A brightly lit scene can contain an infinite number of tonal values. Our vision system ingeniously adapts to such conditions; however, no cameradigital or otherwise is capable of recording the entire tonal range of such a scene in a single shot. In this section, I'll show you how to use Photoshop's new Merge to HDR filter to create digital images with incredible dynamic range.
Let me show you what I mean by limited dynamic range. If you look at this test image, which was taken with a Nikon D200 digital camera, the histogram indicates that the details in the highlight areas of the image are clipped or missing. Figure 5-22
There isn't much I can do with Camera Raw's Adjust controls to fix this image, because the data just isn't there to work with. In this section, I'll show you an how to create an image with great dynamic range. Short of buying another digital camera, it's the best possible solution.
--------------------------------------------------------------------------------
Note: Some digital cameras, such as the Fuji FinePix S3, boast improved dynamic range capabilities, and it is safe to say in a matter of time digital capture will surpass the capabilities of print film. Even when this occurs, the various techniques outlined in this chapter will be useful for those who want the ultimate control of the tonal range of their images.
--------------------------------------------------------------------------------
Using Merge to HDR
One of the newest additions to Photoshop is Merge to HDR (High Dynamic Range). Created by John Petersen, the same brilliant engineer responsible for Photomerge, Merge to HDR allows you to blend or merge three or more files taken of the same scene with different exposures and create a new file capable of storing an almost infinite number of tonal values. Figure 5-23
Few displaysand no printersare capable of utilizing all this data. You'll need to rely on another new Photoshop feature, HDR Conversion, created by Chris Cox. HDR Conversion automatically appears when you change modes to a lower bit rate and provides a variety of ways to squeeze the dynamic data into a useable form.
The advantage of using Merge to HDR is obvious: by using different multiple shots at different exposures, you can extend your dynamic range much greater than is possible with a single shot.
There are obvious limitations to the multiple shooting technique often referred to as "bracketing." First, it's most effective with static scenes. Second, it works best if you carefully frame each shot, preferably using a tripod. Third, it uses up more memory both in the camera and on the hard drive. As you'll see, it also takes some effort to properly use HDR Conversion and get optimal results.
Shooting for HDR
Merge to HDR will work a lot better if you shoot with it in mind from the start:
Use a tripod for consistent results. You can get decent results hand holding your camera, but it becomes hit or miss.
Don't use your digital camera's autofocus! Even the slightest change in focus affects results and autofocus systems are notoriously imprecise.
Vary your exposure by changing the shutter speed using your camera's manual or Aperture Preferred Mode. Figure 5-24 Changing the f-stop would alter the depth of focus and quality would suffer. Don't change ISO between shots either. If you use a flash, use it consistently between frames.
Vary your exposure by at least one stop between frames. (Adobe actually recommends 2 stops between shots; that's fine but it really depends on the capabilities of your camera. Some digital cameras capture less dynamic range than others, and I like to err on the safe side.) Figure 5-25 The bottom line is, if the exposure values are too close, Merge to HDR won't have much effect and if they are too far apart the final image will become posterized. HDR can handle such an extreme range that you can basically shoot from complete black to complete white. (One photographer told me he successfully used 26 images, shot 2 stops apart!)
Don't rely on automatic exposure bracketing. Many cameras offer this feature, however the range is usually limited, typically 23 stops plus and 23 stops minus. I use my camera's +/- exposure controls instead. Refer to your camera manual if you don't know how to set your camera to over-or underexpose.
If you are shooting outdoor scenics, take the shots in rapid sequence. Clouds move, trees and flags wave in the wind, and every change between frames will affect the final image. Figure 5-26
Merging with HDR
Ok, you've got your shots. Here you go.
1. Select File > Automate > Merge to HDR. Browse to your images. (You can also access Merge to HDR directly from Bridge. Just select the files you wish to merge, hold the Shift or Cmd/Ctrl keys to select multiple files, then Select Tools > Photoshop > Merge to HDR.) Figure 5-27
If your images were shot without benefit of a tripod, be sure to select Attempt to Automatically Align Source Images.
