0n our article (Gilbert and Richards, 2000), we showed that a method involving digital imaging may be used to effectively solve some of the problems associated with visually documenting microscopic marks left on bones and teeth. The response to our paper by Hirsch (2001) addresses some of the specific procedures we outline and also the more general debate over analog and digital photography. We address the specifics first.
As Hirsch (2001) suggests in the response, the techniques in our paper can be accomplished with an analog film camera if appropriate lighting and exposure are used, but the difference between technically possible and universally practical is big here. Light metering and effective bracketing become very difficult when shooting small objects while a camera's iris is very tight. The exposure times are long and the lighting intense. For many researchers, the level of technical expertise required for doing this efficiently is restrictive.
Many digital cameras make this process much easier, as they can display a live or nearly live version of the image being captured via a monitor. Thus, the final image becomes the metering tool; contrast and lighting adjustments are made by referring to previous shots. This allows the advanced amateur to produce publishable images of difficult-to-light specimens quickly by trial and error without spending time and money on development and film.
The quality of any imagery is in many ways subjective, being only partially based on the information capacity, or resolution, of its medium. This caveat given, the absolute resolution of an image taken with the Nikon D1™ is indeed lower than one taken using Kodakchrome 64™ 35-mm slide film. Comparing resolution of analog and digital images is a tricky matter, however, for the chromatic units of film emulsion and pixels take different shapes. Roger Clarke (http://www.users.qwest.net/∼rnclark/scandetail.htm) has made an excellent comparison of the two by scanning slides at progressively higher resolutions until further increases in scanning resolution had no apparent effect on the microscopic emulsion pigment patterning of the photos. Clarke found that the pigment particles of fine-grained 35-mm film become blurred when scanned at resolutions of less than between 6,000 and 8,000 dpi. While his comparative method tends to overstate the resolution of chemical film (film emulsion renders objects as mosaics of colored particles, so very fine scanning only enhances the detail of the pigment particles, not necessarily enhancing the details of the objects they evoke), his is one of the best proxies available. The Nikon D1 has a resolution of ∼2,127 dpi, in actual pixels, across the CCD. Fine grained 35-mm film is, if the limitations of chemical chromatic units are taken into consideration, capable of a something more than twice the resolution of a high-end digital camera like the D1.
Practically, however, the question is not over of which is absolutely better in terms of resolution. If resolution were the only consideration, then we would all be using Hasselblad™ 500-series cameras for every photo we took. The question is whether the benefits gained by using a digital camera offset the discrepancy in resolution, and this depends on the application. Digital cameras may not be the way to go for every application, but the quality is more than adequate for capturing the features of microscopic bone and tooth modification. Moreover, without the flexibility provided by the digital input method, it would be restrictive in both time and money to obtain the images needed to properly document bone modification, as demonstrated in our paper (Gilbert and Richards, 2000).
Permanence of the imagery's storage medium is not directly related to its input method. Just as with analog images, those obtained digitally may be archived as high-quality hardcopy. Electronic archival is also possible, but on this Hirsch (2001) raises a valid point. We have all come to learn that if we should partake of advances in technology we must also be part of a cycle of novelty and obsolescence that requires constant monitoring. Digital storage media have not been stable over the last 20 years. But most scientific groups have had to come to terms with this fact over the past two decades, and most regularly upgrade their data storage devices. Another thing to keep in mind is that one of the archive formats possible for images obtained digitally is analog film!
We do not agree that changes in lighting like those suggested by Hirsch (2001) can easily allow high f-stops with conventional film cameras. This is not because conventional film cameras are technically incapable of smaller apertures; rather, it is the difficulty involved in metering magnified shots of three-dimensional objects that require high contrast that restricts the use of analog film for imaging bone or tooth modification. Light must be unidirectional and often come from numerous, specific angles. The difficulties imposed by conventional photography are what have kept archaeologists and paleoanthropologists spending money and time in SEM laboratories; these are the restrictions our method addresses.
It would be wrong to say that digital photography is better than conventional photography. Indeed, both of us still use chemical film and prefer it in most situations. We do, however, suggest that when imaging bone and tooth modifications, digital photography provides a relatively inexpensive, readily archived, and effective alternative to the SEM.