Intensity of Light - Measuring Exposure (4.1a)

Requirement

• Part 1 - 4/6 images, each using exposure compensation
• Part 2 - 30/36 images, 5 x 0.5EV brackets of 5/6 scenes

Purpose

• To learn how to measure and adjust exposure

Technical Learning

Intensity of light

• An important skill for a photographer is to isolate 'light' from other elements of a composition
• Light has 2 dimensions: (a) intensity (see this section) and (b) quality (see 4.2)
• Cameras' exposure meters are designed to work best in normal daylight
• The brightness of daylight varies for 2 main reasons: (a) the position of the sun in the sky (itself a function of (i) time of day - see 4.3, (ii) time of year and (iii) latitude of location), and (b) the weather (especially cloud) conditions
• At brightness below normal daylight (e.g. indoors or in artificial light) you need to adjust exposure. You can do this in one of three ways, but always at a price: (a) increase ISO, at the risk of raising noise, (b) reducing shutter speed, at the cost of greater motion blur, or (c) widen aperture, at the price of shortening depth of field

Metering systems

• Camera metering systems work on the principle of averaging the brightness in the scene and taking this average as the mid tone of the scene, whether or not this is the case. This average is the key around which all other values are set. e.g. in a snow scene the meter will tend to read the whiteness of the snow as neutral grey
• Most cameras have three metering methods: (a) centre weighted, (b) predictive and (c) spot
• Centre weighted averages the brightness of the central part of the image, so excludes what it is assumes are the less relevant edges of the frame
• Predictive examines the brightness values across the scene and anticipates the type of scene based on the data sets stored in the camera. The result varies by the sophistication of the camera make/model 
• Spot uses the reflected brightness from the chosen part of the scene

Dynamic range and working with metering systems

• The camera sensor lacks the dynamic range of the human eye 
• The key tasks in addressing this limitation is to avoid burning out highlights whilst keeping as much as the desired level of detail in the shadows as possible
• This challenge requires making best use of the camera's metering system, either (a) to accept the effect of averaging or (b) to adjust the returned exposure value
• Methods to adjust exposure include (a) switch to manual, (b) use the camera's EV compensation tool
• Other less direct ways of dealing with the challenge are (a) shoot in RAW and the adjust in the conversion software, (b) use bracketing and either select the best image or blend the images (perhaps using HDR software)

EV compensation guidelines where meter averaging is unlikely to produce the best result

• The range of adjustment usually falls between +/ - EV 2.0
• The EV compensation required depends on (a) the brightness of the subject and (b) the level of contrast within the scene
• In high contrast scenes, use (a) predictive metering and/or (b) exposure to the required treatment of the subject
• In low contrast scenes, make the EV adjustment up to the level that avoids compromising the subject

Exercise Instructions

• Part 1 - shoot 4/6 images with exposure set purposefully either darker or lighter than average
• Part 2 - take 5/6 scenes, each bracketed at intervals of 0.5EV

Images and Review

Part 1 - shoot 4/6 images with exposure set purposefully either darker or lighter than average

Underbelly of the new Blackfriars Bridge

Shot as per meter reading
Detail in the highlights under arches approaches burn out.
This has its merits: it creates the
impression of light flowing through the bridge, it also helps to pulls
eye through the image from dark to light (as do the series of girders and
the tapering perspective)

Minus 0.5 EV
Recovers detail under arches

More London, Workers Walking to Work

Shot as per meter reading
Shadows are too dark, highlights are burnt out

EV plus 1 stop
Reveals some of the  detail behind glass and
what is reflected in it as well as of people's backs,
yet strengthen the idea of people walking
towards the white light

Thames Path after rainstorm

Shot as per meter reading
Reflected light burns out the colour of
the paving stones

EV minus 1 stop
Reducing exposure saturates the colour of
 the paving stones, without reducing glint
of sunlight on water

Part 2 - take 5/6 scenes, each bracketed at intervals of 0.5EV

Hats

Minus 1 EV
Creates a low key image, which highlights
the expression on the models faces

Minus 0.5 EV,
balances mood of the models expressions
with some of the detail of the hats and faces

As metered
The dullest image in the set

Plus 0.5 EV
Shows most detail of the hat in the foreground,
yet still retains some of the tones on the models

Plus 1 EV
Creates a high key image,
brings out the detail of the background hat 
(in other images this sits in the shadows)