Photography is an art, plus science. Whilst everyone likes the art part, not all love the science behind it. Most beginners in photography use the Program exposure mode offered by their cameras.
While there is nothing wrong in that, the question is, why do they use the Program mode? There could be two reasons:
1. The Program mode lets the user concentrate on the picture taking rather than getting involved
in technicalities.
2. The user is not scientifically inclined, and is therefore not very comfortable using the other modes.
This article is designed to overcome this fear psychosis of those who fall into the later group.
We need to set 3 parameters on our camera before we can take a photo.
1. The aperture (which controls the quantity of light hitting the film/sensor)
2. The shutter speed (which controls the duration of the light that is allowed to act on the film/sensor – and hence indirectly, in combination with the aperture, the quantity – that is allowed to act on the film/sensor)
3. The ISO (the sensitivity of the image recording medium).
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ISO sensitivity: We first need to set the sensitivity for the recording medium we are using. After all, unless you tell the camera how responsive the recording medium is to light, it cannot guide you about the amount of light (that is controlled by the combination of the aperture and the shutter speed) that should fall upon the recording medium.
Aperture or shutter speed: The next step gives the user a choice: Either set the aperture or set the shutter speed. If you set the aperture, the camera exposure meter will guide you about the shutter speed to use; if you prefer to set the shutter speed first, the exposure meter will guide you about the aperture. If you set the aperture and the camera dictates the shutter speed, you are said to be using Aperture Priority mode(Av). If you set the shutter speed first and the camera shows you the aperture to use, you are said to be using the Shutter Priority mode(Tv). You may call these two modes, semi-automatic, if you like, because part of the decision is taken by the camera.
But what about those users who like to be in total control? Well, there is an exposure mode for them too. It is known as Manual exposure mode(M). In this mode, the user sets the aperture as well as the shutter speed. You have a choice: first select your aperture or select your shutter speed. But what should be the basis of this selection? If you want greater depth of field (zone of sharpness), you select a narrow aperture (like f/11 for example) or opt for a wider aperture like f/2.8 if you want a very shallow DOF. If stopping action is what you want, you first select a fast shutter speed (say 1/1000 sec) or select a slow shutter speed like 1/4 second if you wish to show movement (like flowing water).
If you first decide your aperture, the next step would be to find the corresponding shutter speed.
If you first decide your shutter speed, the next step would be to find the corresponding aperture.
You do the second step by what is known as ‘nulling the meter’. After setting the aperture or the shutter speed as desired, you bring the cursor to zero position on the manual exposure scale in the viewfinder or the status LCD.
The Exposure Meter: As mentioned earlier, once the ISO sensitivity is set, the exposure meter is your guide that advices you as to what aperture/shutter speed to use. In other words, it tells us the exposure to use under a given lighting condition. There are different types of exposure meters (within the camera) that you should know about:
1. Evaluative/Matrix/Multi-segmented
2. Center weighted
3. Center-weighted average
4. Partial
5. Spot
Evaluative/Matrix/Multi-segmented/ Honeycomb/Multi-zone metering
This is a very complex metering system wherein the metered area is divided in various segments (called Matrix. Plural- Matrices). If a particular segment covers an extremely bright or extremely dark area of the scene (which can cause the exposure to go wrong), that particular segment will temporarily go dead. It stands to reason that if the error-causing area is not ‘read’, the exposure cannot go wrong. The meter then checks the remaining segments and depending on the contrast and brightness, selects a exposure (from many thousands that are stored in the camera’s data-base) that closely matches the contrast and brightness of the metered area. Note that most manufacturers do not give away their secrets, and as such, the explanation given here could defer.
Evaluative metering is considered to be an all-round solution for most day- to- day metering situations.
Center-weighted metering
In this type of metering, greater emphasis (about 60-80 percent) is placed at the center of the focusing screen. In other words, this type of exposure meter reads more from the center of the screen and feathers (tapers) its sensitivity as it nears the corners.
Center-weighted metering is useful when the subject and background tones differ a lot.
Center-weighted average metering
Center-weighted average metering is used by Canon. Like the center-weighted meter, it too has greater emphasis at the center, but then averages the reading with other brightness in the scene.
Partial metering
This type of metering is somewhat similar to spot metering but about 10-15 percent of the frame (at the center) is measured. Partial metering is used in Canon cameras.
