Surefire Techniques for Archaeological Photography in Dark Places

Transcription

1 Surefire Techniques for Archaeological Photography in Dark Places ABSTRACT Experimenting with the mode settings included on most digital SLR cameras is an excellent way to learn photographic skills useful for recording images in low-light situations often associated with archaeological excavations. In most cases, the use of sports mode settings provides the optimal balance of high ISO and shutter speed needed to capture detailed excavation images. Variations of these settings will be used to illustrate how to minimize digital noise in archaeological photography. Introduction During the SHA conference in Colonial Williamsburg in January 2007, the author spent an afternoon enjoying the sights and sounds at the glasshouse in the Colonial Jamestown settlement. While photographing some of the glass produced that day, several people stopped to ask how to take better photographs in the dark environment of the workshop. These encounters encouraged the evaluation of the camera settings being used, and it was realized that the soft, somewhat hazy light in the glasshouse was similar to the difficult lighting situations experienced when shooting large indoor sporting events. Realizing the similarity of the challenges, the decision was made to set the camera settings to sports mode, a mode seldom used by professional photographers; this single decision made all the difference in the quality of photographs recorded that day, and a world of new photographic options became evident. Suggested Technique for Low-Light Photography One of the least successful ways to use a digital SLR camera in less-than-perfect archaeological site environments is in automatic mode. Images will be obtained, but there is little or no control over their integrity. More than once, archaeologists have photographed important sites with automatic settings, only to come away with poor-quality images that inadequately illustrate key aspects of their work. The camera does not care about the content being photographed. It is only a tool that records images according to settings determined by the user. Selecting the sports mode setting on the camera may not seem like an obvious choice when photographing artifacts, but artifacts are often located in environments with variable light conditions; sports mode is designed for shooting in just such situations. A quick introduction to some photographic terms will be useful before any explanation of the beneficial uses of sports mode for archaeological site photography in variable-light situations. Regardless of whether the camera being used is a film or digital camera, ISO or International Organization for Standardization is one of the main ways to control and manipulate light. Adjusting ISO is how photographers control relative sensitivity of film. The same is true with digital cameras, except that in the digital world film is replaced by a sensor. Higher ISO numbers mean greater light sensitivity. A setting of ISO 100 to ISO 200 is generally best for bright daylight photography. In a darker environment such as a cave site or excavations in a heavily wooded area, a higher ISO of 500 or even 1000 may be required to ensure that sufficient light is in contact with the sensor. At indoor sporting events, it is generally necessary to shoot at ISO 1600 or higher to capture detail and account for the variability of indoor light. It is important to note that as the ISO setting on the camera increases, the amount of digital noise inherent in the captured images also increases. Digital noise is roughly equivalent to film grain. Generally, it is desirable to use the lowest ISO setting possible to minimize digital noise or graininess in images. At dimly lit sporting events the use of higher ISO settings is required to capture sufficient light and fast-moving action scenarios. For archaeologists, sports mode shooting ensures capturing sufficient light while enabling the photographer to shoot at a shutter speed fast enough to eliminate the need for a tripod. Technical Briefs In historical archaeology, 2009, 4: 41 43

