Or, What’s All This HDR Stuff I Keep Hearing About?
Ever wonder why your camera often doesn’t capture a scene the way you actually saw it?
When you view a scene, like a landscape with buildings and trees in the foreground and a bright blue sky with puffy white clouds in the background, your eye can pick out many of the details in both the shadows and highlights at the same time. But, when you snap a picture of the scene with your digital camera, with even a proper exposure, something’s way off. The shadows reveal no detail and they’re really dark, if not outright black. The highlights are washed out and also show no details. They may even be solid white.
What’s going on?
It’s all about dynamic range. Dynamic range in a scene refers to the ratio of the brightest parts of the scene to its darkest parts. The greater the dynamic range, the more tonal information is contained in the scene. In an image, dynamic range translates to the amount of tonal detail that can be seen in both the shadows and highlights in the image.
A typical real-world scene can have a very high dynamic range of around 100,000:1.
Although the human eye has a tremendous dynamic range of 1,000,000:1, it can can only accommodate a scene with a dynamic range of approximately 10,000:1 and still make out good details in the shadows and highlights (bright areas like the sky) at the same time.
Here’s the real problem…
Your digital camera has an effective dynamic range of less than approximately 1,000:1 (even less for low-end cameras and a bit higher for high-end cameras). That’s far less than what your eyes can handle. So, when you shoot a scene with a high dynamic range, your camera loses much of it. To compensate, you either expose for the shadows, and see the highlights become blown out (i.e., clipped), or you expose for the highlights and find that the shadows are clipped and are totally dark. If you expose for the average or mid tones, you tend to get disappointing results in both the shadows and highlights.
It gets even worse when you display the image on your monitor or make a print, as these devices have really low dynamic ranges of less than 500:1! That’s why parts of your picture look completely dark and other parts look washed out.
Now, there are all sorts of editing techniques you can use to selectively brighten/darken areas that are clipped, especially if you shoot in RAW format (I’ll cover RAW format in a future post). Dodging and burning are two such techniques. Painting with light is another technique. They each have their pluses and minuses. In this post, I’ll introduce High Dynamic Range (HDR) photography.
HDR photography is a fantastic way to create images that represent the full range of tones that you actually saw in the scene you photographed. HDR may sound new, but it’s actually a very old technique and was used by some of photography’s pioneers back as early as the 1850’s.
In digital HDR photography, we seek to create images with 32 bits per color channel, rather than the 12, 14, or 16 bits per channel used by most digital cameras. Without going into a detailed technical explanation, let’s just say that with 32 bits per color channel, we can create an image that represents the entire tonal range of a scene.
Now, you’re probably asking yourself, “if my camera has such a low dynamic range and can’t capture images at 32 bits per channel, how do I actually create an HDR image?
First, what you need to do is get around the limitations of your camera and its inability to handle the large dynamic ranges of real world scenes in a single exposure. Take, for example, an interior scene like a room with a window overlooking a sunlit meadow. To capture all the tonal information in the scene, you need to take not one, but multiple shots of the same scene using different exposures. The technique is called exposure bracketing. Here’s how to create three bracketed exposures, assuming you have a digital SLR camera:
- Use a tripod, whenever possible, to keep your camera absolutely still from shot to shot.
- Set your camera to Manual (NEVER use Shutter Priority or Auto for HDR!)
- Compose and focus the shot.
- Set the aperture and shutter to achieve the proper exposure.
- Take one shot (This “proper” exposure will best capture the details in the mid-range).
- Decrease the shutter speed 1, 2 or 3 stops (i.e., longer exposure time) and take the second shot. This exposure will capture the details in the shadows but the highlights will be clipped.
- Return the shutter speed setting to the proper exposure, but don’t shoot.
- Now, increase the shutter speed by exactly the same number of stops as in step 6 (i.e., shorter exposure time). This exposure will capture the details in the highlights at the expense of the shadows.
You now have a set of three bracketed exposures, which together cover an expanded dynamic range.
If your digital SLR or point-and-shoot camera offers auto exposure bracketing (AEB), simply set the camera to Aperture Priority mode, set the aperture to achieve the depth of field you desire and set up the AEB feature to take three images at ±1, ±2, ±3, or even ±4 EV intervals. Then, with AEB turned on, three bracketed images will be taken sequentially each time you depress the shutter release.
Actually, you can use 3, 5, 7, or 9 bracketed exposures and use any EV interval you like as long as your camera supports it. You have to experiment as different situations work better with more or less exposures and with smaller or larger intervals. For example, to create the image of Hildene, above, I shot the following five exposures at -4 EV, -2 EV, 0 EV, +2 Ev, +4 EV. Notice how the “properly” exposed (0 EV) image shows little detail in the highlights and how the sky is washed out:
You now have a set of bracketed images. Together, these images cover a much higher dynamic range than any single exposure. But, somehow we have to combine these separate images to create our single 32 bit HDR image. That’s where HDR software comes in.
HDR software programs, such HDRsoft’s Photomatix PRO or Adobe Photoshop (Photoshop has built-in HDR processing), allow you to import your bracketed images and then combine them into a single 32 bit HDR image. Now you’re wondering, “if the dynamic range in an HDR image is so much greater than my monitor and printer can render, how can I output the full range of tones?” After all, it’s nice to know that the HDR image contains all the tonal information we want, but unless we can display it or print it, what good is it!?!
The answer is tone mapping. And, this is where the fun really begins. HDR software takes the 32 bit HDR image it just created and then compresses and maps its full range of tones to a much lower dynamic range image that can be fully rendered on your monitor or printer. This process is called tone mapping. Even better, tone mapping makes it possible to produce some really interesting effects, as in my somewhat surreal image of Hildene, above, and my image of the 1940 Ford V8, shown here.
My 1940 Ford V8 image was made from three exposures at -1 EV, 0 EV and +1 EV (see the set below) and was shot dramatically with a wide angle lens. Notice how the details in the car, reflections, palm trees, building, doorways, sky, clouds, and even the underside of the car are all clearly visible in the tone mapped HDR image.
I will not go into all the myriad possibilities, but tone mapping an HDR image is a lot of fun and makes it possible to produce a final image that can take on a realistic, surrealistic, exaggerated, painterly or even downright spooky look!
BTW, many smartphones (e.g., iPhone) as well as digital cameras have a built-in HDR feature. When turned on, each time you snap the shutter, the phone or camera actually takes three bracketed image behind the scenes (pun!) and creates a tone-mapped HDR image. While these tend to be limited and offer little/no control over the output, auto HDR can come in handy when faced with a backlit shot or a scene with both shadows and highlights.
So, we discussed why photographs don’t always capture the full range of tones that you actually see in a given scene. We introduced HDR photography and how tone mapping an HDR image can produce some very striking images.
Now, you know why your photos don’t always look as good as the scene you saw. And, now you know what you can do about it.
Let me know if you have any questions by commenting on this post below. Thanks!