A side note: The Photoshop CS2 help entry on its Merge to HDR function states, “In general, don’t use your camera’s auto-bracket feature, because the exposure changes are usually too small.” This may apply to digital point and shoot cameras, however all DSLR’s should support exposure stepping up to 2 EV or more when auto-bracketing.

If you shoot digital, I recommend that you shoot RAW for HDR work. The point is to capture DR. If you shoot JPEG out of the camera, some amount of DR has already been sacrificed within each given image as the camera compresses the sensor data (usually 12 bits) down to 8 bits. Further, the camera applies a tone curve, compressing shadow tones in order to favor highlights. Since I personally find that shadow tones contribute a lot to my HDR work, I would not like to sacrifice them. Finally, introducing JPEG compression artifacts (however slight) into the HDR process may degrade image quality.

I understand there are arguments in favor of shooting JPEG. Typically the benefits raised are to get more continuous frames, more storage capacity, and potentially minimize subsequent workflow effort in RAW conversion. However, in my opinion, these factors are not particularly relevant for HDR work, especially landscape photography as discussed here. However, if you wish to do so, you can shoot JPEG images and process them using the HDR tools described in this article.

For most other settings, the rule of thumb is to keep the camera in manual mode for each function. This ensures that nothing the camera does will vary the image sequence in a way you do not purposely intend. When shooting the sequence, you essentially want only the exposure changing to capture the range of tonality you need. Everything else should remain constant to prevent more work later, or loss of image quality.

Here are a few final points:

• For digital shooters, set the ISO to as low a setting as conditions will allow. Any noise introduced by higher ISO may be exaggerated by the HDR tone mapping process. If the source scene contains moving elements that blur at slower shutter speeds, you can try increasing ISO to get faster shutter speeds, and take your chances with noise reduction tools later.
• Ensure white balance stays the same across the images fed into the HDR process, since it is important to preserve consistent color. If you are shooting RAW, leave the camera set for automatic white balance and later convert all input images with the same white balance setting. Or you can set a specific white balance in the camera and adjust later as required.
• If you are manually bracketing any portion of the exposure sequence, use manual focus. If auto-focus is left enabled, a different focal point may be selected between one frame and the next. This will create image combination problems that cannot be resolved by the HDR software.

Determining the Exposure Sequence

How do you know what that best exposure sequence is? Clearly this depends on the source scene. It also involves how much shadow and highlight detail you decide to capture – “high” DR does not necessarily mean “all the DR there is.” Finally it depends on how many images at what exposure interval you choose to shoot with your preferred bracketing technique.

Examine the scene looking for shadow and highlight areas. In those areas, use an external light meter, the camera’s spot meter if it has one, or some test shots consulting the histogram (if you shoot digital). This will give you some potential exposure values on each end of the DR spectrum that you ultimately need to capture. You will then have to do some quick exposure math to figure out how many frames at what EV stepping you will need to capture the scene.

You can cover the scene’s DR in fewer exposures by stepping at a higher EV interval, but it is not desirable to use a larger interval than 2 EV even if your camera supports it. Stepping by 1 EV may be preferable depending on the HDR tool. As with any software function that is interpolating reality between one recorded point and another, more data points sampled closer together produce better and smoother final results.

For example, if a given scene’s highlights meter at 1/2000s at your chosen aperture and the shadows meter at 1/4s, then ten exposures taken at an interval of 1 EV would cover the range. You could possibly eliminate one or two of the exposures on the far ends of the range if you shot RAW and can depend on your camera for good shadow and highlight detail capture. Five exposures shot 2 stops apart also would do the job, at the risk of providing a bit less smooth result.

With only a little effort, you can extend the automatic bracketed approach described in the previous section to cover five shots at a 2 EV interval, even if your camera is limited to a three shot burst using auto-bracketing. Enable auto-bracketing at +/- 2 EV and select a good central exposure based on the camera’s meter. Prior to shooting, dial in -2 EV exposure compensation and shoot three frames. Now quickly dial up to +2 EV compensation and shoot three more frames. At this point you have six frames, two of which are exposed the same; discard one of them later. This leaves you with five frames covering -4 to +4 EV, roughly 16.5 stops, which approaches DR of 100,000:1. If your camera supports exposure compensation of +/- 3 EV, you can use the same technique without throwing away one of the exposures. You end up with six frames covering -5 to +5 EV, or about 18.5 stops of DR.

Here is an example showing the same scene processed twice via HDR, from two image sequences taken seconds apart. The first sequence contains three exposures covering -2 to +2 EV, while the second contains five images covering -4 to +4 EV. Both sequences have the same central exposure, and both use an interval of 2 EV. Both were tone mapped with the same parameters in Photomatix Pro, with no further retouching done.