2 Surefire Techniques for Archaeological Photography in Dark Places Both the automatic mode setting and the Aperture- Priority (sometimes listed as AV or AE) settings on the majority of digital SLR cameras are designed for radically varying light conditions. For instance, in the Aperture- Priority mode the user sets the desired aperture and the camera automatically selects the shutter speed. In the highly variable circumstances of archaeological sites, one photograph captured with a short exposure time may be crystal clear while several other shots are out of focus because the camera moved slightly during the long exposure times set by the camera. People often comment that their cameras seem unpredictable because shutter speed for one shot seems fast while in another circumstance the shutter speed is slower. Shooting in Aperture-Priority mode is generally more successful when working with a tripod, since the camera is less likely to move while the shutter is open. For occasional photographers and for archaeologists working in less-than-perfect environments, selecting the sports mode settings on even an inexpensive digital camera will yield surprisingly good results. Often, as was the case at the Jamestown glassworks, visitors do not bring a variety of lenses, tripods, and supplemental lighting for their photography. Even if this equipment is brought along, some archaeological sites, museums, and museum workshops will only allow photography without the use of supplemental lighting, and in general the use of tripods and reflector screens is never permitted. As a common courtesy, one should always check with venue management before taking any photographs. After ascertaining that photography is permitted, the challenge becomes one of capturing sufficient light to create successful images while shooting handheld in a low-light environment. Most photographers require a shutter speed of at least 1/100 of a second to eliminate motion blurring in their images. In low-light situations, a well-practiced photographer will adjust to a higher ISO setting, remembering all the while that at higher settings, image quality suffers. Figure 1 is a series of images of a glass bottle. Notice that in image (a), shot at 1/100 sec./ f2.8 and ISO 500, the image is dark. Image (b) taken at 1/100 sec./f2.8 and ISO 1250 is slightly brighter. By comparison, the diagnostic attributes of the bottle are much clearer in image (c), although surface texture and coloration are still questionable. Image (d), taken at ISO 3200, has better surface detail and clarity. Images (e) and (f) were taken in sports mode. Both are slightly brighter and more detailed. Figure 2 is a photograph in its unedited state. This image was taken in a low-light environment at 1/50 of a second, f2.8 aperture, and ISO 800. Although no color correction has been attempted, the image of the glass bottle is clean and focused. By comparison, Figure 3 is an image of the same artifact photographed at 1/50 of a second, f2.8 aperture, and ISO Notice that as the ISO setting for this image is increased, digital noise is apparent and the general clarity of the artifact image is compromised. In this case, the ISO setting of 800 proved to be the best setting to maintain clarity of the image while maintaining a low-noise factor. Using the through-the-lens light meter feature of Figure 1. Comparison of ISO and sports mode images. Image (a), shot at 1/100 sec./f2.8 and ISO 500 is dark; (b) is also dark. In (c), diagnostic attributes of the bottle are clearer; (d), an image taken at ISO 3200, has better surface detail and clarity. Images (e) and (f) were taken in sports mode. Both are slightly brighter and more detailed. (Photographs by author, 2007.) 42 Technical Briefs in historical archaeology

3 Figure 2. Taken in a low light environment at 1/50 sec./f2.8 aperture, and ISO 800, this image of the glass bottle is clean and focused. (Photograph by author, 2007.) the camera, this setting was used in sports mode to obtain the best-quality image possible in this low-light setting. With practice it is possible to develop a shooting strategy that allows quick adaptations to changes in light and other variables that affect the quality of images. The beauty of digital images is that after taking an image the camera screen provides immediate feedback on the camera s preview screen. This can be a deceptive for gauging the quality of a shot, however, because on such a small screen it is impossible to determine graininess, quality of exposure, and accuracy of colors. A better way to determine the success of a given photograph is to use the histogram feature of the camera. With practice, reading a histogram becomes a much better measure of photograph quality and successful recording. Conclusion The nearly instantaneous recording ability of digital cameras has improved archaeological documentation efforts in the field. The price of digital cameras and the speed with which the usability of the images is verified Figure 3. Shot at an ISO setting of 1600, the surface texture and clarity of the artifact are compromised. (Photograph by author, 2007.) has made film cameras all but obsolete for most archaeological photography. In fact, low pricing of SLR digital cameras has made them a mainstay of archaeological excavation sites. Gone are the days when an unsuspecting graduate student would be handed a film camera and assigned the task of documenting site activities, only to find out when the film was developed that few usable images were recorded. All archaeologists should develop some nominal photography skills. Digital cameras have taken some of the guesswork out of controlling light and capturing images. A small amount of experimentation will lead to better images to supplement fieldwork experiences. Experimenting with the sports mode settings of the digital SLR camera is an excellent way to augment photographic skills in variable or low-light archaeological environments. Center for Maritime Archaeology and Conservation Department of Anthropology, Anthropology Building Texas A&M University College Station, TX Technical Briefs in historical archaeology 43