Figure 5-28 If you are working from Bridge, you don't have a choice: your images will open without benefit of auto alignment. This is fine if your images were shot using a tripod, but it'll be a problem if they were not.(Officially, you are required to use multiple photos shot with different exposure values. However you can fool Merge to HDR if you strip the EXIF information from your single RAW file.
It can take some time for the images to open in the Merge to HDR window, especially if you have selected Attempt to Automatically Align Source Images. How long it takes depends on the number of images you selected, the resolution, and of course, the processing speed of your computer. Figure 5-29 Keep in mind that Merge to HDR uses Camera Raw to convert RAW files. However, it overrides Camera Raw's exposure, shadow, brightness, and contrast controls and relies instead on the camera's EXIF data for these values. It also converts the RAW data to 8 Bits/Pixel. If you change other Camera Raw parameters (such as Sharpen) be sure to do the same for all RAW files. Otherwise, quality may suffer.
2. Preview the results of your merge. Use typical navigation keyboard shortcuts, or control the viewable size by selecting from the percentage values at the bottom of the window. However, since most monitors are capable of displaying only 8 Bits/Channel of color data, you are only seeing a slice of the full HDR range. You can use the slider under the Set White Point Preview to vary the visible slice. This doesn't do anything to the HDR data; it just controls the preview. Figure 5-30
3. Adjust the effects of the merge by deselecting images and removing them one at a time. Figure 5-31 To do this, un-check the box below the thumbnail image. Each time you deselect an image, the process of recreating a new merged image takes time, as indicated by the status bar at the bottom left of the window.
4. When you are ready, you can select OK, or choose a bit depth. I suggest for now you just select OK, and save the full 32-bit HDR data. You can select a lower bit depth later, if that is what you want.
Saving your HDR file
You can save the HDR file in any file format capable of retaining floating point data, which includes: Photoshop (.psd), Portable Bitmap (.pfm), TIFF (.tif) and OpenEXR (.exr). Figure 5-32 You can't save a HDR file as a JPEG unless you convert it first to 8 Bits/Pixel, but then it won't be HDR anymore. Remember HDR files are quite large. For example, an HDR file created by merging images from my 6MP camera weighed in at 68.8MB.
Working with HDR files in Photoshop
If you wish to work on the HDR file in Photoshop, you'll find the tools limited at this time. The Histogram palette is not functional, although the Info palette is. Levels and Curves aren't an option, but Channel Mixer, Photo Filter, and Exposure are. Figure 5-33 Multiple layers are not an option, but the History Brush and Clone Stamp tools are. You can crop and resize and change the canvas size. You can apply arbitrary rotations and flips, as well as free transform and warps. Many filters are available, including Gaussian Blur, Unsharp Mask, Smart Sharpen, and Add Noise. You may find it easier to simply convert your HDR file to a lower bit range and do your serious editing then. If you thought Smart Sharpen was slow on a 16-bit file, just try it on a HDR file!
Changing the Preview for your HDR File
You can change the preview to reflect different exposure and gamma settings without affecting the underlying data. Select View > 32-bit Preview Options from the main menu bar and this dialog box will appear. Figure 5-34
You can compare multiple versions of the same image by choosing Window > Arrange > New Window from the Photoshop menu. Then use the 32-Bit Exposure slider found at the bottom left of the image window to preview different settings for different windows and do a side-by-side comparison. Figure 5-35
Converting HDR Files to a Useable Form
For all practical purposes, you'll want to convert your HDR files to a usable tonal distribution and practical bit depth. The way you squeeze all that data into a more limited space is critical, and sometimes it takes a bit of work to get it right.
To convert your file:
1. With your HDR file open, select Image > Mode > 16 Bits/Channel or 8 Bits/Channel. (Choosing Grayscale only discards color values and doesn't reduce the bit value.)
2. When you do this, you'll get this dialog box. This is where you'll make all the hard choices. Figure 5-36
Your options are found in the Method pop-up menu. They include Exposure and Gamma, Highlight Compression, Equalize Histogram, and Local Adaptation. Figure 5-37
Exposure and Gamma, as the name implies, gives you straightforward control over brightness and contrast. Don't be mislead by the Toning Curve and Histogram. The histogram displays the inherent values, but the Toning Curve isn't functional. (It's functional only when you select Local Adaptation.)
