How to manage exposure using your camera

Some of us might have wondered why some of our shot turned out quite bad such as overexposed skin tone or the underexposed sky. While there are a number of factors affecting, it will be good to know some of the camera limitation as well as the type of function that the camera offered in order to get a well exposed picture.

Limitation
Unlike our human eyes, a camera can take a certain amount of dynamic range. A dynamic range refers to the range between the highest and the lowest value in a single frame. Dynamic range in digital camera is captured by individual photosite that makes up the sensor and has a value from 0 -255. Think of this individual photosite as a bucket. When the amount is full, it overflows. Hence details are lost. When the value has exceeded, then the user has to decide which part of the scene should have the right exposure. You can’t have the best of both worlds unless you take a series of shot and use a technique call high dynamic range imaging to achieve it. Dynamic range can be reflected in histogram to tell you the range brightness and darkness area in the scene. Most cameras are calibrated to 18% gray tone. So no matter which type of metering you are using, it will always be given the standard 18% gray reference point in any shooting scenario. You can override them by switching your camera to manual mode and select the shutter speed and aperture that can give you an accurate exposure.

Having explaining the dynamic range and metering, you probably might wonder what the link between these two. The histogram contains a range of brightness and darkness value. Divide the histogram into half, and you will have you will have your dark area at the left side of the histogram and the bright value at the right side of the histogram. If you overexposed it, then the histogram will be biased towards the right side. If you underexposed it, the histogram will be on the left side. The thinking part of the camera will either controlled by the camera or manipulated by the user. You can use photo manipulating software to help you correct the dynamic range.

The camera’s functions
When you purchased your camera, you are not sure by the unfamiliar logo found on your camera. I shall touch on some of the common functions that are related to exposure.

1. Metering
There are different types of metering available on the camera. Spot/partial metering allows you to meter a small area of the scene. The difference between these two is the amount of area metered. Partial metering has a slightly larger area compared to spot metering. This form of metering is useful when you have a contrast scene such as a back lit subject or dark skin tone subject and a white wall. Centre-weighted metering takes reading on the centre of the frame while ignoring the surrounding bright and dark area. This is useful if you are interested in metering the centre of the frame while not including the edges of the frame. Evaluative metering divides the scene into different segment and then compute the metering based on the selected segment using the camera’s intelligence. While this metering is sufficient for most cases, due to the unpredictable selection, your shot either come out overexposed or underexposed. Hence it should not be used in area where there are a lot different exposure values.

2. Exposure lock
This function allows you to lock a specific exposure value when you want to expose a specific area of the scene. Depending on the brand of camera you are using, different manufacturers may assign different logo to indicate exposure lock.

3. Exposure compensation
This comes in the form of +/-EV in your camera. This allows you to deliberately overexpose or underexpose a selected scene. Usually this form is only available in most scene mode except in manual mode.

4. Histogram
After you have taken your shot, it is time to take a look at your histogram to determine whether your shot has been properly exposed. Unless you know which area to be exposed, a well exposed scene generally should have a range of brightness and darkness contained in the histogram.

5. Exposure bracketing
Exposure bracketing allows you to take a series of different exposure shots: underexposed normal and overexposed. You use that function if you are unsure of what values are needed to get a well exposed picture. You can also use it to create a series of shot that allows you to achieve high dynamic range imaging which can be done in photoshop.

6. Manual selection of aperture and shutter speed
Some cameras offer manual selection. To know whether your picture is properly exposed before you press your button, there should be an indicator usually in the form of +/-EV bar. If the slider is at the left side of the bar, your picture will be underexposed or vice versa. You can ignore the bar and choose the settings you think will be the correct exposure.

So now that you knew that there are so many ways to manipulate your exposure, why can’t I set auto in my camera and let the camera’s intelligence to decide. That is because no system is foolproof!! No matter how advanced the technology is, there is always a loophole in everything. It is a matter of how to overcome its weakness so as to maximize it strength. While auto mode works fine in most scenarios, there are certain situations which will fool the camera. Here are some of the scenarios:

1. Taking under a bright sunlight will certainly leave some shadow at certain part of the object. This will give the camera a range of brightness and darkness value which the camera has to decide which area should be chosen to give a correct reading. If you leave your camera in auto mode, you may get some good result. In this case, you can use fill-in flash to take away the shadow.

2. If there is too much white in the scene the camera will either underexposed the scene. The camera will then be fooled into thinking there is a lot of light in the scene. Hence, it thinks that underexposing the scene will give a good exposed picture. The reverse is also true when there is a lot of dark area. In this case, switch to manual mode to manipulate your camera setting.

3. If the contrast is high for a particular scene, the camera will unable to decide which area to be properly exposed. For example, if you have an area with a lot of white area and an area with a lot of dark area, most likely the dark area will lose a lot of details. In this case, you can use a spot metering to meter the dark area or you can shift your object to a less contrast area.

I have written most of the stuff that you need to know to manage your exposure. Hopefully it can help you to understand better.