4 Expanded Spectrum Photography and Archaeological Conservation ABSTRACT Archaeological conservation requires a broad spectrum of analytical tools to assist in the development of successful conservation strategies. In spite of advances in radiography, magnetic resonance imaging (MRI), and computed tomography, traditional photographic skills, adapted for digital cameras, are easy-to-use and surprisingly effective diagnostic tools for assessing the metallic state of iron objects recovered from marine environments. Expanded spectrum photography (ESP), which is an offshoot of high dynamic range (HDR) imaging, combines an extended spectral range of images to accentuate oxide dispersion and other diagnostic attributes within the matrix of a concretion. While HDR images may be artistic in nature, ESP images are designed to illuminate the distribution of metallic oxides and organic materials within an artifact. Resulting images offer unique perspectives on an artifact that are useful when developing a conservation strategy. Introduction In some cases of artifact imaging, variances in exposure and development techniques mask and obliterate the subtle visual cues that may be important when assessing the state of an encrusted object. Pretreatment assessment of an artifact is critical. The data gathered during such an assessment forms the basis upon which a treatment strategy is developed. If the conservator does not have sufficient data to form a conservation plan, the longterm stability and success of conservation treatment may be compromised. High dynamic range (HDR) imaging broadens the visual spectrum range of a given image by combining three or more images; the end result is a crisp, almost surrealistic final representation. By design, expanded spectrum photography (ESP) processing of digital photographic images uses the basic components of an HDR image to create pictures which may not be aesthetically pleasing, but serve to capture and convey visual data which can easily be obscured using conventional visible spectrum digital imaging. All cameras function to capture a spectrum of light reflected by an object. This captured light is translated into an image by a digital film plane (sensor). Most photographers attempt to record perfectly balanced exposures of an artifact, assuming that these well-balanced images are always best. Depending on variables such as ISO setting (adjustable sensitivity to light), aperture setting (size of the lens opening), exposure compensation (an adjustment to increase or decrease image lightness or darkness), and shutter speed chosen, both film and digital cameras are designed to record only the visible spectrum of light. This spectrum is approximately from 400 nm ( m) to 700 nm ( m) in wavelength. Compared to the total spectrum of light, including ultraviolet and infrared light, the visible light spectrum is very small. All cameras allow the photographer to take multiple exposures of a given subject. In the case of artifact photography, most archaeologists will take multiple images of a single artifact, changing aperture/iso settings for each series of images recorded. This practice is referred to as taking bracketed exposures. In the days when film was used exclusively, there was a time lag between the taking of photographs and the final development and printing of the film. In this situation, bracketed exposures ensured that at least one image from the set would be successfully recorded. Digital cameras have eliminated the time-lag factor in creating photographic records of an artifact before and during the conservation process. Modern digital processing, often called the electronic darkroom, provides new tools for creating information-rich artifact photographs. A bracketed series of photographs (a series of images taken at different exposure settings) holds a greater potential for producing good diagnostic images, because when combined or stacked together, the dynamic spectral range Technical Briefs In historical archaeology, 2010, 5: 16 19

5 is greater than the spectral range of a single image. It is this expanded spectrum which encompasses more light and therefore more potential data. When these images are combined into one image, the visible spectrum of data will be greater than the visible spectrum of data available in one conventional image. In order to capture a greater dynamic spread of the visible light spectrum, the exposure settings for the bracketed images can be increased. After determining which exposures may be effective for an HDR image, the next step is to address other critical components of the photographic system setup. Since HDR/ESP images rely on combining several images to create one super-broad spectrum image, it is critical to ensure that the artifact is framed in the viewfinder in exactly the same position for each shot in the sequence. To do this, a high-quality tripod is essential. Because budgets for archaeological projects are usually tight, many cost-conscious archaeologists eliminate things such as tripods. If the bracketed sequences are not identical in artifact placement, one cannot successfully record HDR images; in this case, the tripod will pay for itself in the form of information gained from the image during later analysis. Remote trigger systems or extension release cables are useful tools for minimizing camera movement while taking a series of images for use in ESP imaging. Digital cameras also have a locking mirror function, which minimizes vibrations in the camera caused by the movement of the mirror when taking an image. Most single lens reflex (SLR) digital cameras have such a camera lock, so photographers should consult their camera manuals and become familiar with this invaluable tool. Once data acquisition (image capture) is complete, tools in the digital darkroom can be used to create ESP images. A number of companies make software that assists in turning bracketed images into diagnostically important images. Two such programs are discussed here. Under the File directory within Adobe Photoshop (from CS2 through CS5) there is a subdirectory called Automate. A left click on this directory will reveal a subdirectory called Merge to HDR. After left clicking this menu heading, the program will ask for source files; after three or more images are selected, the program can start the largely automated process of creating an HDR image. Another powerful software tool for creating HDR images is PhotoMatrix Pro. This software is simple to use and only requires a few practice images to build familiarity with the process. When the program starts, a menu entitled Workflow Shortcuts will be visible. Select Generate HDR Image. Once selected, a box entitled Open will appear. Identify the location of the desired bracketed images and select Open. The selected images will appear in the Generate HDR-selecting source images box. Simply select OK, and the program will provide a long list of parameters which may be used to adjust the images. The best way to understand the parameter adjustments is simply to play with the settings and compare the results. Once a pleasing image is attained, set additional parameters for processing and select either Tone Mapping or Tone Compressing. The program will compile the image in a short period of time. To create ESP images, parameter settings within a program such as PhotoMatrix Pro are deliberately manipulated not to create a hyperrealistic image but to accentuate the diagnostic features of an artifact. The illustrations provided for this technical brief show an encrustation containing the remains of a very rusty nail. Figure 1 is a traditional digital image of this artifact. Clearly, the iron nail has seriously decayed, and much of the detailed surface of the artifact is obliterated by orange corrosion products. A series of three images of the encrustation was taken for the purpose of creating an ESP image. These images range from being underexposed to being overexposed. Figure 1. A traditional digital image of an encrustation showing a cross-section view of a corroded iron nail. Note that corrosion products (generally orange in color) have permeated the matrix of the surrounding concretion. As a result it is more difficult to identify potentially recoverable metal. The corroded mass is 4.25 inches across its largest dimension (Photograph by author, 2010.) Technical Briefs in historical archaeology 17

