HDR Tutorial Part 2: Merging and Tone Mapping

In part 1 of this tutorial we discussed about the how’s and why’s of HDR including how to properly capture multiple exposures of the same scene as building blocks for our final image. Here we will cover how to merge and tone map our shots.

Refer to the photos above where we took multiple exposures of the same scene: EV0, EV-2, and EV+2. These are unedited photos taken at dawn. I only cropped them at 8×10 format. Look closely at their corresponding histograms. The “standard” shot EV0, although acceptable, did not fully capture what I saw when I took the shot. The sky actually had a beautiful magenta and bluish tint. The sky looks gray because it is overexposed. If we look at the histogram we can see lost of pixels to the extreme right. By taking another shot with an exposure that is two stops lower, EV-2, we can now see the colors of the sky that I was talking about. The problem with this shot is underexposure. Most of the pixels are now flushed to the left of the histogram that is why the image is too dark. Let’s look at EV0 again. Notice that not only is the sky overexposed, portions of the wreck are also hidden in deep shadows. Actually, this isn’t too big of a problem but there are interesting stuff in there that I want to show. So I took another shot but this time I compensated two stops higher, EV+2. This is just enough to show the structures hidden behind the shadows but the sky is now just pure white.

With AEB, those three images can be captured in under five seconds. My EV0 was a bit under one second at f11.

Most of the time, three exposures are enough to get acceptable results for HDR. Always check your histogram and make sure that your EV+ shots properly expose the shadows and the EV- shots do not have blown up highlights. Some high end cameras can take 5, 7 or 9 shots in AEB mode. Although not necessary, this makes HDR photography so much more pleasing and the output are very realistic.

HDR Merge

At this point we now open our photos in Photomatix to generate our HDR. Click on the “Generate HDR” button and select the images. Let’s look at some of the options here. I have ticked the box “Align source images” and the option for horizontal and vertical shifts. Not really that important if you used a stable tripod but it does not hurt if enabled. Photomatix has built-in chromatic aberration and noise reduction which you should enable. I find them not very effective so I still correct them later in post processing. Ghosting may occur if the scene has movements such as flowing water or swaying trees. Up to a certain point, Photomatix can also reduce these artifacts. One important option here is the “Color primaries HDR based on” which I set to sRGB. The reason I chose this is that my final image will be processed and saved in sRGB color profile. The default is AdobeRGB which is actually better than sRGB. The problem is that most monitors and websites such as Flickr and Facebook use sRGB. If you save in AdobeRGB and upload to these websites your photos will look dull and lifeless during the automatic color profile conversion.

Click on “Generate HDR” to start the merge. Depending on your computer this may take about 15-45 seconds. Once finished you will now have a “transformed” photo that looks terrible. This is your HDR radiance file. Mouse over the dark portions and you will notice that they automatically lighten. Do the same for the bright parts and they automatically darken. You want this automatic lightening and darkening to be persistent so now click on “Tone mapping”.

Tone Mapping

Click on the photo on the left to view it large. There are two tabs and I use “Details Enhancer”. Have a look at the settings I use. Note that these settings will not work for all photos. This is where Art takes over Science. There is no recipe for proper tone mapping but there are some basic guidelines.

Strength refers to how aggressive Photomatix will perform tone compression: how much it darkens the bright parts or brighten the dark parts. I noticed that this is tightly coupled with the “Smoothing” option where you have several choices: Min, Low, Mid, High, Max. Experiment with these settings but never go lower than “High” smoothing otherwise you will start getting “halo” artifacts which look really yucky. The most realistic setting is with “Strength” set to about 90-100 and Smoothing set to “Max”. The image will look flat though. Sometimes by lowering the Smoothing to “High” I could achieve a more dramatic or natural lighting effect. I also refer to “Smoothing” as “wacky option #1”. If you want wacky HDR photos, set this to Mid or Low. A lot of people actually like this “wacky” effect so try it if you want.

Keep “Microcontrast” low. Don’t push it beyond 4. Think of this as something similar to the “Clarity” option in Adobe Camera Raw.

There are options that you should not be touching. “Saturation” is one of them. Do NOT tweak it beyond 50. You can do this later on with your photo editing software which is more flexible. And even before you start tweaking saturation, try adjusting contrast first for a more subtle touch on color. I call Saturation “wacky option #2”. Drag this to 100 for that cartoon effect. Some people wish they can set this to 200 🙂

Let’s discuss the Luminosity option. It makes the darker portions of the image even more bright. Think of this as some kind of fill flash. The problem is that if you push this too much it will introduce noise on the shadows. Try shifting your histogram to the right using the “Gamma” slider instead. If you really want to play around with “Luminosity” don’t push it beyond 4. Since I have mentioned Gamma, this option will tweak the overall “brightness” of the image.

Do you see that “Color Settings” group? Stay away from it. Again, these options are better adjusted from within your photo editing tool. Now you might ask, if we are not meant to use it then why is it there? The answer is that Photomatix tries to make sure that the output image is ready to use after tone mapping without the need for further post processing. Just like Adobe Lightroom. It’s supposed to be the only tool you will ever use but who are we kidding? So my advise is, use only Photomatix to adjust dynamic range and NEVER for color correction. There are better tools for the latter.

Lastly, there’s the “Miscellaneous Settings”. A very important option here is “Micro-smoothing”. I wish they included this option in the “Smoothing” group. It locally tweaks the transition of intensity levels. Drag this to the right for the most realistic effect. This will minimize the halo effect of HDR as well. There are situations though that keeping this low will still give a clean result with a more dynamic lighting feel so play with it. It might be good to zoom out of your HDR view because halo artifacts are more obvious when the photo is small.

Notice that we did not use a lot of the options available in Photomatix. There are better tools for these options. Most of the time, the output of Photomatix is not ready for posting yet. Firstly, it tends to be noisy. Secondly, it’s not easy to crop or rotate. But for expanding the dynamic range of our photos, Photomatix does it very very well. We will discuss the final post processing in the next installment of this tutorial. For the mean time, have a look at the final output of Photomatix tone mapping:

Wreck HDR
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