High dynamic range images

The dynamic range (ratio between dark and bright regions) in the visible world far exceeds the range of human vision and of images that are displayed on a monitor or printed. But whereas human eyes can adapt to very different brightness levels, most cameras and computer monitors can reproduce only a fixed dynamic range. Photographers, motion picture artists, and others working with digital images must be selective about what’s important in a scene because they are working with a limited dynamic range.

High dynamic range (HDR) images open up a world of possibilities because they can represent the entire dynamic range of the visible world. Because all the luminance values in a real-world scene are represented proportionately and stored in an HDR image, adjusting the exposure of an HDR image is like adjusting the exposure when photographing a scene in the real world.

In Photoshop, the luminance values of an HDR image are stored using a floating-point numeric representation that’s 32 bits long (32‑bits-per-channel). The luminance values in an HDR image are directly related to the amount of light in a scene. By contrast, non-floating point 16‑ and 8‑bpc image files store luminance values only from black to paper white, reflecting an extremely small segment of dynamic range in the real world.

In Photoshop, the Merge To HDR Pro command lets you create HDR images by combining multiple photographs captured at different exposures. Because an HDR image contains brightness levels that far exceed the display capabilities of a standard 24‑bit monitor, Photoshop lets you adjust the HDR preview. If you need to print or use tools and filters that don’t work with HDR images, you can convert them to 16- or 8‑bpc images.

Keep the following tips in mind when you take photos to be combined with the Merge To HDR Pro command:

• Secure the camera to a tripod.

• Take enough photos to cover the full dynamic range of the scene. You can try taking at least five to seven photos, but you might need to take more exposures depending on the dynamic range of the scene. The minimum number of photos should be three.

• Vary the shutter speed to create different exposures. Changing the aperture changes the depth of field in each exposure and can produce lower-quality results. Changing the ISO or aperture may also cause noise or vignetting in the image.

• In general, don’t use your camera’s auto-bracket feature, because the exposure changes are usually too small.

• The exposure differences between the photos should be one or two EV (exposure value) steps apart (equivalent to about one or two f‑stops apart).

• Don’t vary the lighting; for instance, don’t use a flash in one exposure but not the next.

• Make sure that nothing is moving in the scene. Exposure Merge works only with differently exposed images of the identical scene.

You can use the following tools, adjustments, and filters with 32‑bpc HDR images. (To work with more Photoshop features, convert a 32‑bpc image to a 16‑bpc or an 8‑bpc image. To preserve the original image, create a copy with the Save As command.)

The Merge To HDR Pro command combines multiple images with different exposures of the same scene, capturing the full dynamic range in a single HDR image. You can output the merged image as a 32‑, 16-, or 8-bpc file. However, only a 32-bpc file can store all the HDR image data.

HDR merging works best when photos are optimized for the process. For recommendations, see Take photos for HDR images.

Move the slider below the histogram to adjust the white point preview of the merged image. Moving the slider adjusts the image preview only; all HDR image data remains in the merged file.

The preview adjustment is stored in the HDR file and applied whenever you open the file in Photoshop. To re-adjust the white point preview at any time, choose View > 32‑Bit Preview Options.

HDR images contain luminance levels that far exceed the dynamic range that 16‑ or 8‑bpc images can store. To produce an image with the dynamic range you want, adjust exposure and contrast when converting from 32‑bpc to lower bit depths.

Choose one of the following tone-mapping methods:

If images have different content due to moving objects like cars, people, or foliage, select Remove Ghosts in the Merge To HDR Pro dialog box.

Photoshop displays a green outline around the thumbnail with the best tonal balance, identifying the base image. Moving objects found in other images are removed. (If movement occurs in very light or dark areas, click a different thumbnail where moving objects are better exposed to improve results.)

Response curves indicate how camera sensors interpret different levels of incoming light. By default, the Merge To HDR Pro dialog box automatically calculates a camera response curve based on the tonal range of images you are merging. You can save the current response curve and later apply it to another group of merged images.

• In the upper-right corner of the Merge to HDR Pro dialog box, click the response curve menu , and then choose Save Response Curve. (To later reapply the curve, choose Load Response Curve.)

If you originally created a 32-bit image during the Merge to HDR Pro process, you can later convert it to a 16- or 8-bit image.

The dynamic range of HDR images exceeds the display capabilities of standard computer monitors. When you open an HDR image in Photoshop, it can look very dark or washed out. Photoshop lets you adjust the preview so that the monitor displays an HDR image whose highlights and shadows aren’t washed out or too dark. The preview settings are stored in the HDR image file (PSD, PSB, and TIFF only) and are applied whenever the file is opened in Photoshop. Preview adjustments don’t edit the HDR image file; all the HDR image information remains intact. Use the Exposure adjustment (Image > Adjustments > Exposure) to make exposure edits to the 32‑bpc HDR image.

The HDR Color Picker allows you to accurately view and select colors for use in 32‑bit HDR images. As in the regular Adobe Color Picker, you select a color by clicking a color field and adjusting the color slider. The Intensity slider allows you to adjust the brightness of a color to match the intensity of the colors in the HDR image you’re working with. A Preview area lets you view swatches of a selected color to see how it will display at different exposures and intensities.

With a 32‑bpc image open, do one of the following:

• In the toolbox, click the foreground or background color selection box.

• In the Color panel, click the Set Foreground Coloror Set Background Color selection box.

The Color Picker is also available when features let you choose a color. For example, by clicking the color swatch in the options bar for some tools, or the eyedroppers in some color adjustment dialog boxes.

The lower part of the HDR Color Picker functions like the regular Color Picker does with 8‑ or 16‑bit images. Click in the color field to select a color and move the color slider to change hues, or use the HSB or RGB fields to enter numeric values for a particular color. In the color field, brightness increases as you move from bottom to top, and saturation increases as you move from left to right.

Use the Intensity slider to adjust the brightness of the color. The color value plus the intensity value are converted to 32‑bit floating point number values in your HDR document.

You can edit and add effects to HDR/32‑bpc images using any of the following Photoshop tools: Brush, Pencil, Pen, Shape, Clone Stamp, Pattern Stamp, Eraser, Gradient, Blur, Sharpen, Smudge, and History Brush. You can also use the Text tool to add 32‑bpc text layers to an HDR image.

When editing or painting on HDR images, you can preview your work at different exposure settings using either the 32‑Bit Exposure slider in the document info area or the 32‑Bit Preview Options dialog box (View > 32‑Bit Preview Options). The HDR Color Picker also lets you preview your selected foreground color at different intensity settings, to match different exposure settings in an HDR image.