High Dynamic Range (HDR) Imaging

Back when we used to shoot slide film, any photograph we made could contain about 4 stops of light. Since many scenes contain a much higher light range than this, a common tool in any serious photographers bag was a Neutral Density filer.

A Neutral Density filter (or ND Grad) is basically a piece of glass that is dark on one end and clear on the other. By positioning the dark part of the filter over the light portion of the image, you could reduce the light variation enough to make a photograph. At least this was the theory. Often there wasn’t a good way to do this because the transition from light to dark on the filter was a straight line. Since straight lines don’t show up in nature that much, this would lead to ugly compromises like burying the light/dark transition in a tree line and loosing a bunch of detail. Sometimes it just meant that you couldn’t make an image at all.

In the digital world, we have more options when we have scenes that contain more light than the sensor will capture. The two primary techniques are:

• Digital ND Grad: Shooting two images at different exposures and masking them together using Photoshop.
• High Dynamic Range: Shooting a number of images at different exposures and using specialized software such as HDRMax or Photmatix to combine them into one image.

In the rest of this article I’ll walk through capturing images for HDR, using Digital ND Grads, and using HDR software packages. Finally I’ll spend a little time talking about how I use photoshop to “finish off” HDR images.

As an example I’ll work with a set of images I took of “Three Brothers” one morning in Yosemite.

Best Single Exposure	Digital ND Grad	HDR
This image is a bad compromise on all levels. The sky is blown out, the foreground is dark. This doesn’t even make a good snapshot.	This isn’t bad. It looks like a traditional ND Grad photograph. You could have done exactly the same thing with an ND Grad filter but it is quicker and less error prone to do it digitally.	An HDR image made from 5 exposures. Detail in the sky, detail in the foreground, detail everywhere. It’s different to the ND Grad image. Whether it is better or not is up to you.

Capturing Images for High Dynamic Range (HDR) Photography

The most important tool for HDR capture is a tripod. It is critical that all of the component images align correctly and have the same focal plane. When you capture images your goal is to get a good exposure for all elements of the scene. For Digital ND Grad you need two images, for HDR you need between three and five.

Since digital capture is free, I have a very simple solution for both Digital ND Grad and HDR capture. I take a 5 shot bracket that is 1 1/3 stops apart. This allows me to have 5 images to chose my “best two” from for a Digital ND Gras, and it gives me a great exposure range for HDR. The process I use is:

• Compose the image on a tripod
• Make sure I have a cable release attached so I don’t bump the camera
• Shoot in RAW – this gives you the most image data
• Put the camera in Aperture priority mode. This is really really important. If you change the aperture the focal plane will shift and the images won’t line up.
• Take an evaluative (or Matrix) reading of the while scene.
• Put the camera in bracket mode for a 5 shot bracket with the images 1 and 1/3 stops apart
• Take 5 images
• Done!

The hardest part of this is remembering which images are part of the exposure bracket. One thing that helps me is to take an exposure with the lens cap on both before and after the bracket. That way, when I’m going through the images in Lightroom later I will see a black frame, 5 images, and another black frame.

Here are the five source images from “Three Brothers”:

Digital ND Grad – Using Photoshop to merge exposures

To create the an image using the Digital ND Grad technique, you need to select two images. One should have the light parts of the image exposed correctly (e.g. the sky) and the other should have the darker parts of the image exposed correctly. For this image, I chose the following images:

Dark (sky is correct)	Light (grass is correct)

Once you have selected your two images, load them both into photoshop.

Now select the lightest image (select all, copy) and paste it on top of the lighter image. This will give you two layers (circled below).

Select the top (light) layer and click "Add Vector Mask" button (circled below). This will create a mask on the top layer that will allow you to let some of the bottom layer show through.

Click on the mask (the white rectangle in layer 1), then select the brush tool. Set the foreground color to white and make the brush opacity about 50% and the flow 75%. This makes the brush go on more slowly so you have more control. Now, just brush over the lightest parts of the image (blown highlights) and the correctly exposed portion from the bottom image will show through.