6 Expanded Spectrum Photography and Archaeological Conservation Figure 2. A series of three images was taken of the encrustation: (A) an overexposed image (-1.4) emphasizing the lighter spectrum; (B) a relatively normal exposure (0) with balanced highs and lows; and (C) an underexposed image (+1.4) emphasizing the darker spectrum. The graphs accompanying each image show the brightness value of all the pixels in the image. (Photograph by author, 2010.) Notice the spectra associated with each image in Figure 2. Figure 2A is a greatly overexposed image and its corresponding graph indicates that the darker spectrum of data is missing. The image is excessively bright. Figure 2B is a relatively normal exposure. Its corresponding graph indicates that the image includes balanced highs and lows. Figure 2C is dark, containing more low-spectrum data and sparse high-spectrum data. The resulting ESP image (Figure 3) includes a much broader spectrum of data, resulting in an image that is less aesthetically pleasing than a traditional photograph, but which better defines the remains of recoverable iron within the concretion. Figure 3. The resulting HDR image clearly defines the outline of the remaining iron running through the center of the concretion. The large areas of corrosion around the head of the nail are better defined in this image. Generally, iron that has been bent or hammered during the manufacturing process is more susceptible to corrosion. (Photograph by author, 2010.) Equipment Suggested for HDR and ESP Photography Because of the nature of HDR and ESP photography, several pieces of equipment and camera setup configurations are recommended. Under normal circumstances, small errors in exposure settings are not important. When compiling three, four, or even more images into one ESP image, it is essential to prevent camera movement. Most digital cameras offer a mirror lockup setting. This feature is essential, because under normal shooting circumstances, each exposure taken includes movement of the mirror assembly within the camera. For a single image, this may not be important. For a series of images that will be compiled, slight movements will lead to errors in the final compilation. Check the owner s manual and be familiar with the camera lockup settings. The simplest way to prevent general movements when taking photographs is to use a high-quality tripod; for ESP imaging this is essential. Additionally, the photographer should consider investing in a remote triggering device for archaeological photography; this is the best way to ensure well-focused images. Remote trigger systems can either be tethered or radio-frequency controlled. In either case, these devices prevent excessive handling during the taking of the photograph. It is always important to record the finest details of an artifact; this includes color. Accordingly, a grayscale card and a color registration chart should be readily available and used as part of the photographic process. Doing so will 18 Technical Briefs in historical archaeology

7 ensure that the photographer has a known set of colors by which individual images can be adjusted during later computer processing. Digital SLR cameras often include a very handy setting that allows the adjustment of the white balance. To do this, routinely take a photograph and select it as the white balance for continued shooting. Even without a color calibration chart, this will ensure the recording of images which are consistent with one another. It is essential to take a new white balance image every time a new location is selected, or when the artifacts being photographed are changed. The camera manual will have very detailed information about successfully using and adjusting white balance settings. Center for Maritime Archaeology and Conservation Department of Anthropology, Anthropology Building Texas A&M University College Station, TX Technical Briefs in historical archaeology 19