Spot metering
As the name suggests, a very small area (typically 1-5 percent of the screen area) in the center of the screen is used for metering. This allows for pin-pointing an area for metering. It is worth noting here that while spot metering is the most accurate form of metering, it is also the most difficult to use since it involves subjective judgment from the user
(more on that later).
Also note that the area covered by the metering circle will depend on the focal length of the lens. Let’s say that the metering circle covers 2 degrees. When using a 24mm wide-angle lens, the area covered will be much smaller when compared to the area covered by the same meter when a 200mm telephoto lens is used.
Think it Over
Even though the camera provides us with an exposure meter, exposures often go wrong. Have you ever wondered why? Most people in this category would like to blame the camera but actually, the reason is your lack of understanding how the meter works.
How Does it Work?
An exposure meter (also known as a ‘light meter’ or just plain ‘meter’) is designed to turn whatever you point it at (a single tone, not a multiple of tones) to a mid tone. This means that if you point the meter to a dark blue tone, the result will be a mid-blue tone. If you point it to a light blue tone, the result will again be a mid-blue tone. And of course, if you point the meter to a mid-blue tone, it will remain mid-blue! Similarly, if you point the meter to a dark green tone, it will become a mid-green tone. If you point it to a light green tone, it will again become mid-green. If you point the meter to a white tone, it will become middle-white (gray); if you point it to black, it will become mid-black (gray). And of course, if you point the meter to mid-gray, it will remain mid-gray!
Now that you know the secret of the light meter, you can meter off whatever tone you desire – just remember to apply the required correction.
How do I apply that correction?
Let’s say your subject/scene has a variety of tones (it always has!). If you meter off an area that seems mid-gray to you, no correction is required. If you meter off a really white tone, you’ll need to ‘open up’ by 2-stops. If the tone you meter from is light (not as white as ‘white’), you will have to ‘open up’ by 1-stop. Similarly, if you meter from a black tone, you’ll have to ‘close down’ by 2-stops; close down by 1-stop if the metered area is dark (but not black).
Now let’s say you are photographing a plant that is completely green. Some areas could be dark green, some areas could be light green, while some areas may be what you would call mid-green. From which tone should you meter? From any tone you like! Just remember, if you meter from dark green, you may need to ‘close down’ by about 1.5 stops; if you meter from light green area,
you may need to ‘open up’ by may
be 1-stop.
Remember
If you want your image tone to be like your subject tone, apply the following corrections:
A note on Spot metering
Tone Correction
Black, dark blue, – 2 stops
dark green,
dark red etc.
Dark (but not very – 1 stop
dark as the above)
Mid-tone Nil
Light, but not white + 1 stop
White, extremely + 2 stops
bright colors
Why is it suggested that beginners should stay away from spot metering? The reason is simple. After taking a meter reading, you need to know by how much you need to modify the exposure. But doesn’t that hold true for all types of metering? Yes it does, but the degree of error with the spot metering can be greater.
Let’s simulate the effect of the three widely used in-camera exposure meters using this example of a bird:
Evaluative
The light being soft, the evaluative type of metering would have no problem in ascertaining the correct exposure. The bird itself has enough contrast and the evaluative metering would compare this contrast and brightness with its in-house stored data and pick up the one that closely matches the scene.
Center-weighted
Greater metering emphasis would be applied to the center of the frame. The green grass and part of the bird (light brown tones) would be medium tone. The remaining dark areas and the whites would average out (more or less) to a mid-tone. Depending on how much weightage is placed in the center of the frame (60 percent or 70 percent or 80 percent), the final exposure may
differ slightly.
Spot metering
If you were to take a spot meter reading from the green grass or the mid-brown (marked A) near the eye (both mid-tones), you need not apply any correction.
If you were to take the spot meter reading from the white (B), you would have to open up by about 1.5 stops (otherwise the whites would turn gray, indicating underexposure). If the bird was in sunlight, and you metered from the whites, you might have had to open up by 2 stops.
If you were to take the reading from the dark brown area in the center of the bird (C), you would probably have had to close down by about 0.5 stop.
If you had to take the spot meter reading from the orange legs (D), you would have had to open up by may be 0.5 stop.
So you see, you can take the spot meter reading from any area you wish, as long as you know what correction to apply, if any. And believe me, it is not difficult. Try it out.
Rohinton Mehta