If you brush too much, just select white as the foreground color and brush over your mistakes. The way masks work is that the white portion of the mask shows the top image, the black portion shows the bottom image, and the gray areas blend the two images.

Now you have a traditional ND Grad image without the hassle of lining up filters and carrying extra gear.

Using HDR Software to create an HDR Image

HDR software is like Digital ND Grad on steroids. You can use more than two images and the software uses some really heavyweight math to give you lots of control over the exposure blending.

There are a number of HDR packages on the market. The one I use is HDR Max from Ariea (http://www.ariea.com/products/hdrmax/). Photomatix is popular as well but I have found that HDR Max gives me more control over the final image.

Using HDR Max, select File, New HDR. When the dialog box opens, select the five images from the bracket and click OK. The files will take a while to load so be prepared for a wait.

Once the image has loaded, don’t be surprised if it looks awful. All five images have been sucked into one master image and now you need to tell it what is important (this is easier than it sounds).

To start out, I usually reset all of the sliders. Set the Tone sliders to .50, the color sliders to the middle, and the Black and White sliders to .25.

Next, I work on Color sliders. Adjust the Gamma, Luminance, Lighten, and Darken sliders until the image is properly exposed. Don’t worry about image detail, that will come later. To understand what a slider does, pull it all the way to the left, then all the way to the right and see what changes.

Even at this point, you probably have something better than if you just did a Digital ND Grad. This makes sense because you have far more exposure data to work from. Now move up to the Tone sliders to bring out detail. I usually adjust the Intensity and Magnitude sliders before the Smoothing and Micro Detail.

The final step is to export the image as a 16 bit TIFF so that you can do final processing in Photoshop. I find that I have to do post processing on every HDR image so I just build this step into my workflow.

Photoshop – Post Processing the Generated HDR or ND Grad File

At this point both the HDR and the Digital ND Grad images are good raw material but both of them need some work. For now I’ll just work on the HDR image but the same workflow applies to pretty much any image. This isn’t meant to be an exhaustive tutorial in Photoshop, just an illustration of my post processing workflow. If you haven’t learnt about curves, you need to. They are my primary post processing tool.

Open the image in Photoshop. You’ll see that the color is slightly different to what you had in HDR Max. Also notice that the grass is a way too green, some of the image is a little dark, some of the image is a little light. We’ll fix all of this on our way to the final image.

First, create a new Curves Adjustment Layer (Layer, New Adjustment Layer, Curves) and call it “Black/White Point”. On the histogram, move the top and bottom points of the curve so that the top touches the right hand side of the histogram and the bottom touched the left hand side. This makes sure that you are using all of your available color range.

Next, we’ll deal with the overly green grass. Create another Curves Adjustment Layer but this time, when the New Layer dialog opens, select “Saturation” as the mode. Create a mask that just covers the grass and de-saturate by pulling down the right hand side of the curve.

Now, while most of the sky is OK, there is a little bright spot on the right hand side. Create a Curves Adjustment Layer with a mask for just the bright spot and pull the right side of the curve down a bit.

Finally, we need to brighten the darker parts of the image just a little. Create another Curves Adjustment Layer and click on the line about 1/3 of the way down from the top right. Click again 2/3 of the way down the curve and drag the curve up slightly. You will see the darker part of the image lighten and the light part stay the same. Magic huh? Go learn about curves!

At last, we’re done. Save the image, resize it and export it as a JPEG so that you can show it off on your web site.

Final Thoughts

Since HDR is relatively new, some people question whether it is a legitimate technique or not. I remember these exact same arguments back when Digital SLRs first came out. HDR is just as legitimate a tool as ND Grad Filters, flashes, and dodging and burning. All of these techniques simply modify the amount and quality of light that makes up the final image.

It is clearly early days for High Dynamic Range imaging. There has been tremendous progress over the last few years and I’m sure that there are lots of innovations still to be done.

Here is an example of more HDR images

This entry was posted on Thursday, September 10th, 2009 at 11:47 am. It is filed under HDR, articles, post processing. You can follow any responses to this entry through the RSS 2.0 feed.