Today I’m pleased to present an introduction to High Dynamic Range Imaging (or HDR PHotography) that has kindly been written by Jason Bouwmeester.
An Introduction to High Dynamic Range Imaging (HDR)
HDR, or High Dynamic Range Imaging seems to be all the rage these days. HDRI is described as:
In image processing, computer graphics and photography, high dynamic range imaging (HDRI) is a set of techniques that allows a greater dynamic range of exposures (the range of values between light and dark areas) than normal digital imaging techniques. The intention of HDRI is to accurately represent the wide range of intensity levels found in real scenes ranging from direct sunlight to shadows. (from Wikipedia)
More accurately however, the images that are commonly seen and referred to as HDR or HDRI images are tone-mapped.
Tone mapping is a technique used in image processing and computer graphics to map a set of colours to another; often to approximate the appearance of high dynamic range images in media with a more limited dynamic range. Print-outs, CRT or LCD monitors, and projectors all have a limited dynamic range which is inadequate to reproduce the full range of light intensities present in natural scenes. Essentially, tone mapping addresses the problem of strong contrast reduction from the scene values (radiance) to the displayable range while preserving the image details and color appearance important to appreciate the original scene content. (from Wikipedia)
Definitions and technicalities aside, I decided to look into HDR and tone mapping a bit closer to see if there really was a difference between different processes. I was curious to see if there was noticeable differences between generating HDR/tone-mapped shots from a single RAW, multiple RAWs, multiple JPGs from the camera, and multiple JPGs generated from a single RAW. For the purposes of the rest of this post, I will be referring to my final images as HDR images (even though we all now know that’s not exactly correct). Here is the original, straight out of the camera image shot with my Canon Digital Rebel XT/350D.
Honestly, not a bad image for SOOC! Anyways, the recommended way to produce HDR is to take multiple exposures using your camera’s Auto Exposure Bracketting (AEB) setting. I’m not going to get into the details on this, I’m merely posting my comparison results here.
In the past, I’ve used the ReDynaMix Photoshop plugin to generate my HDR images from a single RAW file. It really can’t be beat for the $16 price tag. It worked pretty decently, but I’d heard that Photomatix was a much better program to use. Below is the image above run through the ReDynaMix plugin.
Not too shabby.
Photomatix is a much more robust, and more expensive ($99), program to use but allows for blending of multiple exposures into a single HDR file, as is recommended. I’m going to be up front here. I don’t walk around with my tripod in my back pocket, so taking multiple exposures without getting movement is very difficult. I tried Photomatix ages ago, but for some reason I guess I just wasn’t steady enough. Fortunately, this set of multiple exposures turned out and aligned nicely when I imported them into Photomatix. The three shots were taken at an exposure value of 2, meaning I had a shot that was properly exposed, one that was underexposed by 2 steps and one that was overexposed by 2 steps. Because the Canon Digital Rebel series only allows for 3 photos in AEB mode, that is all I am going to use. Using the .CR2 (RAW) files out of the camera I created the HDR image you see below (note, clicking this image takes you to the Flickr page). I processed it as I normally would process for HDR, tracked the settings and made some final curves and unsharpen mask adjustments in Photoshop.
It’s pretty close to the ReDynaMix version, but definitely crisper and more variation between the dark and light areas. This is especially noticeable in the BMW logo area, below the BMW logo on the grill, the upper right corner, the dirt/water streaks and between the buildings near the top. A much better looking HDR image in my opinion. As a comparison, I created an HDR image using the corresponding JPG files from the camera. I used the exact same Photomatix settings and Photoshop Curves and Unsharpen Mask adjustments. The image below is the result.
Still crisper than the ReDynaMix version, slightly washed out I think from the RAW version, but not much.
Camera JPG or generated JPG?
If you’re still reading, thanks for sticking with me this far. This is what I was really wanting to test out. Are there noticeable results between exposures generated in camera, and exposures generated using Photoshop’s Camera RAW plugin from a single RAW? Using the Camera RAW plugin for CS3, I can generate JPG files with various exposure values ranging from -4 to +4. For the intent of this test, I created a -2EV and +2EV from the 0EV RAW file out of the camera, and used the 0EV JPG from the camera as well. Below are comparison shots of the in camera EV JPG files (on the left) and the Photoshop generated EV JPG files (on the right). The top image is +2EV, the bottom -2EV.
As you may be able to see, the Photoshop generated JPG files are a bit lighter and washed out, but only slightly. Using the two newly generated EV JPG files and the originally exposed JPG from the camera, I recreated the HDR image, again using the same settings as before. The images below are a comparison between the in camera JPG HDR file (top) and the Photoshop generated JPG HDR file (bottom).
Can you see the difference? Sometimes I think I can, then other times I think I can’t… there are some areas that are a little lighter and perhaps washed out, for example the area below the BMW symbol, but all in all, it isn’t much.
The final test: 3xp or 7xp?
And that concluded my test. Or so I thought. I see people producing HDR images with 3, 5, 7 and even 9 exposures set to various EVs from 1/3, 1/5, 1 and 2. So I thought I’d do a quick comparison between an HDR using the 3 Photoshop generated exposures and 7 Photoshop generated exposures with an exposure value of one. In layman’s terms, the bottom image above uses +2, 0 and -2 EV JPG files, the image below uses +3, +2, +1, 0, -1, -2, and -3 EV JPG files.
Some obvious differences between the two images, there’s a bit more range in the 7 exposure version, which is understandable.
Thanks for hanging in there, hopefully I saved you some time with having to go through this process yourself. The image below (clicking will take you to the Flickr page, which you may have found this article on) is the 4 final HDR versions in a mosaic for easier comparison. Top left: 3 exposure, 2 exposure value, in camera RAW; top right: 3 exposure, 2 exposure value, in camera JPG; bottom left: 3 exposure, 2 exposure value, generated JPG; bottom right: 7 exposure, 1 exposure value, generated JPG:
Seeing the images together like this, you can see subtle differences between the three 3 exposure shots (top left, top right and bottom left), and you can see a bit more color variation in the bottom right 7 exposure shot. I do think that the HDR generated from the camera .CR2 (RAW) files turned out the best, I wish my camera was capable of doing 5 or 7EV shots without having to change the settings, and I also wish I was a human tripod. That being said, neither is going to happen and if I showed you each of these images separately, unless you are an HDR geek/expert/guru, you’d be hard pressed to see the differences or tell me one looked better than the other. That being said, I’ll most likely use the Photoshop generated JPG files for my HDR’s unless I have a tripod or can be certain I won’t budge between exposures. Besides, I’d never get an HDR like this based on 3, 5, 7 or 9 EV exposures in a camera: Thanks for stopping by, and I hope you found this informational and useful! Happy HDR’ing.