Accessibility

Why shoot raw?

Page 2 of 9
Previous Page
Next Page

Table of Contents

  1. Introduction
  2. Using all the bits
  3. White balance control
  4. Colormetric interpretation
  5. Exposure
  6. Detail and noise
  7. Raw limitations
  8. Adobe Camera Raw
  9. The Digital negative

Using all the bits

Most of today’s cameras capture at least 12 bits per channel per pixel, for a possible 4,096 levels in each channel. More bits translates directly into editing headroom, but the JPEG format is limited to 8 bits per channel per pixel: So when you shoot JPEG, you trust the camera’s built-in conversions to throw away one-third of your data in a way that does justice to the image.

When you shoot raw, though, you have, by definition, captured everything the camera can deliver, so you have much greater freedom in shaping the overall tone and contrast for the image. You also produce a file that can withstand a great deal more editing in Photoshop than can an 8-bit per channel JPEG.

Edits in Photoshop are “destructive”—when you use a tool such as Levels, Curves, Hue/Saturation, or Color Balance, you change the actual pixel values, creating the potential for either or both of two problems:

  • Posterization can occur when you stretch a tonal range. Where the levels were formerly adjacent, they’re now stretched apart, so instead of a gradation from, for example, level 100 through 101, 102, 103, 104, to 105, the new values may look more like 98, 101, 103, 105, 107. On its own, such an edit is unlikely to produce visible posterization—it usually takes a gap of four or five levels before you see a visible jump instead of a smooth gradation—but subsequent edits can widen the gaps, inducing posterization.
  • Detail loss can occur when you compress a tonal range. Where the levels were formerly different, they’re now compressed into the same value, so the differences, which represent potential detail, are tossed irrevocably into the bit-bucket, never to return.

Figure 1 shows how the compression and expansion of tonal ranges can affect pixel values. Don’t be overly afraid of losing levels—it’s a normal and necessary part of image editing, and its effect can be greatly reduced by bringing correctly exposed images into Photoshop as 16-bit/channel files rather than 8-bit/channel ones—but simply be aware of the destructive potential of Photoshop edits.

Page 2 of 9
Previous Page
Next Page