TABLE 1.1

1.1

Limits of Photoshop’s Support for Bit Depths Above 8

16-bpc Images 32-bpc Images

Cannot use Bitmap, Duotone, or Indexed

Color mode

Can use only RGB and Grayscale modes

Cannot use Art History Brush tool Cannot use some blending modes

Cannot use Variations image adjustment Cannot use some commands from the Image ➪ Adjustments

menu

Cannot use some filters Cannot use some filters

Can save these formats only:

Photoshop (PSD, PSB)

Radiance (HDR)

Portable Bit Map (PBM)

OpenEXR

TIFF

Cannot fully use a few commands from the Image menu

Cannot use a few commands from the Layer menu

Cannot use a few commands from the Select menu

Cannot use some of the tools in the Toolbox

Eight-bpc RGB images are sometimes called 24-bit images (8 bits x 3 channels = 24

bits of data for each pixel). This can be confusing. For example, sometimes 8-bpc

RGB images are called 8-bit images and sometimes they are called 24-bit images.

Thirty-two-bpc HDR images can represent the entire dynamic range of the visible world, unlike the

lower bit depths, which can represent only a portion of it. Among the advantages of HDR is that

adjusting its exposure is like adjusting the exposure when photographing a scene in the real world.

Blurs and other real-world lighting effects look realistic in HDR images. Motion pictures, 3-D

work, and some high-end photography are mediums that sometimes use HDR images.

HDR images contain brightness levels that a standard monitor cannot display, so Photoshop lets

you make choices about the appearance of the on-screen preview of HDR images by choosing

View ➪ 32-bit Preview Options.

The Merge to HDR command lets you create HDR images from multiple photographs with different

exposures, thereby gaining the larger dynamic range that 32-bpc images can support.

Following are the steps to merge multiple images to HDR format:

1. Choose File ➪ Automate ➪ Merge to HDR.

2. In the Merge to HDR dialog box, click Browse, select the images, and click Open.

3. If you held your camera in your hands when you photographed your multiple

images, select the Attempt To Automatically Align Source Images option.

4. Click OK.

5. Set view options by clicking the minus or plus button for zoom in or out, or choose

a view percentage or mode from the pop-up menu below the preview image.

6. (Optional) Select or deselect check boxes under the filmstrip thumbnails to specify

which images to use in the merge.

7. Choose a bit depth from the Bit Depth menu. Choose 32 Bits/Channel if you want the

merged image to store the entire dynamic range of the merged images.

8. Move the slider below the histogram to preview the merged image (this does not

remove any image data). This preview adjustment is stored in the HDR image file and

applied when the image is open. It can be changed later by choosing View ➪ 32-bit

Preview Options.

9. Click OK to create the merged image.

You can convert a 32-bpc HDR image to a 16-bpc or 8-bpc image, but some of the image’s information

will be lost in the conversion. You will need to make choices about what information to

keep and what to throw away based on what you want the converted image to look like.

Following are the steps to convert from 32-bpc HDR to 16- or 8-bpc:
1. Choose Image ➪ Mode ➪ 16 Bits/Channel or 8 Bits/Channel.
2. Make sure the Preview box is checked and choose one of the following four methods
for adjusting the brightness and contrast in the image until the image’s appearance
and tonal range meets your requirements:
n Exposure and Gamma. Move the Exposure slider to adjust the gain, and move the
Gamma (midtone) slider to adjust the contrast. If the result is acceptable, click OK to
complete the conversion.
n Highlight Compression. This is an automatic method that compresses the highlight
values in the HDR image so they fall within the range the lower bit depth can support.
If the result is acceptable, click OK to complete the conversion.
n Equalize Histogram. This is an automatic method that compresses the dynamic range
of the HDR image while trying to preserve some contrast. If the result is acceptable,
click OK to complete the conversion.
n Local Adaptation. Move the radius slider to specify the size of the local brightness
regions. Move the threshold slider to specify how far apart two pixels’ tonal values
must be before they are no longer part of the same brightness region. You can also use
the toning curve to make adjustments. It’s usually best to uncheck the corner option
so the curve will be smooth. (For more about using curves, see Chapter 11.) If the
result is acceptable, click OK to complete the conversion.
NOTE With the HDR dialog box open, you can use the Save or Load button to save or load
your custom HDR conversion settings.