Capture More Light: Getting the Most Out of Your HDR Images

March 28, 2011

Never underestimate the effect of even a gentle breeze. You will find some nasty artifacts in your pictures.

This is the final article of a 3 part series:

  1. HDR (High Dynamic Range) and how to create HDR images (click to read)
  2. High speed bracketing method (click to read)
  3. Getting the Most Out of Your HDR Images

It sounds so easy to combine images -- and, technically, it really is. This article covers some of the major challenges towards getting excellent HDR photos.

We cover the following aspects:

  • Camera Settings (Exposure)
  • Alignment
  • Ghosting
  • Chromatic Aberrations (CA)

As mentioned in the previous articles, the main concern for HDR photography has to be all kinds of movement. This means that most of the challenges are related to dealing with different kinds of movement.

Camera Settings

For a proper exposure, these factors should be constant for all exposures in the series:

  • F-stop (shutter speed with vary for each exposure; we suggest aperture-priority mode)
  • Focus (sometimes using manual focus can be a plus)
  • White balance (can also be changed later in post processing; we use Auto WB and then change the WB later when processing the RAW images)
  • Movement (the camera should not move or shake -- use Live View, if available, to lock the mirror up when shooting from a tripod)
  • ISO (should not change)

The only parameter that should change is the shutter speed.

Alignment

Perfect alignment is key to a good HDR image generation. If the images are not perfectly aligned, it is like blurring your photos. You don’t want to use a high quality digital SLR and then throw away the resolution by poor alignment. Today’s HDR software is very effective to correct the alignment.

Photomatix does a good job aligning the images -- but in some cases we use Adobe Photoshop CS3 to align our images. Because it is not common knowledge how to perform the alignment in CS3, we cover it here.

Image Alignment in Photoshop CS3

Photoshop CS3 does a very good job aligning images. Here is how to align images for the purpose of later HDR generation in Photomatix, or other HDR software.

To align these three images you use the CS3 function File -> Scripts -> Load Files Into Stack. The following dialog shows up (after you selected your source images):

After clicking OK, CS3 will load the three files as three layers in a single file. If you've shot RAW images, opening them in Photoshop automatically converts the RAW files using Adobe Camera Raw (note that the layers retain the .CR2 extension, but are no longer RAW files). We then select all three layers:

Now you use the CS3 function Edit -> Auto-Align Layers and get this Auto-Align Layers dialog window.

Use the Auto option and click OK. CS3 will then align the layers.

This is not just a simple shift operation. CS3 can perform:

  • Shifts (up/down or right/left)
  • Rotations (if the camera gets tilted)
  • Morphing to stretch (if the camera gets closer or further away)

The result is quite remarkable. Once the layers are aligned you should inspect the result (making layers visible/invisible) and crop the image to the parts that are present in all your bracketed photos.

Here is the key part: We want to create new TIFF file that represent the layers after alignment. Fortunately CS3 has a neat little function: File -> Scripts -> Export Layers to Files

We only mention the important parameters.

  • Destination: Folder for the new files
  • File Type: TIFF (you don’t want to lose quality using JPEG; The bit depth is the same as the source images, and best is 16 bit)
  • Include ICC Profile: Always checked

That is all there is to it. You now have a new TIFF files that you can use to generate HDR images. If you then use Photomatix, do not check “Align images” because they are already perfectly aligned. There is no general rule when Photoshop may align more perfectly than Photomatix. That decision has to be made on an image-by-image basis (the differences can be very subtle, but we aim for optimal resolution).

Ghosting

While misalignment is related to (minor) camera movements, ghosting is caused by moving objects in the scene.

Here some common causes:

  • Moving branches and leaves
  • Flying birds
  • Flags
  • People and larger animals
  • Cars
  • Clouds
  • Water

We are not too enthusiastic about the tools that can help to remove ghosting. If you want a crisp and sharp image, ghosting may spoil your party. One of the only truly effective and preventative shooting techniques is to take many, many pictures of the same scene -- then, it may be possible to remove elements like people, cars and animals because (if you're lucky) one of the frames may not show the object.

Let’s have a closer look at some of the causes (and suggested solutions) to ghosting:

Moving branches and leaves

Never underestimate the effect of even a gentle breeze. You will find some nasty artifacts in your pictures. The ghosting feature in Photomatix may reduce the effect a bit. In some cases it may save your day (especially from B&W photos).

Flying birds

If the birds are just in the sky try to use spot healing in Photoshop CS3 to clean them up.

Flags

The only option we see is to clone the flag out of the picture. Can be tricky.

People and larger animals

If the ghostly appearance of people fits into the picture (e.g. in the distance) all is fine. Otherwise this maybe a lost cause.

Cars

If you have a replacement shot fine. Otherwise forget about it.

Clouds

Clouds are often no problem at all if they do not interfere with other elements in the photo.

Water

Water can work if it is slow moving. Surf often looks odd.

 

Chromatic Abarrations

Often, you may see artifacts in your images, like these below -- this is a result of chromatic aberrations (sometimes called color fringing):

What is chromatic aberration (CA)? Most lenses are not perfect, and can sometimes focus the red, green and blue channels on different planes (relative to the sensor). This can produce green/purple fringes at higher contrast edges. The effect is stronger towards the corners than in the center. Even top rated zoom and wide-angle prime lenses can show this artifact, especially at higher contrast edges. Remember, we use HDR to cope with capturing higher contrast scenes; this means we have to deal with chromatic aberration. Besides misalignment, we find CA one of the major show stoppers for our HDR photography. Fortunately, CA can quite easily be fixed.

A tone mapped, or blended, image shows often stronger chromatic aberrations than the source images (cumulative effect). Careful inspection of the source images is needed to reveal CA.

Fortunately, modern RAW converters like Canon Digital Photo Professional (DPP) can help to fix chromatic aberrations. Here is the simple method to correct CA in DPP (you’ll need to do this with each RAW file in the HDR sequence): Open the Lens Aberration Correction function, and just checking the Chromatic Aberration box does the trick:

Be sure you correct all the images in your bracketed series (please note that DPP software is only compatible with RAW images from Canon digital SLRs).

Summary

For most of the challenges the HDR photographer faces, there are good solutions to fix them:

  • Alignment
  • Exposure
  • Chromatic Aberrations

Ghosting, unfortunately, is not that easy to fix. You are much better off to carefully examine the scene before you shoot, and avoid moving elements in the first place whenever possible.

The CDLC contributors are compensated spokespersons and actual users of the Canon products that they promote.

All images are copyright Uwe Steinmuller

http://learn.usa.canon.com

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