CIS 421 › Photoshop > Photomerge

merge several photos into HDR image

What is HDR?

HDR = High Dynamic Range -- at this point, it means the results of merging together several photos (Camera RAW for the best results) of the same subject, exposed at bracketed shutter speeds into one 32-bit image.

Future digital cameras may do this within the camera.

32-bit images are currently used in film and 3-D production. Printers and computer monitors cannot display 32-bit color because there are too many colors, so for photos, the process is used for creating photos that have detail in both the light and dark areas of a scene.

Why create HDR images?

  • Sometimes you take a photo of a place that has very high contrast (dark areas and light areas), and the camera cannot expose correctly for both, so either the dark area has no detail and the light area does, or the dark area has detail, but the light area is burnt out.
  • HDR lets you merge photos of the same subject that are taken with different exposure settings, so that the resulting image has detail in both the light and the dark areas.
  • You hold the aperture constant and vary the shutter speed which affects how long the lens stays open). That way the depth-of-field stays constant and the photos are taken the same picture.

What you need to create images for HDR merge

  • The best results are from camera raw images that have a full range of colors and values, usually taken with a digital DSLR camera.
  • NOTE: You can use JPEG images, but when you take the photos, avoid automatic exposures, use one aperture setting, and bracket the shutter speeds of the images.
    • letting the camera automatically change the aperture or ISO results in lower-quality results, because detail at some distances may vary.
    • Turn the Flash off because differences from Flash may alter images
    • The camera's auto-bracketing feature makes exposure changes that are too small (+-1/3 or +-1/2 stop).
  • You need to bracket your photos, usually by 1 exposure stop -- need a minimum 3 exposures. Photoshop recommends 5 to 7 exposures to get the full dynamic range of the scene. At minimum, you need:
    • One shot is taken at normal exposure
    • One shot is taken 1 step up
    • One is taken 1 step down
  • A tripod helps hold the camera steady so that the photos can stack on top of one another without too much difficulty.
  • A remote controller to take the pictures works even better because pressing the shutter release button jiggles the camera.
  • Photos of objects with straight lines that do not move much work better than ones that move, such as children or pets.

How I took photos for merging to HDR

  1. I found a subject that had very light and dark areas and straight lines.
  2. I set my white balance to the appropriate type of light, which was daylight so there would not be color casts. It lessened the need for color adjustments before merging.
  3. I set the ISO to 200 (the lowest for my camera/lens combination) because the day was bright, and the photos would be sharper at a lower ISO.
  4. I focused the camera lens on infinity, since it was a street that stretched a long way, and the image would have greater depth of field (the image would be sharp over a longer distance).
  5. I set the camera on a tripod, looked through the viewfinder and composed the image.
  6. I set the camera mode to aperture priority so the aperture would stay constant and I could vary the exposure.
  7. I fixed the aperture to its highest setting F/9, so the image would have the greatest depth-of-field and more of the photo would be in focus.
  8. I pushed the Shutter-release button halfway down so the camera could to its calculation of the correct exposure for aperture F/9.
  9. Then I switched to Manual Mode so I could set the exposure setting myself.
  10. I bracketed 8 exposures (overkill, but I wanted to try the maximum I could) set at:
    1. aperture = F/9.0, shutter speed = 1/160
    2. aperture = F/9.0, shutter speed = 1/200
    3. aperture = F/9.0, shutter speed = 1/250
    4. aperture = F/9.0, shutter speed = 1/320
    5. aperture = F/9.0, shutter speed = 1/400
    6. aperture = F/9.0, shutter speed = 1/500
    7. aperture = F/9.0, shutter speed = 1/640
    8. aperture = F/9.0, shutter speed = 1/800

