The art of HDR photography part 3
Chapter 3: Essential HDR Workflow
|Zion Lone Tree|
Scene Analysis for HDR
"To HDR or not to HDR" is often the question while photographing a scene. Regardless of whether a particular scene requires HDR to accurately capture it's essence, it is pretty much always beneficial to have bracketed photos to work with. Having multiple exposures allows you to chose the best single shot or to go the HDR route. Here is a short checklist to assist you in making that decision.
Here are some situations in which attempting to use HDR techniques would produce unusable results:
- People or animals in motion
- Birds in flight
- Sports or other high motion activities
- Foliage in windy conditions
- Ocean with strong surf (if detail should be visible)
We almost always shoot bracketed exposures. We always have the middle exposure as the first shot and have the option to just delete the other exposures later if we don't need them. The first shot will be the normally exposed shot taken at the moment you hit the shutter. Of course bracketing shots uses more frames and so for fast action should always be turned off to increase responsiveness.
Scene Dynamic Range Evaluation
Evaluate the scene for the dynamic range you would like to capture.
- Overcast: No HDR needed in most cases (unless you plan to show details in darker shadows).
- Midday sun with strong shadows: Take 3 shots at about 1 ~ 1.33 EV apart.
|Mission San Antonio in California|
In these cases a single middle exposure photo often works fine but bracketing for HDR can give you more options for nice highlights and open shadows. As with the photo above we often shoot these scenes handheld, because there is enough light to get a fast shutter speed (even for the overexposed image).
- Dark rooms with some light through the windows: Here we usually take at least 5 bracketed shots with about 2EV apart. Using a sturdy tripod is a must.
|Fort Point: Officer's Bed|
- Dark room with detail in bright scene outside the windows: Indoors in low light you may need to bracket for 7 or more exposures. In this situation a tripod is of course is a must. The use of a manual programmable remote control allows you to change the exposure time without moving the camera.
Note: These types of images only pay off if there is an artistic balance between the interior and exterior zones of the image.
Highlight/ Shadow Challenges
Always make sure you know how to capture the details in the highlights and how to get the shadow details you want. While highlight clipping should always be avoided it is your artistic decision how open the shadows should be.
Two factors will help you decide wether to use a tripod or not:
- Your personal style of photography (tripods are stable but handheld gives you more flexibility).
- If the most overexposed photo of the set can't be taken without a tripod (without inducing motion blur), the only option is to use a good tripod or forget about HDR.
Capturing Exposures for HDR
After analyzing the scene (which normally only takes a few seconds) you should know how many brackets to shoot. Depending on your personal shooting style you can then decide on whether to use a tripod or shoot the scene handheld.
Normally we rely on the camera's AEB (Auto Exposure Bracketing) feature. Here the camera controls the exposures of all the shoots needed.
Here is how it looks on more recent Canon and Nikon cameras.
|Canon 3 shots @+/- 2EV||Nikon 5 shots at 1EV apart|
Note: It's unfortunate that some camera manufacturers limit some features which would be useful to HDR photography:
- Limited maximum EV step of 1EV (in some cases it's limited to 2/3EV). We've pushed for change in our reviews for years but manufacturers have yet to take note.
- Limited the maximum number of bracketed shots to 3 photos (except in the higher priced cameras).
- All cameras we know of limit the maximum exposure time to 30 seconds. I don't believe there's a solid technical reason for this any more. Yes, I know long exposures can create noise, but many cameras work quite well beyond 30 second exposures. The photo below stressed the 30 second limit quite a bit.
HDR Brackets from the Tripod
For HDR a tripod is even more essential than with single shots, to avoid any movement between photos.
- Sturdy tripod, heads, clamps and plates are needed
- Use a remote if available or the camera's built-in timer
- Using live view or mirror lockup can reduce camera shake when taking longer exposures.
