Chapter 4 ExposureThe Exposure Triangle and ISO

The Exposure Triangle The three elements of the

exposure triangle Aperture Shutter Speed ISO

These three elements working together determine the exposure of the image.

Aperture The size of the opening of the

lens Determines how much light hits

the sensor (CDD) Is Measured in f-stops

Shutter Speed Refers to the amount of time the

sensor is exposed to (or sees) light. Shutter speed is measured in

seconds or in most cases fractions of a second.

Shutter speeds available to you on your camera will usually double (approximately) with each setting.

1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000

ISO The measurement of the sensors

sensitivity to light. The higher the ISO number the

more sensitive the sensor is to light.

Measurements 80, 100, 200, 400, 800

The Exposure Triangle

The Results of Exposure Normal Over Exposure Under Exposure

Metering Mode The metering system in a digital

camera measures the amount of light in the scene and calculates the best-fit exposure value based on the metering mode set in the camera

All you have to do is select the metering mode, point the camera and press the shutter release. Most of the time, this will result in a correct exposure.

Metering Method The metering method defines

which information of the scene is used to calculate the exposure value and how it is determined.

Three of the most widely used metering methods are; Center-weighted Average Metering Spot (Partial) Metering Matrix or Evaluative Metering

Center-weighted Average Metering

The most common metering method implemented in nearly every digital camera and the default for those digital cameras which don't offer metering mode selection.

Assesses the whole image area but assigns the most importance to the central 40 to 60%. 

This system is accurate in average lighting conditions, but can be fooled by excessively dark or light areas.

Spot (Partial) Metering Allows you to meter the subject in

the center of the frame (or on some cameras at the selected AF point).

Only a small area of the whole frame is metered and the exposure of the rest of the frame is ignored.

This type of metering is useful for brightly backlit, macro, and moon shots.

Matrix or Evaluative Metering The most complex metering

mode, offering the best exposure in most circumstances

The scene is split up into a matrix of metering zones which are evaluated individually.

The overall exposure is based on an algorithm specific to that camera and manufacture.

ISO Continued Almost all cameras have the

ability to change the ISO setting. Auto ISO, the camera will look at

the amount of light in the scene and change the ISO appropriately so that the shutter speed doesn’t get too slow.

Manual ISO settings ISO 50-100. Suitable for bright light

(like outdoors on a sunny day). ISO 200. Great for overcast or

cloudy days. ISO 400 and 800. Use these values

when the light is getting dim but it is not yet night.

ISO 1600 and above. Use for indoor or night shots. Also useful to freeze the action in sports shots. These values will produce the most noise.

ISO and Noise Image noise is the by-product of

using a high ISO setting on your digital SLR camera — it is also often called "grain".

When a photo has image noise, areas of color that should look smooth have a mottled or speckled appearance.

The higher the ISO setting the more noise introduced into the picture.

At 400 ISO you may begin to see noise.

ISO Noise Comparison

ISO 100 ISO 200

ISO 400 ISO 800

Simulation Web Site The address below will take you

to a Web site that simulates Exposure control

http://www.photonhead.com/simcam/