Exposure Basics

Modern digital cameras have almost made the task of getting correctly exposed photographs a no brainer. YES I said 'almost'.

So what is exposure?

Exposure is the amount of light you allow to fall on the camera image sensor or film (if you still use it). If you allow too little light, the image will be dark or 'under exposed'. If you allow too much then you'll end up with an over exposed image.

How do you control exposure?

A long long time ago, the photographer would ask the subject to sit still, remove the lens cap and keep the lens open for a certain duration depending on how much he/she wanted to expose the film. These days with fully automatic cameras you just press the button and the camera takes care of the rest. Most cameras offer some degree of control over exposure while some offer full manual override. 

Why bother with manual control?

Before we talk about it lets look at how a camera controls exposure or

'The Exposure Triangle'

1. Shutter speed: Very simple- just like our ancient photographer friend, the longer you keep the shutter open, the more light you allow to fall on the sensor. But keeping the shutter open too long could get you blurry photos because the kids move and your hands shake. Shutter speeds are indicated as a fraction of a second- so a 1/200 shutter speed simple means 1/200th of a second. Shutter speed is not always a fraction of a second and many cameras will allow you to expose a frame for more than a few seconds. Sometimes you will find 'B' or bulb setting available which means that the shutter remains open as long as you keep the shutter release button pressed - A good option for shooting fireworks!

Coming back to our main topic- if you are outdoors on a sunny day, a very fast shutter speed will do the job, but indoors or late evenings, you may need a slow shutter speed to get the right exposure. To avoid blurry photos a good rule of thumb is that minimum shutter speed should be inverse of the focal length of the lens. So if you are shooting with a 50mm lens, try not to go less than 1/50 shutter speed. What if your camera wants to use a slower shutter speed? what else can we change?

2. Aperture: Aperture is an adjustable circular opening between the lens and sensor/ film that allows more or less light on to the sensor. A large aperture and fast shutter speed could get you similar exposure as a small exposure and slow shutter speed. Now you know that you can avoid using too slow shutter speed and blurs by increasing the aperture. A 'fast' lens is a lens with a large maximum aperture. In photography aperture is expressed as F-stop number. The smaller the F-stop number, the larger the aperture. Yes - smaller F-stop number = larger aperture. An f2/8 or f/2.8 lens has a larger aperture than f5.6. Increasing aperture  also reduces the 'depth of field' resulting in an image with a focussed subject and blurred out background and foreground.

Depth of field is also a function of sensor size and focal length of the lens.

3. ISO: refers to the sensitivity of the film or sensor. Higher ISO number indicates more sensitive film or sensor that could be useful in low light conditions where you want to use a higher shutter speed to avoid blur or for sports photography where you want to capture action by using very fast shutter speeds. Higher ISO setting/film also results in more noise. An ISO 800 film will result in more grainy pictures compared to an ISO 100 film. In digital cameras, small sensor cameras (compacts) have very poor high ISO performance but digital SLRs and mirror-less interchangeable lens cameras with bigger sensors perform much better. Digital cameras with adjustable ISO setting provide a third way to control exposure.

Now that we know how exposure works, lets go back to the question on why to bother about all these settings? here's why:

1. Camera sensors are stupid: yes- they can't differentiate between a boy and a potato. They just measure the amount of light and adjust the exposure triangle to get the correct exposure for that amount of light. If you are photographing landscape with a lot of trees or an awesome black sports car, the camera is tricked into 'thinking' that its dark and it'll try to increase exposure resulting in washed out photographs. Similarly, shooting in snow with automatic exposure setting may get you dull photographs with gray snow instead of white because the camera 'thought' it was too bright and reduced the exposure. In most cameras you'll find exposure compensation controls. Try them out!

2. You want to control shutter speed: Photographing fast moving cars and kids can be a challenge that requires fast shutter speeds. Your automatic camera as I said before doesn't know a kid from a stool so you gotta step in and enter 'shutter priority' mode to pick the shutter speed and let the camera calculate everything else.

3. You want to control the depth of field: if you are shooting portrait you may want to open up the aperture to get a shallow depth of field and get nice blurred out backgrounds; but for landscapes you want to keep everything in focus and use smaller aperture. 'Aperture Priority' mode is an easy way to control aperture and let the camera do the rest.

How about going fully manual?

It's exciting and digital cameras allow you to experiment without any financial implications. Why full manual? So you have a 300mm lens and want to capture the full moon. Try all automatic settings and all you'll get is a white disc on a black background. To capture the surface of the moon, imagine photographing a desert landscape in full sunlight- that's what the moon surface is. Try ISO 100, 1/250 shutter speed and f/8 aperture. Adjust aperture and shutter speed till you get your perfect shot!

Vishal Charles

June 28, 2011

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Comment by Robert P Fryer on June 29, 2011 at 4:59pm
I love the statues..

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