HDR Brackets Handheld
Software can work very well to align your bracketed images (more later) so in many cases shooting HDR hand-held can work quite well. For these shots we enable AEB for three exposures (usually about 1.33 EV apart). Then we use the highest possible frame rate to capture all three exposures. To decrease motion blur the fastest high speed shooting possible is desired. Using 7-10 frames per second can produce good results but we also use slower frame rates. The camera should be set to continuous shooting and will normally stop once the bracketed exposures are done. We need to keep the aperture constant so we shoot in Aperture Priority mode.
Keep in mind the order of the exposures as well. When shooting we always try to use the following sequence: normal, under-exposed, over-exposed - if possible. This way the normal exposure is the one we capture nearest to the moment we hit the shutter. We set the EV compensation for Aperture Priority so that in many cases the middle exposure can be used as a single shot. The under-exposed shot can be used as a backup in case the middle exposure shows clipped highlights. The last and over-exposed photo has a higher likelihood to show motion blur from camera shake because it uses the longest exposure. That is why it's important to check that the highest possible shutter speed is fast enough for the +1 1/3EV exposure
We nearly always use cameras with lens image stabilization (IS, VR) to get our photos as stable as possible. Hold the camera steady through out all the shots. We will sometimes even take 5 shots, though only with cameras that can do at least 7 frames per second.
It is very important to keep the White Balance (WB) the same for all shots in your bracket sequence. However, we still use Auto WB because we only use the camera's Raw image format anyway.
- Most often the auto WB will be the same or very close.
- All Raw converters can easily allow to sync the WB settings in your sequence. This does not cause any perceptible loss of quality.
If you plan to shoot JPEG for any reason it's best to set a manual WB for your sequence. Remember to check your WB before you shoot your next scene.
More Issues to check for
Avoid to shooting directly into the sun.
- This could physically damage the sensor
- The flare introduced in the lens and camera reduces image quality
High Speed HDR
We've already covered this to some extent in our section about handheld bracketed shooting. In this section we will go more in-depth on high-speed bracket shooting because it can create an entirely new style of shooting in the field.
With the camera set to the highest burst rate (7 - 10 fps) we capture bracketed shots with different exposures at this speed. This means a 3-exposure bracketed sequence (-1, 0, +1EV) is shot in less than a second.
To ensure success using this method make sure the camera exposure is set to aperture priority and watch that the shutter speed is fast enough to produce sharp photos. If it's not you may need to use a higher ISO or a tripod.
As you can see, the basics are very simple. This is important because in the field we want to get as much technique out of our way as possible so you can concentrate on the subject scene.
Why "High Speed bracketing"?
We have combined the classic bracketing approach with modern HDR techniques.
This proven technique is based on taking multiple shots at different exposure settings to ensure a single best photo. The only difference here is that we shoot these frames so fast that it feels like a single shot (at least once you make it a habit).
Expose to the right
In essence this well known technique means choosing an exposure that allows you to fully recover all of the highlights. If you only grab one shot it may turn out to be unusable if you are not able to recover all highlights. This is not always easy to check in the field. With our technique we always have the underexposed shot as a solid backup.
Since we're not overly worried about the right exposure (one of the three frames will be good enough) we can concentrate on the scene we're trying to photograph. In the past I've checked histograms all the time and even used manual exposure, but now I more or less leave exposure on automatic (Aperture priority). I only avoid automatic exposures when shooting photos for stitching, when all the frames need to have the exact same exposure.
Using advanced HDR techniques
Because we always capture bracketed exposures we always have the option to use HDR if it would help our final image.
It may sound like my intent is to always merge images to HDR. This is not necessarily the case, but we always have the option to do so if necessary - which is why we call it "Capturing more Light". Using this technique we capture about 2-3 f-stops more dynamic range that we can make use of if need be. If you consider that many of the recent top DSLRs capture about 8-10 f-stops in a single frame we can extend the dynamic range to 10-13 f-stops (this approaches the dynamic range of color negative film).
