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Exposure and Stops

Now I will post some basics about Photography Theory: Exposure and Stops since there are enough people in RL asking me about this topic… The good thing is that I don’t need to reinvent the weel, Neil Creek did it for me… Thanks!

What is “exposure”.

In the simplest terms, exposure is: “is the total amount of light allowed to fall on the photographic medium during the process of taking a photograph” (Wikipedia).

Whether it is a digital sensor chip or grains of chemically dosed silver on a film, it is the same thing. The greater the amount of light that falls onto a particular region of the photographic medium, the brighter that part of the recorded image will be when reproduced, whether on screen, print or slide.

The variation of brightness in the real world is absolutely huge, much more than you might think from your subjective experience of it. A subject lit by the midday sun on a beach looks over four thousand times brighter to your camera than the same subject lit by the quarter moon! (fredparker.com)

Brightness is measured in “Exposure Value” or EV.

You might recognize this acronym from your camera’s settings or manual. An EV of 0 is defined an image exposed for 1 second at f1. Steps of one up or down from zero are a change in the light by a factor of two. So an EV of 1 is twice as bright, EV 3 is eight times as bright, and EV -2 is one quarter as bright.

Exposure variationsFig 1.4.1: From a base exposure, the exposure is increased and decreased in one stop steps to +/- 4 stops.

The “Stop”

A step up (doubling) or down (halving) by one EV is called a “stop”.

If you only come away from this lesson having learned one thing, it is this. Photographers talk about light and exposure settings in terms of stops. In photography a stop can refer to different settings in any of the three points of the exposure triangle (more below). One of the most important and useful things you can learn as a photographer is to get an intuitive feel for light levels.

I’m not suggesting that you should be able to walk onto a location and immediately be able to assess the EV of the light and determine the correct exposure settings (although some very experienced photographers can do just that!) – that’s what your exposure meter is for. However, if you can learn to look at a photo you have taken on the back of your camera, and see that the exposure needs to be increased by say 2/3 of a stop, then you will become a much more efficient and successful photographer.

Controlling Exposure

To accommodate the huge variety of brightness levels we see in the real world, we need to be able to control how much light gets to the camera’s sensor. We do this by adjusting one or more of the three points of the “exposure triangle”. These three points are ISO, Shutter and Aperture.

The aperture is an adjustable iris or opening that can be made wider to let in more light, or narrower to let in less. The shutter is the “gate” that allows light onto the sensor, and it can be left open for different lengths of time, to let the sensor collect more or less light. Finally, the ISO once referred to the sensitive to light of the film in the camera. In digital cameras it refers to the “gain”, or amplification of the information collected by the sensor. In film days, changing ISO meant changing films. Today the ISO can be easily adjusted with a dial.

The Exposure TriangleFig 1.4.2: The exposure triangle.

Each of these points will be the subject of future lessons in Photography 101. For now, you need to know that they are there, and that they all work together to control the exposure. At the centre of the exposure triangle is your camera’s light meter. It is by reading this that you determine how to set each of the three points. We’ll cover that in a future lesson as well, probably in Photography 102 – A Basic Course in Taking Photos.

Trade-Offs

Each method of controlling exposure does so in a different way, and as such, has a different effect on the character of the resulting photo. Increasing the shutter speed reduces the light, and freezes motion. Decreasing it allow more light in, but blurs movement occurring while the shutter is open. Closing the aperture decreases the light, but increases the depth of field, meaning sharp focus over more of the image. Opening the aperture lets in more light, but decreases the depth of field, meaning a narrower window of sharp focus. Increasing the ISO amplifies the light collected, but also amplifies the random noise in the chip, which can become visible in photos at higher settings.

It’s important to note that all of these effects can be used for creative purposes in photography. Having a narrower depth of field for example can be an artistic effect in a portrait, a slower shutter speed can convey a feeling of movement. Taking a good photo is the result of the conscious choice of the three points on the exposure triangle in order to get a well exposed image which has a character pleasing to the photographer. Adjusting the settings is a balancing act that affords huge creative options to the photographer.

Homework

  • Put your camera into manual mode, and find the controls to adjust each of the three exposure triangle points: ISO, Shutter and Aperture.
  • Set your camera to full auto, find various scenes, and “half press” the shutter and see what exposure settings it recommends. Now go to manual, make the settings using the manual control and take the photo. Repeat until you feel comfortable adjusting the manual settings.
  • Using the technique above, see what the camera recommends for various scenes, then adjust the photo up or down one stop of exposure with each of the exposure controls. Note the difference in the appearance of the resulting photos.
  • For those already familiar with manual control, find difficult, high contrast scenes (eg: a wall with a window outside, or under a shady tree on a sunny day). Shoot the scene on auto, then looking at the result, try to guess how much you need to adjust the exposure manually to see detail in the over or under exposed areas. Practice to see how close you can get just by estimating.
  • Apply these lessons artistically. Experiment with deliberately over or under exposing photos (high and low-key photos) or tweaking the auto exposed settings to emphasise otherwise under or over exposed areas.
  • If you wish to share your homework photos, upload them somewhere (eg: your Flickr account) and post a link to them in the comments on this post below.

Resources

This entry was posted on July 13, 2008 at 11:07 pm and is filed under Photography with tags , , , , . You can follow any responses to this entry through the RSS 2.0 feed You can leave a response, or trackback from your own site.

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