Exposure (1): Why you shouldn’t always trust the automatic exposure
When you take a photo, the camera determines the right exposure at the time the shutter release is pressed. Unfortunately, it’s not always right: faces are too dark or pale, white snow looks grey, and atmospheric evening scenes look drab and colourless.
In this article, you’ll learn why this is the case, and the situations in which you can’t trust the automatic settings. Because even though it measures the light, there are still constantly times when exposure is faulty.
Let’s think back to what exposure actually is. Specifically speaking, it means “sensor exposure”. The sensor collects the light with all its different levels of brightness, and then converts it into a two-dimensional image.
The amount and duration of the light incidence, as well as the sensor’s sensitivity, determine how bright or dark the image appears. If everything is balanced, i.e. details are still discernible in shadows and lights, it means the exposure is correct or accurate. But how do we achieve this if the camera can’t manage it on its own?
Correct exposure depends on three settings: shutter speed, aperture and ISO value.
In automatic mode, you leave the camera to manage these parameters. It measures the light and selects an appropriate speed/aperture/ISO combination without any work on your part. Advanced photographers want to have more control, and often choose semi-automatic (A/Av or S/Tv) mode, in which they themselves set the ISO value and aperture or ISO value and shutter speed. But even here, you can’t trust the camera to set the crucial parameters correctly.
Five situations in which the camera fails
Although the camera makes the right judgements in many standard situations, automatic mode soon reaches its limits as the light becomes more challenging. Here are five situations in which even pro DSLRs consistently make mistakes:
The reason for the mistakes is simple: The camera tries to evenly distribute the tonal ranges, and thus optimises the exposure to an average grey scale value (with an 18% reflection rate). All exposure measurement systems are calibrated to this grey scale value.
This is precisely why you shouldn’t always rely on your camera’s automatic exposure. Even the smartest camera won’t see the picture as you yourself see it.
In other words, automatic exposure doesn’t mean your image will automatically have the right exposure. In order to achieve the perfect result, you need to manually adjust the exposure.
Next time your photograph doesn’t meet your expectations, correct the exposure, either positive (+1 to +2 EV) or negative (-1 to -2 EV). Over time, you’ll gradually get more experience, and be able to intuitively avoid faulty exposure.
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