While you may not know what HDR photography is, you’ve probably seen plenty of HDR images around the Web. HDR photographs are high dynamic range photographs which show a range of tones in the image that is impossible to get in a single image captured with a digital camera. HDR photography is all the rage right now, so I’ll explain what HDR is, what you need to do to capture images for HDR processing, and how to achieve some great effects with it.
WHY HDR?
The problem with our digital cameras is that they have a sensor which can only record a limited range of light. In any image you will see some very light areas and some very dark areas, and that’s the extent of the range of tones that your camera can capture with a single image. However, there is a lot of detail in the light areas of the scene that the camera has failed to capture, and detail in the shadow areas that is also lost in the typical photograph.
Just because the camera appears unable to capture the full tonal range of a scene in one shot doesn’t mean it can’t do it in multiple shots. Enter high dynamic range photography – this process involves taking a series of photographs at different exposures and merging them into a single image which has an extended dynamic range, so you see details in the highlights and in the shadows.
When shooting for HDR you will typically capture three or five images using a range of exposures. If you’re capturing three images you might do this at -2, 0 & +2 exposure compensation, and for five images you might capture at -2, -1, 0 +1 & +2 exposure compensation. By overexposing the image you capture some of the detail from the deep shadow areas in the scene, and by underexposing the image you get detail from the lighter areas of the scene. Most digital cameras, including both point-and-shoots and SLRs, have features that allow you to capture multiple images at a time and at a series of different exposure settings – typically called auto bracketing. If you don’t know how to configure your camera, check the manual to learn how to set up the camera to take the shots. You will generally hold the shutter release down as all the shots are captured.
When you have the photos downloaded to your computer, it is time to start up your HDR software. One program that you can use, and which offers a free trial, is Photomatix from www.hdrsoft.com Download and install the Photomatix Pro trial software, launch the program and choose the Generate HDR Image button. Click Browse and load the images in your sequence. The program will read the metadata in the
images and will automatically set the exposure settings for each image. If for any reason it does not set the exposure settings, you can enter them manually using minus values for underexposed and plus values for overexposed images.
When the Options dialog opens, select to Align the images if there is any chance the camera moved while you were taking the shots. If you captured the images using a tripod, then this may not be necessary. If there were objects moving from one image to another, choose the ‘Attempt to reduce ghosting artifacts’ option. Click Reduce Noise if the images contain a lot of noise. Click Generate HDR and wait as the image is created for you. The image won’t look very good at this stage.
Click the Tone Mapping option to adjust the image. If you select the Details Enhancer tab you can adjust the image by fi rst selecting the left-most Light Smoothing option to create a painterly-type effect or the right-most option to set it to a more natural look. Use the Strength slider to vary the strength of the effect, and use MicroContrast to adjust the midtones to give the image a little more colour and a sharpness boost. The Highlights and Shadow smoothing tools smooth out some of the halos you get around areas of high contrast in the image. You can adjust the Saturation and Luminance here, or do this in a program like Photoshop or Photoshop Elements later on.
The other alternative is to use the Tone Compressor tab options to fix the image. I found that with the images I was working with, which were shot early in the morning, this option gave the most realistic results by bringing out the detail in the foreground without compromising the sky detail. Here you can adjust the Brightness of the image, the Tonal Range Compression, which lets you compress or stretch the tonal range of the image to bring out more detail. To bring out the lighter parts of this image we used a large positive value and the resulting tone curve can be seen to stretch from one end of the chart to the other, rather than being bunched up on the left as it was.
The Contrast Adaption option allows you to enhance some of the colours through adjusting the contrast, and I used this to boost this image. Use the White and Black point sliders to ensure that you have true white and black in the image. When you use the sliders, keep an eye on the chart, as you don’t want the chart data to go over the edge at either end, but you do want it to be close to the edge.
Use Temperature to warm up or cool down the image – drag to the right to warm the image and to the left to cool it down. The Saturation slider boosts the colours in the image – drag to the right to enhance the colours and to the left to reduce the strength of the colours.
Once you have a result that you like, click the Process button to create the actual HDR image. Choose File>Save As to save your image. You can now use the image as is, or open it in your favourite photo editing program to work with it further. You will find that HDR photography works very well when you are shooting a scene which has different combinations of light. You might encounter this when shooting indoors and where the light outside is very bright. You can use HDR to create composite images that have the outside detail rendered, so you can see it clearly as well as seeing the inside detail – an image impossible to create without HDR software. HDR also works very well when capturing landscapes, cityscapes and any scene with great cloud detail.