Analyze the bracketed Exposures
Dynamic Range coverage: Check the Histogram
It is good practice to check the camera histogram of your sequence after the shoot.
|Under exposed photo|
It is important that the under exposed photo has no clipped highlights, otherwise the final HDR image will also show the same clipping. Look for a good amount of head room as shown in the histogram.
|Over exposed photo|
You do not want too much clipping in the shadows. The histogram below is acceptable, though more headroom in the shadows may have been better.
|Histogram of an over exposed photo.|
|Middle exposed photo
As expected the middle/normal exposure falls in between the other exposures. There is not much to check here.
|Histogram of the middle/normal exposed photo|
Check for Motion Blur
You should also check the images for motion blur. In most cases you will know how these shots pan out at certain shutter speeds based on experience.
Check for moving objects
Check both while shooting and afterwards for unwanted movement in the scene that might cause a problem.
Raw conversion for HDR
We are assuming that you will capture your images in Raw mode. HDR from JPEGs will work but is often not of the same quality.
White Balance (WB)
All shots in the sequence should have the same WB. Correct the WB of the normal exposure in your Raw converter and then synchronize the WB settings to the other exposures.
Chromatic Aberrations (CA) and other Lens Corrections
We mentioned before that CA correction is crucial for good HDR images. We can perform this correction using the Lightroom 3 Lens Correction tool:
|Lightroom 3 Lens Correction Settings|
If your lenses are supported in Lightroom the correction will be applied automatically otherwise you have to use the manual CA corrections. We also set "Defringe" to "All Edges" with the goal being to reduce or remove as much fringing as possible.
Correcting the lens distortions and vignetting is more a matter of preference. However, if you photograph architecture it can be essential.
No major Curve or Basic settings corrections
Keep the images as linear as possible. Merging images to HDR works best with linear files because the HDR images themselves store the tonal values in a linear way. All the HDR tools try to recreate the linear data if they work from TIFF files (it's best if you also use 16 bit data here).
Actually "Merge to HDR" in Photoshop ignores these settings to get the best possible results. It is unfortunate this does not work in cases where you need to perform Highlight Recovery (see below).
Highlight Recovery (HR) can save the day
If your bracketed sequence covered the highlights well you do not need to use this trick but every once in a while it can save the day.
We had a chance to take some pictures at the abandoned Gilroy Hot Springs. These hot springs are is a part of Henry Coe State Park, but most of the time are closed for the public. We had hoped for some overcast skies (having started out with an overcast morning in San Juan Bautista) but when we arrived at the Springs we found the worst light possible (10am Summer sunlight). As usual we shot all our photos bracketed and handheld (here a Sony NEX-5 with its very limited EV spacing of 2/3EV). To be honest we thought these images would mostly be a lost cause (especially the image we feature here). Fortunately the Highlight Recovery in Lightroom 3 saved the day.
|Basic default setting for linear conversion (all are zero)|
As we examine the underexposed photo we can find that even though there is no fatal clipping visible, it is still too much for our taste. Clipping in the underexposed photo cannot be recovered in the final HDR image and results in bad highlights after tone-mapping.
|Basic settings with Highlight Recovery|
Once we are able to produce an image with good Highlight Recovery for the most underexposed photo we then sync the settings to the other photos in the bracketing series.
Note: You cannot use these settings directly with Lightroom 3 using Edit in > Merge to HDR because in this case Photoshop will bypass these corrections in an attempt to access better data directly from the Raw. You should create TIFF files instead and work with them in Photoshop.
As we mentioned earlier, this saved the day and we got the final result we wanted after merging to HDR and tone-mapping. Still, it's best you take proper bracketed exposures to avoid the need for this kind of trickery.
As always, shooting at lowest ISO level possible avoids noise. This is rarely an issue if you use a tripod. But, if you shoot handheld you have to balance Aperture, Shutter-Speed and ISO wisely. Because the Aperture is defined by your lens and intended DOF (Depth of Field) you basically have two parameters to work with:
- Shutter Speed: needs to be fast enough to avoid motion blur on the most over exposed photo.