Try merge to HDR with the photos I took

Download the photos

Open the photos in Adobe Bridge

  • The process is supposed to work best when started from Adobe Bridge, on Camera Raw files.
    • If you use JPEG files, and merge them in Photoshop (File>Automate>Photomerge), be sure to use the files you capture from the camera and not ones that you have changed in some way, because it will be hard to line up altered jpegs of the same scene
    • If the images need some color adjustments it is better to do them in Camera Raw, before you run the Merge to HDR command.
      • select all the images in Bridge,
      • Rightclick to open them in the Camera Raw window
      • Click the Select All button at the top of the stack of images on the left
      • make the color adjustments to one of the images and it will make the same color adjustments to all of them
      • Then click Done and return to Adobe Bridge
      • With the same photos selected, run the Tools>Photoshop>Merge to HDR command
  • From Adobe Bridge: Select all the photos
  • Click Tools>Photoshop>Merge to HDR to open the Merge to HDR window
  • In this more recent experienment, I merged 9 images, taken using Aperture F9, and lens speeds from 1/80 to 1/500. The sky differed in each version, because wind was blowing the clouds.
  • The version I merged using raw images from Bridge differed from another version using the same images saved as PSDs. I lowered the file dimensions in half to reduce the file size.
  • The PSD version had better image quality, better color, and was sharper.

Merge to HDR window shows individual photos and merged photo

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Merge to HDR Window

  • The source images in the left column can be switched off if they are not contributing to the final image because they are blurred or have other problems.
  • The merged image is a floating-point 32-bit image, which exceeds what the computer monitor displays, so the image may not look good in Photoshop.
  • You can adjust the preview settings for viewing in the monitor
    • Move the White Point slider in the right column to adjust the overall tones of the image
    • Preview settings are saved in the HDR image file and applied when the image is opened in Photoshop.
    • The HDR image itself still has all the 32-bit color information in it
    • Click OK to create the single-layer 32-bit HDR image
    • File> Save As -- Preserve the original copy. so you can open it and work with it later, starting with the 32-bit version

What next?

  • The right-arrow at the bottom of the image frame lets you change what image information displays -- change it to see the large document size of the 32-bit image: Show>Document Sizes
  • Not many Photoshop commands work with 32-bit HDR images -- cruise through the menu dropdowsn to see what is enabled in 32-bit mode
  • To work with the image in Photoshop, you need to convert it to a 16-bit or 8-bit image
    • Keep the 32-bit image the same so you can reuse it
    • You can open it later and do other versions
  • Click Image>Mode>16 bits/Channel or 8 bits/Channel to open the HDR Conversion Window. It has 4 options for conversion
    • Exposure and Gamma lets you adjust the tonal range (brightness and contrast) of the image with exposure and gamma sliders --
      • gamma adjusts midtones
      • exposure adjusts highlights
      • adjust the gamma first & then the exposure
    • Highlight Compression compresses the highlight values so they fall within the luminance range of 16-bit or 8-bit images -- in this case it darkens the image
    • Equalize Histogram compresses the dynamic range of the HDR image and tries to preserve the contrast, but does not allow alteration
    • Local Adaptation gives you more control. It calculates the amount of correction necessary for local brightness regions throughout the image and lets you adjust the Toning Curve and Histogram.

Local Adaptation option when converting 32-bit HDR image to 16-bit version

Local Adaptation Settings

  1. Click the down arrows to open the Tonal Curve & Histogram, if it isn't open
  2. In the Histogram, you can clip the histogram at the lower and upper ends (by dragging them in to where the first pixels in the histogram appear on either side) since there is much more image information in a 32-bit HDR image than can be displayed on the computer monitor.
  3. Adjust the curve to make different tonal areas lighter (pull it up) or darker (pull it down).
  4. Radius slider specifies the size of the local brightness regions
  5. Threshold slider specifies how far apart 2 pixels' tonal values must be before they are in separate brightness regions.
  6. If the HDR image has banding, move the radius and threshold sliders to fix them.
  7. Save the 16-bit or 8-bit image with a different file name.
  8. If the image is 8-bit, most of the Photoshop commands work; 16-bit images do not have some commands available.

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Color Picker in 32-bit mode

Color Picker for a 32-bit image lets you adjust colors and boost or reduce the color's brightness in the intensity slider

  • Displays the current color with several different intensity levels.
  • can display color relative to active document's exposure setting.

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Example Result

results of merging 8 photos with different exposure settings into 1 HDR photo

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Resources

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