Real World Camera Raw with Adobe Photoshop CS2: Industrial-Strength Production Techniques

Chapter 2: How Camera Raw Works

35

to the gamma-corrected output space. They work completely differently

from the similarly named Photoshop Contrast and Brightness controls.

Instead, they behave similarly to Photoshop’s Levels and Curves, respec-

tively (Brightness is a midtone adjustment, Contrast is an S-curve) but

with one important difference. The Camera Raw controls use an algorithm

that preserves the original hue, whereas hard curve adjustments to the

composite RGB curve in Photoshop can cause slight hue shifts.

How Much Highlight

Detail Can I Recover?

The answer, of course, is “it de-

pends.” If the captured pixel is

completely blown out—clipped to

white in all three channels—there

is no highlight detail to recover. If

a single channel, (or, better, two

channels) still contain some in-

formation, Camera Raw will do its

best to recover the detail and attri-

bute natural-looking color to it.

The fi rst stage of highlight re-

covery is to use any headroom

the camera leaves by default,

which varies considerably from

vendor to vendor, with some leav-

ing no headroom at all. The next

stage uses Camera Raw’s highlight

recovery logic to build color in-

formation from the data in one

or two unclipped channels. Next,

the amount of highlight compres-

sion introduced by the Brightness

slider is reduced, stretching the

available highlight data over a

wider tonal range. The fi nal stage

is application of a curve to map

the midtones and shadows.

Several factors limit the amount

of highlight data you can recover,

and these vary from camera mod-

el to camera model. The fi rst is the

sensor clipping itself—the point

at which all three channels clip.

You can recover a lot of highlight

data when only one channel con-

tains data, but if you stretch the

highlights too far, the transition

between the totally blown-out

highlights and the recovered ones

looks unnatural. Also, some cam-

eras run the sensor chip slightly

past its linear range, producing

hue shifts near the clipping point,

and these hue shifts get magnifi ed

by the extended highlight recovery

process—if you try to stretch the

highlight data too far, you’ll get

strange colors—so in either case

the practical limit may be lower

than the theoretical one.

Most cameras use analog gain

to provide different ISO speeds,

but some use digital gain in-

stead—a high-ISO image from

these cameras is essentially just an

underexposed image with built-in

positive exposure compensation

applied—so a lot of highlight data

can be recovered by undoing the

positive exposure compensation.

The white balance also has an

effect on highlight recovery, since

it scales the clipped channels to

match the unclipped one. When

you’re attempting extreme high-

light recovery it’s often a good

idea to adjust the Exposure slider

before setting white balance, be-

cause the white balance is likely

to change as you stretch the high-

lights anyway.

In practice, most cameras will

let you recover at least a quarter

stop of highlight data if you’re

willing to compromise a little on

the white balance. Many cameras

will let you recover at least one

stop, possibly more, but the full

four-stop range offered by the

Exposure slider is beyond the use-

ful range for most cameras. I don’t

advocate deliberate overexposure,

but if you’re shooting in changing

lighting conditions, the linear

nature of digital captures makes

it preferable to err on the side of

slight overexposure rather than

underexposure, because underex-

posing to hold the highlights will

make your shadows noisier than

they need be. In these situations,

Camera Raw’s highlight recovery

provides a very useful safety net.