007 - Exposure For Beginners

So finally we get to looking at exposure.  I've been putting it of for a while because I wasn't too sure where to start. Let's try looking at it from this angle, a camera needs light hitting the sensor to be able to record what it can see through the lens. Depending on where you are, what time of day it is, the weather conditions or the lighting conditions if you are inside there will be differing amounts of light available to the camera. The camera needs a certain amount of light hitting the sensor to produce an acceptable image and this is the exposure.

Given that in these differing amounts of available light the camera wants to get sufficient amount of light to get a good exposure, how does the camera control the amount of light? I think in my previous posts I've hinted at some of the ways this is done but essentially it uses what is termed the exposure triangle. The three things the camera can use are the aperture size, the shutter speed and the sensitivity.

The aperture is how far open the lens makes the hole that it lets the light into the camera which it controls using the aperture blades. A tiny aperture will obviously let less light in than a wide aperture, but the aperture also controls the depth of field, or in other terms. how much of the scene is in focus. A small aperture will give a wide depth of field and a large aperture will give a narrow depth of field. As we have discussed in earlier posts we would most likely want a wide depth of field for landscapes and probably a narrower depth of field for portraits.

The shutter speed is simply how long the shutter stays open and it follows that the longer it is open, the more light it lets in. For action shots we would probably want the aperture to stay open a very short length of time to freeze the action. For normal hand held photos we would want the shutter speed to be fast enough to prevent camera shake. If we're using a tripod the shutter speed is less important unless there is movement in the scene we're trying to capture. Sometimes we actually might want the shutter speed to be very slow so that we would purposely blur moving objects or scenes and we will discuss this in a later post.

The sensitivity or ISO setting is how much the camera will in effect amplify the light that is picked up by the sensor. Generally we would want this to be fairly low because as the sensitivity is turned up there is the possibility of the image being adversely affected. Particularly in the darker parts of the scene you might start seeing flecks of light or colour appearing and this is usually termed as noise. I guess it's similar to cranking up you stereo system and getting a buzzing noise through the speakers.

So to get the correct amount of light into the camera it will determine how much available light is present and then balance these three variables to get the correct amount for a good exposure. How it does the balancing act will depend on the type of picture you are taking. The actual setting it uses will depend greatly on the amount of available light. If it is very bright the camera has an easier job because it can use the ideal settings for the type of picture.  However if there is a shortage of available light it will probably need to make some compromises or more than likely try to use the flash to compliment the available light.

For a portrait the camera will open up the aperture to get the narrow depth of field, then it will make the shutter speed fast enough to cut out camera shake and finally will adjust the sensitivity if necessary. If there is too much available light the camera may reduce the aperture from the ideal setting, reduce the sensitivity to the minimum and then use a faster shutter speed than necessary to cut down the amount of light hitting the sensor. If there is insufficient light and the flash is turned off the camera may open the aperture a little more than ideal, then increase the sensitivity as much as it dare and then set the shutter speed to as slow as it needs. Sometimes this can be too slow and you might start to see motion blur from camera shake or the movement of the subject.  If it's too dark the camera may be unable to focus or may refuse to fire a shot when you press the button.

For a landscape the camera will close down the aperture to get the wide depth of field, it will probably need to reduce the shutter speed to compensate for the smaller aperture and will try to keep the sensitivity low. As with portraits, if there is plenty of light there is usually no problem with getting the ideal settings but if the light is poor then compromises need to be made. The aperture will need to be opened up a bit, the shutter speed slowed down and possibly increase the sensitivity.

I think I need to discuss exposure a little more in the future, but I think this post has gone far enough.  I hope I have been clear and that I haven't confused anyone. If you have any questions, please leave a comment or send me an email and I'll try to answer. Next time I think I will go into more detail about the relationship between aperture size and depth of field.