Highlight Compression and Equalize Histogram apply preset equations that may or may not be appropriate. There are no user controls for these options. I've found the Equalize Histogram works reasonably well on some merged images, but I've haven't had much luck with Highlight Compression. Try for yourself.
Use Local Adaptation to gain real control. Local Adaptation and Photoshop's Shadows/Highlight image adjustment command work similarly: they both attempt to lighten or darken an image, based on surrounding pixels. With a localized approach, it is possible to lighten a dark area (or darken an area that's too light) without affecting other areas of the image. Figure 5-38
In the case of Local Adaptation, this is achieved using a combination of Radius, Threshold, and Toning Curve controls. There are no fast rules on how to use these controls. What you do depends on the content of the image, resolution, and tonal range.
As a very general rule of thumb:
Pay particular attention to areas of detail and extreme tonal range as you adjust Radius and Threshold controls. Standard keyboard navigation commands are available. The Histogram palette isn't functional, but you can use the Eyedropper tool and Info palette to confirm your highlights and shadow areas are within range.
A high resolution image with fine detail requires an increased Radius setting. A low resolution image with little detail requires a very low radius setting. Keep in mind, higher Radius values require more processing power, and it'll take longer for your image to convert. (Simply put, Radius controls edge detail.)
An image with high tonal values will require a higher Threshold setting. (The most useful Threshold settings, however, range from .5 to 2. Higher values really don't do much for the majority of HDR images.)
Set your Radius and Threshold values first, then adjust the Toning Curve. (Some images benefit from a further tweaking of Radius and Threshold values after applying a new Toning Curve.)
Here are three different versions of the same image. For the first two images, I adjusted the Radius and Threshold controls to extremes, with just a slight adjustment of the Toning Curve.
The first image is "flat" and very soft. Figure 5-39
The second image has more contrast and detail, but the highlights are actually blown out. Figure 5-40
The third image demonstrates the effect of the default Radius and Threshold settings, with just a slight tweaking of the Toning Curve. It is a nice compromise. Figure 5-41
Note that the 68.8MB HDR file was created with a Nikon D100 6MP camera, mounted on a tripod Figure 5-42 (8 shots, 1 stop apart, merged using Merge to HDR.)
Let me show you what I mean by limited dynamic range. If you look at this test image, which was taken with a Nikon D200 digital camera, the histogram indicates that the details in the highlight areas of the image are clipped or missing. Figure 5-22
There isn't much I can do with Camera Raw's Adjust controls to fix this image, because the data just isn't there to work with. In this section, I'll show you an how to create an image with great dynamic range. Short of buying another digital camera, it's the best possible solution.
--------------------------------------------------------------------------------
Note: Some digital cameras, such as the Fuji FinePix S3, boast improved dynamic range capabilities, and it is safe to say in a matter of time digital capture will surpass the capabilities of print film. Even when this occurs, the various techniques outlined in this chapter will be useful for those who want the ultimate control of the tonal range of their images.
--------------------------------------------------------------------------------
Using Merge to HDR
One of the newest additions to Photoshop is Merge to HDR (High Dynamic Range). Created by John Petersen, the same brilliant engineer responsible for Photomerge, Merge to HDR allows you to blend or merge three or more files taken of the same scene with different exposures and create a new file capable of storing an almost infinite number of tonal values. Figure 5-23
Few displaysand no printersare capable of utilizing all this data. You'll need to rely on another new Photoshop feature, HDR Conversion, created by Chris Cox. HDR Conversion automatically appears when you change modes to a lower bit rate and provides a variety of ways to squeeze the dynamic data into a useable form.
The advantage of using Merge to HDR is obvious: by using different multiple shots at different exposures, you can extend your dynamic range much greater than is possible with a single shot.
There are obvious limitations to the multiple shooting technique often referred to as "bracketing." First, it's most effective with static scenes. Second, it works best if you carefully frame each shot, preferably using a tripod. Third, it uses up more memory both in the camera and on the hard drive. As you'll see, it also takes some effort to properly use HDR Conversion and get optimal results.