- ISO: Higher ISO shows more noise.
We would gladly trade more noise for bad motion blur which would make the exposure unusable.
It's best not to sharpen before we merge to HDR for the following reasons:
- The HDR and tone-mapping process may amplify sharpening artifacts
- Part of the tone-mapping process also extracts more details
- Sharpening as final step is often a good practice anyway
Optimal Image Alignment
In this section we will cover alignment as a a discrete step although most HDR tools include automatic alignment. Unfortunately not all alignment is of equal quality.
We have often found that the alignment in Photoshop CS5 gets the best results, good enough for HDR images of high quality.
We have created a modified CS5 script that you can download for free. The use of this script is at your own risk and does not include any support. This script is only designed to work with CS5. We use this script on our Mac all the time and it should work on Windows as well. You should download this script before continuing with this section as it will be referenced in the process.
The script can be downloaded from here (ZIP File). Copy the script to the Photoshop/Presets/Scripts folder.
The key to any good HDR merge is a precise alignment of all of the images. Photographers often use very sturdy tripods to shoot nearly perfectly aligned images. In this case the HDR software will have a little bit of work to do to perfect the alignment. Bad alignment may not be that easily visible but can show as a loss in resolution, and all we can do is to use the best alignment available to us. For high resolution HDR images proper alignment is ultra-critical.
Try it: Shoot with a 200mm lens on a very sturdy tripod with a camera such as a Canon 1Ds Mark III or Nikon D3x. Follow all the rules about mirror lockup and remote shutter release. Take 3 shots. You will be surprised how much the shutter alone rattles your camera.
On the other hand we shoot a lot of bracketed shots handheld, at which point the task of alignment becomes more complicated. In our experience there is not much of a problem when you use cameras that can shoot 7-10 fps sequences. However, we often use the Canon 5D Mark II or Canon 550D which can only shoot at about 3fps. In this case we've found that some HDR applications can fail to perfectly align all the shots. Remember you may tilt, shift and/or rotate your camera between shots.
In our experience Photoshop CS5 most often performs the best alignment. You may be asking yourself: "why not create the full HDR image in CS5 too?" Unfortunately not all HDR merging software performs equally (e.g. saturation) so we may want to perform the HDR creation in other applications.
Which leaves the following question: How to align images in CS5 and then merge to HDR in an application of your choice later?
Here we will explain how to do it. It makes the workflow a bit more tedious but is clearly worth the effort.
|Three bracketed exposures|
Step 1: Settings in Lightroom (or Camera Raw)
We will perform some basic and important corrections in Lightroom 3 (identical for all images in our bracketing sequence).
- Highlight recovery if needed
- Lens Correction (we want as much CA removed as possible)
- Noise reduction if needed
- White balance
- Keep all tone curves linear
- Black Point to 0
Step 2: Open images as Layers into Photoshop CS5
|Select all images of the sequence in Lightroom and choose Edit in > Open as Layers in Photoshop.|
|This opens all these images as a Layer Stack into Photoshop CS5|
Step 3 + 4: Align and Crop
Note: As a welcome side effect: the alignment will likely be slightly improved over the normal CS5 auto alignment (the script uses an enhanced auto align version used by the CS5 "Merge to HDR" script).
Step 5: Export Layers into files
This is an edited version of the third chapter of an ongoing work by Uwe Steinmueller of Digital Outback Photo, featuring his personal experiences of HDR photography, and will eventually form the basis of a book on the art of HDR photography. If you'd like to find out more about digital imaging workflow from a fine art photographer's perspective then check out the Digital Outback Photo E-book, 'The Digital Photography Workflow Handbook (2010)', by Uwe Steinmueller and Juergen Gulbins, which covers the complete digital photography workflow from input to output. The 540 page prize-winning handbook covers everything from Import to Print (and even backup) and also features Photoshop and Lightroom techniques, HDR, color management and raw editing.
© 2011, www.dpreview.com & Uwe Steinmueller.