Obtaining the Perfect Exposure

Presented by

Stephen Alexander

What is Exposure?

It's the effect of the combination of the camera settings and the scene iluminance.

Most simply defined, exposure = (illuminance at the image plane)*(time)

when photographers say exposure, they're usually talking about this in combination with another factor, which is the sensitivity of the medium.

Why is it important?

Exposure is important because it determines how light or dark your photos are in general.

Exposure of any medium is defined in terms of a range of values, from 100% dark to 100% light.

Too dark and you lose detail in the shadows, too light and you lose detail in the highlights.

So, what makes a perfect exposure?

Fundamentally, the photographer makes the exposure.

Sorry, but there is no perfect exposure; there are always decisions to be made.

It's you against nature: There are external factors that cannot be controlled, but you can control FOR them

The Photographer's Responsibility

The photographer is responsible for the idea of what is to become of the photograph.

The photographer is responsible for understanding external factors that need to be controlled-for

Therefore, the photographer must account for those external factors, plan and execute accordingly.

What are the external factors?

How much light is there to work with?

How fast is the subject moving?

How much range is there between the darkest and lightest element of what is to become the image? (for our purposes today, this is the big one).

How do I control for those factors?

Amount of light: aperture, filters

Amount of time: shutter speed

sensitivity of the medium: film speed, ISO setting

But, as the photographer, you must bear in mind that these controls are limited not only by environment, but also by the specifications of your hardware and/or film.

Exposure Management 101

A photographer must manipulate the controls in an intelligent manner to compensate for the environmental conditions and limitations of the equipment.

Also must bear in mind the limitations of the medium (dynamic range, high-ISO noise) and side effects of modifying camera settings (motion blur, depth of field).

Exposure Management ABCs

Various types of film and the various available digital sensors have different characteristics in terms of how they respond to more or less light.

We'll quickly step through digital and analog media and briefly discuss their various exposure-affecting characteristics.

Bear in mind that this is grossly overgeneralized.

Black and White Film

In general black and white film has very wide exposure latitude.

Most black and white films are probably good for about 12 stops worth of difference between lightest and darkest parts of the image. This includes C41 process B/W films.

B/W has less latitude in the shadows than in the highlights, so expose with this in mind

Color Negative Film

Color film has a narrower range than B/W; perhaps about 7 stops total.

generally the opposite of B/W in that it's easier to blow out highlights than to drop shadows.

Thus, one must measure the highlights and determine the brightest part of the scene in which to retain detail, and expose accordingly.

Slide Film

Has a much narrower useful range; it's closer to 5 stops.

Further limited by the fact that you can't compensate when printing, as you don't print it.

Accordingly, one must determine the appropriate exposure for the midrange and make choices about what's important in the image.

Digital

In general, digital cameras have historically had less dynamic range available than films. This is improving all the time.

In general, it's more likely that you will lose detail in the highlights than the shadows

The tradeoff is that there can be more noise in the shadows

Tools

There are three major tools that will help you dial in your exposures. One conceptual, one hardware, and one software (or firmware if you like). We'll treat them in their own slides, but they are:

Conceptual: Exposure Value

Hardware: Spot Meter

Software/Firmware: Histogram

Exposure Value

Logarithmic scale the units of which correspond to film speed/aperture/shutter speed stops: One step in EV is equivalent to one stop change in film speed/ISO, f-stop, or shutter speed.

EV is an absolute measure, but it can be used in relative terms: EV(x+1) = one stop faster than EV(x)

Spot Meter

Measures the EV for a small area of your scene (usually ~1 degree spot)

Provides a scale to translate the EV into appropriate exposure based on shutter speed, film speed, and aperture

Photographer still needs to combine this information with knowledge of the medium

Histogram

A histogram is a distribution curve representing relative frequency of pixels at a given brightness

Some cameras have histograms for each channel; R, G, B, and Value and will superimpose them

It's just a tool, and there's no correct histogram. However, it can tell you whether you are blowing out or dropping parts of the image.

What does it all mean?

In a nutshell, you are responsible for selecting an appropriate EV for your environment, equipment, and medium; however this means different things for different environments, equipment, and media.

This has been only an intro; to really learn any of this you need to go out and apply it!

Happy shooting.

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