Shooting for HDR
Merge to HDR will work a lot better if you shoot with it in mind from the start:
Use a tripod for consistent results. You can get decent results hand holding your camera, but it becomes hit or miss.
Don't use your digital camera's autofocus! Even the slightest change in focus affects results and autofocus systems are notoriously imprecise.
Vary your exposure by changing the shutter speed using your camera's manual or Aperture Preferred Mode. Figure 5-24 Changing the f-stop would alter the depth of focus and quality would suffer. Don't change ISO between shots either. If you use a flash, use it consistently between frames.
Vary your exposure by at least one stop between frames. (Adobe actually recommends 2 stops between shots; that's fine but it really depends on the capabilities of your camera. Some digital cameras capture less dynamic range than others, and I like to err on the safe side.) Figure 5-25 The bottom line is, if the exposure values are too close, Merge to HDR won't have much effect and if they are too far apart the final image will become posterized. HDR can handle such an extreme range that you can basically shoot from complete black to complete white. (One photographer told me he successfully used 26 images, shot 2 stops apart!)
Don't rely on automatic exposure bracketing. Many cameras offer this feature, however the range is usually limited, typically 23 stops plus and 23 stops minus. I use my camera's +/- exposure controls instead. Refer to your camera manual if you don't know how to set your camera to over-or underexpose.
If you are shooting outdoor scenics, take the shots in rapid sequence. Clouds move, trees and flags wave in the wind, and every change between frames will affect the final image. Figure 5-26
Merging with HDR
Ok, you've got your shots. Here you go.
1. Select File > Automate > Merge to HDR. Browse to your images. (You can also access Merge to HDR directly from Bridge. Just select the files you wish to merge, hold the Shift or Cmd/Ctrl keys to select multiple files, then Select Tools > Photoshop > Merge to HDR.) Figure 5-27
If your images were shot without benefit of a tripod, be sure to select Attempt to Automatically Align Source Images.
Figure 5-28 If you are working from Bridge, you don't have a choice: your images will open without benefit of auto alignment. This is fine if your images were shot using a tripod, but it'll be a problem if they were not.(Officially, you are required to use multiple photos shot with different exposure values. However you can fool Merge to HDR if you strip the EXIF information from your single RAW file.
It can take some time for the images to open in the Merge to HDR window, especially if you have selected Attempt to Automatically Align Source Images. How long it takes depends on the number of images you selected, the resolution, and of course, the processing speed of your computer. Figure 5-29 Keep in mind that Merge to HDR uses Camera Raw to convert RAW files. However, it overrides Camera Raw's exposure, shadow, brightness, and contrast controls and relies instead on the camera's EXIF data for these values. It also converts the RAW data to 8 Bits/Pixel. If you change other Camera Raw parameters (such as Sharpen) be sure to do the same for all RAW files. Otherwise, quality may suffer.
2. Preview the results of your merge. Use typical navigation keyboard shortcuts, or control the viewable size by selecting from the percentage values at the bottom of the window. However, since most monitors are capable of displaying only 8 Bits/Channel of color data, you are only seeing a slice of the full HDR range. You can use the slider under the Set White Point Preview to vary the visible slice. This doesn't do anything to the HDR data; it just controls the preview. Figure 5-30
3. Adjust the effects of the merge by deselecting images and removing them one at a time. Figure 5-31 To do this, un-check the box below the thumbnail image. Each time you deselect an image, the process of recreating a new merged image takes time, as indicated by the status bar at the bottom left of the window.
4. When you are ready, you can select OK, or choose a bit depth. I suggest for now you just select OK, and save the full 32-bit HDR data. You can select a lower bit depth later, if that is what you want.
Saving your HDR file
You can save the HDR file in any file format capable of retaining floating point data, which includes: Photoshop (.psd), Portable Bitmap (.pfm), TIFF (.tif) and OpenEXR (.exr). Figure 5-32 You can't save a HDR file as a JPEG unless you convert it first to 8 Bits/Pixel, but then it won't be HDR anymore. Remember HDR files are quite large. For example, an HDR file created by merging images from my 6MP camera weighed in at 68.8MB.