Three Image Sequence	Five Image Sequence

In this case, there is not much difference between the resulting tone mapped images. The one based on the five image sequence is a little more contrasty due to slightly deeper blacks. For that reason it has slightly better definition in the clouds. But on the whole the two are close, and the three exposure version likely could be touched up to become even closer. For this scene three exposures at +/- 2 EV were sufficient to capture the available DR. In part this is because the sun is not directly within the frame, and there are no extreme shadows or highlights.

Outdoor scenes can have DR of 100,000:1 or more, but many have less. Learning to read the scene can reduce the need for metering and calculating a longer exposure sequence. Likewise, gaining that experience improves on the “just shoot a bunch of exposures and hope for the best” approach.

Note that by shifting the exposure range, for example by moving the base exposure up or down when using auto-bracketing, you can bias the eventual results towards the darker or lighter tones. If manually bracketing, you can choose to leave off some of the exposures on the bottom or top end of the range. You can also shoot the exposures and then simply not include them in the HDR input. This lets you adjust the mood, block up some shadows or clip some highlights for creative reasons.

Some experimentation and experience with a variety of scenes will give you a baseline for the number and stepping of exposures you need to produce pleasing results. So far I have been using three or five frames shot at a 2 EV step and have been happy enough with my results. With only three frames, some highlights and shadows are lost but often even this small a sequence produces pleasing results.

RAW Conversion

For those who shoot in RAW mode, another setup issue that must be considered involves how to configure the RAW converter. There are numerous RAW conversion applications out there, each with its strengths, weaknesses and proponents. Which one you use is not really that important for the purposes of HDR processing. What is more important is how you do the conversion.

One key factor, as previously indicated, is to ensure that all of the images in a sequence are processed with the same white balance. Tone mapping HDR images is challenging enough without throwing different color balances into the mix. Pick a representative exposure from the input sequence, get its color temperature and tint right, and then apply the same settings to the entire sequence. Do the same with any other color enhancements that you make in the RAW conversion. Many RAW converters with strong workflow support make it easy to copy settings across a series of images.

The main point of HDR tone mapping is to set levels, apply a tone curve and make contrast enhancements to the final image containing DR information compiled from all input images. Therefore it is best to avoid making any significant exposure changes to the RAW files during conversion. If the initial exposures were set up reasonably well, most of the changes that you might make to an individual input image will be trumped by the changes later made during HDR processing.

The main reason you might be tempted to adjust exposure in the RAW conversion is to shift the entire input sequence up or down, to bias the final tone mapped HDR image. As with many digital processing functions, use a light hand. This is especially true if you are increasing the exposure as this will bring out noise that the HDR processing may emphasize further. However before even trying this, recall that shifting the exposure during RAW conversion can not bring out any more real luminance information than existed in the original RAW data. Since the HDR image is going to include all of that data by mapping all of the input images, it may be less work to perform a straight forward RAW conversion, process and tone map the HDR image, and then make final exposure adjustments to the end result.

Single Frame Scenes vs. Multi-frame Stitched Panoramas

For stitching multiple frames to work well as input to HDR processing, it is even more important to ensure that camera support is stable and all non-essential camera functions are on manual as recommended above. Stitching software can compensate for a number of things that are not quite synchronized between images across the field of view. However, the more work the software has to do, the more the quality of the final product may be jeopardized. Keeping in mind that HDR tone mapping may exaggerate undesirable details in the input images, you do not want stitching artifacts to be introduced and subsequently be magnified.

One other thing to consider when planning for both stitching and HDR work on an image sequence is the shooting time factor. If you are shooting outside at sunrise or sunset for example, the light may be changing relatively quickly in your critical shooting window. The more fiddling you have to do to capture the sequence across the field of view, the more likely it is that the light quality may change perceptibly between the beginning and the end of the sequence. It is also possible that moving elements such as clouds or water will shift enough that seamless processing will be made more difficult.

Some examples of things that can help decrease the time taken to shoot the entire sequence include:

• Use automatic exposure bracketing with a larger EV interval to speed up the capture of exposures taken at each rotation point. Fewer manual exposure changes means less time taken.
• Make sure you have chosen a good focal point that works across the field of view, and disabled auto-focus on the lens. You do not want to wait for auto-focus to hunt for a new lock at each rotation point.
• Use a lens focal length that gives a wider angle, which may cut several one or more rotation points from the sequence. You can crop later for compositional reasons if you do not mind losing the resolution. The risk here is getting an increased amount of rectilinear distortion with some lenses.
• If you normally shoot vertical frames for stitching, consider whether the composition will support horizontal shots instead. This will cover the field of view even faster.
• Ensure that your exposure sequence is not filling up your camera buffer faster than it can write. Find ways to shoot fewer exposures, in order not to be waiting on the camera while the light is changing in front of you.

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