spot metering mode trys to calculates exposure based only on what is in the selected spot area. Rest of the frame can be over or under exposed.
  • The only time you are NOT using Camera's metering is when you go in full "Manual" mode, when you have to select both aperture and shutter speed.This makes it very important that you understand metering modes properly.

    [UPDATE : In all the other exposure modes like full auto, other programmed modes, or priority modes like aperture priority or shutter priority, the camera is selecting at least one parameter for you. So it has to use its metering algorithms based on the metering mode you have selected.

    In manual mode, you are selecting everything, so you are not really using camera's metering capabilities inherently. However that does not mean it becomes unavailable. Modern digital cameras show some form of indicator regarding how you are doing based on your selected parameters, compared to cameras metering. i.e. It may show you, how many stops above or below you are with respect to cameras metering. You don't have to use it, but it's there for reference. ]
  • Pay attention to subjects in your scene and make sure that you are not losing details in the shadows. Play around with various metering modes and try to get the details right.

The Backlit Subject
Above the elephant is heavily backlit subject. And we played around with spot metering to get the details out correctly, but at the cost of washing out rest of the frame in white over exposure! Well, there is a better way to take this picture and get oth the background scene as well as the main subject (a mainly black body) without losing the fine details. We can achive this by illuminating the elephant... I am talking about firing the flash!

Evaluative metering with Fill in Flash
As we saw, we were either losing details in the elephant or in the background. But if we fire the flash, then due to the extra light, the artwork details come out and our background scene which is very bright also comes out nice! This technique is commonly referred to as "Fill in Flash" as the Flash is used to fill in the details. Compare this image with the Evaluative Metering image and you can see how the flash is filling inthe details.
This is a very simple but very effective techniques while taking pictures at Sunset or in other backlit conditions where you can't use spot metering.

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