Working with HDR files in Photoshop
If you wish to work on the HDR file in Photoshop, you'll find the tools limited at this time. The Histogram palette is not functional, although the Info palette is. Levels and Curves aren't an option, but Channel Mixer, Photo Filter, and Exposure are. Figure 5-33 Multiple layers are not an option, but the History Brush and Clone Stamp tools are. You can crop and resize and change the canvas size. You can apply arbitrary rotations and flips, as well as free transform and warps. Many filters are available, including Gaussian Blur, Unsharp Mask, Smart Sharpen, and Add Noise. You may find it easier to simply convert your HDR file to a lower bit range and do your serious editing then. If you thought Smart Sharpen was slow on a 16-bit file, just try it on a HDR file!
Changing the Preview for your HDR File
You can change the preview to reflect different exposure and gamma settings without affecting the underlying data. Select View > 32-bit Preview Options from the main menu bar and this dialog box will appear. Figure 5-34
You can compare multiple versions of the same image by choosing Window > Arrange > New Window from the Photoshop menu. Then use the 32-Bit Exposure slider found at the bottom left of the image window to preview different settings for different windows and do a side-by-side comparison. Figure 5-35
Converting HDR Files to a Useable Form
For all practical purposes, you'll want to convert your HDR files to a usable tonal distribution and practical bit depth. The way you squeeze all that data into a more limited space is critical, and sometimes it takes a bit of work to get it right.
To convert your file:
1. With your HDR file open, select Image > Mode > 16 Bits/Channel or 8 Bits/Channel. (Choosing Grayscale only discards color values and doesn't reduce the bit value.)
2. When you do this, you'll get this dialog box. This is where you'll make all the hard choices. Figure 5-36
Your options are found in the Method pop-up menu. They include Exposure and Gamma, Highlight Compression, Equalize Histogram, and Local Adaptation. Figure 5-37
Exposure and Gamma, as the name implies, gives you straightforward control over brightness and contrast. Don't be mislead by the Toning Curve and Histogram. The histogram displays the inherent values, but the Toning Curve isn't functional. (It's functional only when you select Local Adaptation.)
Highlight Compression and Equalize Histogram apply preset equations that may or may not be appropriate. There are no user controls for these options. I've found the Equalize Histogram works reasonably well on some merged images, but I've haven't had much luck with Highlight Compression. Try for yourself.
Use Local Adaptation to gain real control. Local Adaptation and Photoshop's Shadows/Highlight image adjustment command work similarly: they both attempt to lighten or darken an image, based on surrounding pixels. With a localized approach, it is possible to lighten a dark area (or darken an area that's too light) without affecting other areas of the image. Figure 5-38
In the case of Local Adaptation, this is achieved using a combination of Radius, Threshold, and Toning Curve controls. There are no fast rules on how to use these controls. What you do depends on the content of the image, resolution, and tonal range.
As a very general rule of thumb:
Pay particular attention to areas of detail and extreme tonal range as you adjust Radius and Threshold controls. Standard keyboard navigation commands are available. The Histogram palette isn't functional, but you can use the Eyedropper tool and Info palette to confirm your highlights and shadow areas are within range.
A high resolution image with fine detail requires an increased Radius setting. A low resolution image with little detail requires a very low radius setting. Keep in mind, higher Radius values require more processing power, and it'll take longer for your image to convert. (Simply put, Radius controls edge detail.)
An image with high tonal values will require a higher Threshold setting. (The most useful Threshold settings, however, range from .5 to 2. Higher values really don't do much for the majority of HDR images.)
Set your Radius and Threshold values first, then adjust the Toning Curve. (Some images benefit from a further tweaking of Radius and Threshold values after applying a new Toning Curve.)
Here are three different versions of the same image. For the first two images, I adjusted the Radius and Threshold controls to extremes, with just a slight adjustment of the Toning Curve.
The first image is "flat" and very soft. Figure 5-39
The second image has more contrast and detail, but the highlights are actually blown out. Figure 5-40
The third image demonstrates the effect of the default Radius and Threshold settings, with just a slight tweaking of the Toning Curve. It is a nice compromise. Figure 5-41
Note that the 68.8MB HDR file was created with a Nikon D100 6MP camera, mounted on a tripod Figure 5-42 (8 shots, 1 stop apart, merged using Merge to HDR.)