UNDERSTANDING EXPOSURE – The Essential Lesson

17 01 2010

I’ve decided to stray from the business tips that this section usually offers to address a critical issue that I have been approached about regularly by up-coming photographers in the past few months.

This being the fundamental basics of understanding how the camera works and why advanced cameras have ISO, Aperture and Shutter Speed settings and what the difference is between manually changing these settings and letting the camera automatically adjust them.

My advice when getting a new SLR or advanced compact is always to switch it to Manual and experiment with it so that you have a proper feel for its capabilities and restrictions.

Learning these basics seems difficult when starting out but with this knowledge you’ll be able to set your camera instinctively to get the type of shot you want and have an overall understanding of its’ capabilities.

I initially tried to just find an article that was simple and concise that explained these basics using terminology that an average person would understand but I’m still looking, so here’s my explanation:

ISO (ASA), Shutter Speed (or “Time Value” for Canon users) and Aperture are the 3 primary variables on your camera which contribute to a correct exposure, each one when used affects the overall image so it’s important to understand the relationship between them. An overexposed image is white, an underexposed image is dark. There are no “correct” exposures; this is simply a guide to understanding how to control them.

ISO (ASA) refers to the sensitivity of the camera sensor (digital cameras) or film.

As your sensitivity level raises (ISO value increases), your image will be more exposed (lighter).

Shutter Speed refers to how long the shutter is open when taking a shot.

The shutter opens to allow the image to be captured by the sensor, how long it is open determines how much light gets in.

Shutter speed values are often measured in fractions of seconds, so a “higher value” on your camera might mean an increased speed which results in less light.

Aperture Value refers to the amount of light that a lens allows into a camera.

As the lens is “opened” it allows more light in.

Aperture values are also measured in fractions so the “higher” aperture settings mean that there is less light being allowed in. Measured in “f/stops”.

By understanding the relation between the 3 settings above one can quickly adjust the settings based on the type of image to be taken.

A good analogy is the one of sun-tanning:

The ISO would be the sensitivity of your skin, some people have a low sensitivity and won’t burn (expose) easily. Others have a high sensitivity to light (high ISO value).

The Shutter Speed relates to the length of time your skin is exposed to the sun (light), a long period will increase exposure.

The Aperture would be the sun-block level. A high sun protection factor (f/stop) would allow less light to expose the skin.

A picture depicting an open (low F number) and closed (high F number) aperture.Similar to SPF factors in sun-block, a high “f/Stop” number will allow less light to penetrate.

There is of course a 4th variable which is the intensity of the light source, which in this instance would be the sun.

If any of the 4 factors are altered then one or all of the others would need to be equally adjusted to ensure the EV (exposure value) remains the same.

When your camera is set to shoot on “pure” Manual then you can adjust each of the 3 settings individually based on the strength of the light source (whether light is from the sun or an external source such as a flash). Be sure to check that “auto ISO” is turned off.

Once you understand the relationship between all three settings you will find that with the same light source (in this example F13) it’s possible to get a similar exposure from numerous different settings: e.g.

ISO 100 + S/S 200 + F5.6 will give you a similar exposure to ISO 400 + S/S 100 + F16 or ISO 200 + S/S 800 + F4

You will soon find that there are numerous setting options available to create the same exposure; now all you need to do is decide what settings work best for the type of image you need to create.

An interesting do-it-yourself chart can be found here http://www.stacken.kth.se/~maxz/files/jiffy.pdf

Now that you have these varied setting options available to you it is possible to adjust your settings according to your subject; each setting has its’ own set of uses and restrictions:

Freezing action: You may want to increase your shutter speed in order to “freeze” moving objects, a slower shutter speed will experience motion blur. Often a controlled slower shutter speed can give life to the image however.

Depth: Lower f/stop values (aperture) create a shallower DOF (depth of field), often used to blur out subjects in the background of an image. Similarly zoom and sensor size can also contribute to this phenomenon. Higher f/stop values are necessary when everything needs to be in focus. Very low f/stop values are often only achieved with specific lenses, primes and professional series lenses.

Low light: Usually high ISO settings are required for low light conditions when very low shutter speeds are not possible (i.e. moving subjects, no tripod etc). A high ISO will allow enough light to get a shot that would otherwise be underexposed. Generally however as you raise your ISO level the image quality tends to degrade, often becoming “noisy” (think of the example above, sensitive skins exposed to sunlight often develop freckles). Some camera ISO ratings can already go as high as 102400!

To test the effect of different settings I suggest doing the following:

In daylight (with a digital camera) set your ISO to 200, your Aperture to F8 and adjust your Shutter Speed previewing every image as you go.

Now set your Shutter Speed to 400 and adjust your Aperture in the same way, finding a correct exposure but also exploring the extremes (over-exposed and under-exposed).

Do the same with your ISO.

Now move into shade and do the same experiment.

By doing the above a few times you will quickly get used to the relationship between the settings and how they affect the exposure.

A light meter will advise what settings to use based on the amount of light on your subject, but an understanding of the above variables will enable you to use the meter reading to obtain different results.

In some situations you may find it impossible to achieve a satisfactory image using just your Aperture, ISO and Shutter Speed…

For example: you’re shooting your children playing on the swings in the garden at sunset, you cannot reduce your Shutter Speed too much as they are moving quite quickly and you want a sharp image, your Aperture is open as wide as possible and pushing the ISO level causes too much noise. In this situation you may need to use flash or switch on the garden lights to increase the light on your subject to an acceptable level in order to reach a suitable exposure.

Don’t be shy to experiment though, try to use a tripod (or rest the camera on something solid) and reduce your shutter speed considerably to take in as much light as possible. Still objects will remain sharp while moving objects will blur.

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