|Posted by caseycrockerphoto on April 22, 2010 at 6:14 PM|
Photo Fusion Two:
High Dynamic Range (HDR)
(written for Camren Photographic's September 2009 Newsletter)
Two special fusion techniques are unique to digital photography and widely accepted for their sleight of hand and illusion. These fusion techniques are the Panoramic Stitch and the High Dynamic Range merge. Last month, the Panoramic fusion (stitching) was discussed and now the High Dynamic Range fusion (HDR) will be explained in what aims to be an easy to apply purpose and methodology. The purpose of HDR is to reduce contrast of high contrast scenes. Previsualization is required. If this does not occur, exposure issues or image overlay proplems could easily arise, resulting in a less than successful image. In the case of HDR, exposure bracketing might pose timing problems, so fine tuning an approach helps circumvent possible problems.
Early on in photography's history, photographers have tried to reduce the contrast of high contrast scenes. As early as 1850, a man maned Gustave LeGray developed a technique to bring out detail in areas that were too dark while also keeping areas from becoming too light. When photography was young and film quality was low LeGray accomplished a new technique that we use today in HDR . To photograph his seascapes (picture at left) without overexposing the detail in the sky and without underexposing the detail in the shadows, he took two pictures. One exposure was for the sky and another was for the sea. He then sandwiched the two together during print production and generated positive prints that maintained both areas of the scene, creating the first know High Dynamic Range photographs. The approach of HDR photography of yesterday is remarkably similar to that of today.
The same technique is necessary for digital HDR fusions and this tutorial will use a process similar to LeGray's. By clicking on the above image of the dragonfly, images and screen shots of the HDR process will appear in two pages. The example image above is a HDR fusion consisting of two images. Even though a classic HDR contains more than two images, assembling two images is as easy as a fusion of three or five images. The idea behind this illustration is the importance of technique.
Even the simple act of using a polarizer will help reduce glare and increase the dynamic range within a single image. (Without doubt, LeGray would’ve loved a polarizer). Some photographers simply use the Shadow/Highlight controls to boost shadows and or regain highlight detail, but as figure 1 will show, the results can be mediocre because this adjustment will promote image noise because it amplifies detail. HDR fusions add information rather than amplifiy and a simple HDR fusion can be made with a minimum of two images, such as the example of the dragonfly above. Photographers certainly have been known to use more than five photos within a fusion to expand the image's tonality. This does tend to give the final photo a painted effect, full of bold colors and a certain surrealism. This dragonfly HDR is something more practical, simple, and describes the nature of an expanded dynamic range.
A way to consider HDR is as if it is a multiple exposure. Different exposures of the same subject of varying lightness and darkness are taken and then digitally placed on top of one another to be blended into a single shot. At least one image for shadows (an overexposure) and another for highlights (an underexposure) are needed. In-camera bracketing is a useful tool for HDR because the succession of exposures happen quickly. Manual brackets might be hard to time quickly because each exposure must be taken as close to the same instant as possible. Otherwise, things like clouds or trees may move too much and result in a blurry HDR.
This tutorial expects that several, ready to fuse photographs already exist. However, here is a quick start to shooting for HDR: shoot in RAW for the best detail and to counter-act file compression. (A RAW conversion program may be required to open the RAW files). Use a tripod so nothing compositional changes. A polarizer is helpful and will lessen the total number of shots needed for an HDR and using the camera's lowest ISO will maximize the camera's color capturing potential. Set your aperture and shutter values however you wish, but be sure to change the exposure setting over and over for each image taken. It is recommended here that you prioritize either the aperture or shutter speed. Keep one of the two values constant, then make exposure sacrifices to the other value at a single stop difference. (I.E., keep the aperture at f/11 and fluctuate the shutter speed). Still images, such as landscapes, architecture, and flowers are a great start. The dragonfly worked out simply because the insect did not move between the two quick exposures. A tripod and polarizer were used and the ISO was set to a constant 200. The shadow detail image was at f/8 at 1/8th of a second and the highlight detail image was at f/8 at 1/250th of a second.
The main reason for doing this process is to maximize bit-depth. Bit-depth is the amount of color the camera will produce. (Thinking of bit-depth as a box of crayons would be a good metaphor). A JPEG image (which most people shoot) are not designed for HDR. JPEG images must be saved as 8 bit images, meaning they are low on the color reproduction scale, but still capable of gathering 256 individual colors. Not bad, but RAW images and Tiff images can automatically record and save far more color. Tiff files can be captured in 16 bit color, but not all cameras offer the option of recording a Tiff. (Tiffs are notoriously slow, and the popularity of the Raw file has become the professional standard). Raw files can gather between 12 and 14 bit depth (check your camera for the 12/14 option). This means that theoretically a straight out of the camera 12 bit image can contain 4,096 colors and a 14 bit image can contain 16,384 colors. Ideally, an HDR image should be shot in Raw file mode and at a maximum bit depth. This way, the camera is prepared to produce images that contain 64 times more potential colors than a JPEG. (This alone is a great reason to shoot in Raw mode). In the same respect an ISO (film speed) setting also dictates how many colors an image contains. 100 ISO contains twice the color potential as 200 ISO. So on and so forth. By the time 1600 ISO is compared to 100 ISO we are only using 8 colors of crayon as opposed to 128. (This alone is a good reason to keep your ISO low when possible).
Computer programs allow the HDR photographer to assemble multiple images together quickly and with little effort. A program is required to assemble HDR images, although clever layering and masking could be put into play. Photomatrix, Artizen, and Photoshop offer HDR fusion. Adobe Photoshop CS2, 3, and 4, have the HDR capability. CS3 & 4 allow you to simply open the pictures then run an automation (click file, then automate, then "Merge to HDR"). Any adjustments you want to make to your captured RAW files must be done first. Photoshop will then prompt for a type of HDR exposure adjustment. At this point, make a light adjustment if you wish. The end result will look different, as this is a preview of what the HDR will become. After hitting "ok," the program will blend the two together, allowing the best parts of each image to come through. You'll see an increase in shadow and highlight detail a shadow/highlight adjustment cannot offer a single image. You'll notice an increase in color information that a saturation adjustment cannot produce because now there are truly more tones and colors. This process is a fairly simple and highly effective way of improving your overall image quality. The HDR fusion will often grant the eye a more pleasing contrast and popularly gives the photographer something that better matches what is in their mind's eye. 
Click here for illustrations: