Photography is the art of producing an image from the physical world through an electro-mechanical means. From the first sliding box cameras to todays professional digital systems, all cameras throughout history have striven for perfection in visual reproduction. Alas, the world is not perfect.

Today there is a huge debate among professional photographers as to whether film or digital cameras provide the best or most accurate images. The following is a brief primer on the equipment of photography in general and a discussion of the advantages and disadvantages of both film and digital imaging. The goal of this page is to show you that regardless of the original media that is used for imaging the artwork, the same amount of information available for forensic analysis can exist in the photographic image of your piece.

The Photographic Art

Whether you are using a film or digital camera, there are some aspects of photography that are going to be the same. First of all is the availability of lenses; there are a host of different lenses available for both digital and film cameras. Everything from zoom lenses to panoramic lenses to lenses that deliberately distort the imaged object in some specific way are available. While the type of lens you use will make a difference in the quality of your photo, the easy availability of lenses means that they have no bearing on our topic of film versus digital.

The second aspect that is common to both types of imaging systems is filters. Filters are placed in front of the lens of the camera and are designed to alter the incoming light in some fashion. Filters can be used to polarize light to provide greater contrast, deliberately block some specific color of light, or any of an innumerable number of other uses. The purpose of filters is to deliberately block some light so as to more easily see the details or colors you are most interested in.

An image from Kodak photo services of a train taken with a star filter. Notice the rainbow highlights that the filter generates with the image.

A very good example of filter use is with infrared reflectography (IRR). In IRR, an image is lit with an infrared source (light waves of 1.0 to 3.5 microns), and the resultant reflected light is imaged. For IRR, you are generally not interested in the visible spectrum of light at all (0.4 to 1.0 microns). Therefore, it is common to use a filter that blocks all light in the visible spectrum with an IRR system. You have lost some light and resultant information at the gain of making the infrared image much more visible.

It is important to note that with todays software, some types of filtering can be done by a computer on a digital image. This is discussed in much greater detail in our section on digital processing techniques.

Film Photography

Until only a few years ago, film photography was still unconditionally accepted as always being a superior type of photography to digital. Film photography is the process of exposing a light sensitive material and then developing and fixing the material so that it is no longer light sensitive. The developed image can then be used by a film printer to print larger sizes of the image.

The quality of the light sensitive material, commonly referred to as film, thus greatly determines the quality of image that can be taken with a film camera. There are a number of different sizes of film that can be used with different types of film cameras; the most common size is the 35mm by 24mm film size, but sizes up to 8" by 10" are also available. Generally, the larger the film, the better the resulting photo can be at a correspondingly higher price. There is also both regular film that creates a negative of the image captured, and slide film, which creates a positive image of the object.

Films also come in different speeds, which is an indication of how quickly the light sensitive material reacts to the incoming light. Choosing an incorrect film speed for your photos will mean that your image will be over or underexposed and may introduce excessive noise into the image. Finally, films also have a granularity associated with them. The light sensitive element of most films is silver halide, and this element exists on the film in micron sized crystals. The smaller the size of these grains of silver halide and the more evenly the distribution of the grains, the higher the resolution the resulting image can be when using that film.

A slice through the developed emulsion layer of Agfa APX100 film and showing the exposed silver halide grains.

The creation of a color film image is a chemical process that is inherently unstable. This means that even the most perfectly preserved film negative or slide is going to have its color degrade over time. This can become a liability when conservators wish to keep a long term record of an artwork so as to track any changes with the piece over time. Degradation can be minimized, however, by choosing to use an archival film that has a retarded degradation process, and storing the film in sub-zero temperatures.

Fading of a photograph due to the cyan layer partially bleaching away with aging.

The one absolute feature that film has over digital photography is in the graceful way film deals with overexposed objects. If a particular area of an image has too much light, a film photo of the object will saturate the overexposed area in the same way that our eyes deal with to much light, making the image look more natural. Digital photos, because of their inherent nature, will saturate at a level and all saturated areas will be at that same level of intensity.

In the end, a large format film print of an object will have more potential information available in the image than any digital image that can be made with todays technology. Even a 35 mm film photo is equivalent to a 25 megapixel (MP) camera, digital imaging technology that is only just possible. The amount of information in an image, however, is only one half of the equation; the quality of information is just as important. To go further, it is the quality of information and amount of information necessary for the task at hand that is important. An exquisite 8"x10" format film image may not be necessary for the task of something like microphotography. This idea of sufficient information will be looked at further after a discussion of digital imaging.

Digital Photography

There are two different types of digital imagers available in the market today; Complementary Metal Oxide Semiconductors (CMOS) and Charge Coupled Devices (CCD). Both types of sensors convert photons into electrons in individual pixels and a count of the electrons becomes a measure of the intensity of the light. The difference between the two technologies is in the way the electrons are shuffled around the sensor and for our purposes here, are largely irrelevant; we will simply discuss digital imagers as a whole and ignore the differences between CMOS and CCD imagers.

Each pixel of a digital sensor acts as a storage bin for the incoming elections and pixels are generally between 2 and 10 microns in length and width. The total number of pixels on the imager gives the resolution of the image that can be achieved and is given in megapixels. Each pixel is only sensitive to the intensity of light throughout the entire visible spectrum. For digital cameras to produce color images, the imager is covered with a colored filter in a specific pattern called a color or filter matrix.

A Bayer color matrix, the first type of color matrix developed, which distributes red, green and blue colors to individual pixels. Other types of color matrices include CMYK filter schemes, and others RGB derivatives.

A color matrix filters the incoming light so that specific pixels will be sensitive to only a specific color of incoming light. Once the image has been captured, algorithms within the camera process all the pixels, assigning to each pixel a specific color based on that algorithm.

The algorithms used by camera companies are highly proprietary and not available for study. The effectiveness of the algorithms can be rigorously analyzed, however, by imaging known color samples and performing a chromatic analysis. Again, the wars rage as to whether film or digital can provide better color.

As mentioned in the film photography section, a digital imager cannot gracefully deal with over exposed areas. On the other end of the spectrum, however, a decent digital camera will have very good algorithms to deal with underexposed areas and low visibility photos.

Prameter	Film Images	Digital Images	Advantages *
Resolution	Dependant on film grain. Standard 35mm film negative approximately equals 25 MP digital resolution. Medium format film equals 100 MP.	Dependant on pixel size and total number of pixels.  25 MP sensors just becoming available.	Film
Color accuracy	Three color process. Color negatives are unstable and colors degrade over time.	Bayer matrix and on camera algorithms used to create color.  Color accuracy dependent on in camera software.	Neither
Sharpness and contrast	Affected by the combination of film grain and photographic paper used for the print.	On board algorithms can help provide better contrast and sharpness.	Neither
Noise	Dictated by correct use of film speed for available light.  Difficult to swap in correct speed film at a moments notice.	On sensor correlated double sampling and noise reduction. Effective equivalent film speeds available and easily changeable.	Digital
Highlights	Over saturated areas degrade gracefully and similarly to our eyes.	Saturation is at a defined point and all areas of even high intensity will be the same white color irregardless of intensity level.	Film

*As with any highly contentious issue, the advantage of any given parameter is hotly contested and many arguments are given for both sides. The advantage given to one type of photo or the other is based on what seem to be generically accepted views in the photography world.

Currently, there is no clear advantage to using either film or digital photography. The only consensus seems to be, "It depends on what you are doing". Since forensic analysis encompasses a number of different methods, what we wish to use the photo for can also vary greatly. In the next couple of sections you will see how digital images, either from a digital camera, or scanned from a photograph, can always contain sufficient information for the forensic analysis of a piece of art.

Digitization

When you digitize something, you are selecting bins for each datum of information. In order to do that without a one to one correspondence, each bin has a range. Therefore, some pieces of information in a film photo that are not quite identical are made to BE identical in the digitization process; some information is lost.

A waveform in both its analog form and what the waveform looks like after it has been digitized. Each step is a bin chosen by the digitization process.

The digitization process can happen at many different levels; in the camera itself with a digital imager, taking an analog signal from a video and converting it, or scanning a film photo to have a digital record. All of these processes are digitization processes and thus information is lost.

This binning process is the basis of the term resolution when discussing digital images. For CMOS and CCD imagers, the binning process occurs with the photons being binned into each individual pixel on the sensor. With the conversion of a video signal, the binning occurs through the selected electronics performing the analog to digital signal conversion.

For scanning film photos into a digital image, the resolution is measured in Samples Per Inch, or SPI. SPI is the effective resolution of the scanner that is used to convert the film photograph and is sometimes mistakenly referred to as Dots Per Inch (DPI) which is actually a measure of printer resolution. In a scanner's case, the resolution is a measure of how many individual areas the scanner can observe in a given linear inch of the film photo being scanned.

There are a number of different consumer and professional level scanners with resolutions between 400 and 10,000 SPI. As always, the quality of the scan is by and large a factor of money. That being said, the highest resolution scan may not be strictly necessary as we will soon see.

The Advantages of Digital Photography

In the end, a film photo of an artwork will always theoretically contain more information than a similar digital photograph or a digital scan of the film photo; this loss of information is inherent in the digitization process. Of course, the first thought obviously becomes, "Hard copy photo images must be better than digital for the study and analysis of my artwork. Why would a company work with digital images?" There are three key elements that make digital images a superior choice over film photos for forensic analysis:

1. Not all information is necessary.
2. A digital image can always be created that has the necessary information.
3. digital images can be analyzed and manipulated in ways that are just not possible with film photographs.

Each of these are very important concepts, which deserve some further consideration.

1. Not all information is necessary.

A perfect example of unnecessary information is the filter example described previously. Some light, and thus information, is blocked from becoming part of the photograph so as to enhance other aspects of the image. Another example is the cropping of a photograph, either digitally or manually.

Perhaps, the information that is important is easier to see if it is in the center of the picture whereas the original photograph has the focal object off to one side. By removing some information, the primary object becomes more of a focus, and thus overall, the quality of information has increased.

A portion of Bellini's Feast of the Gods cropped and photographed using both ultraviolet and infrared lighting and filters. Notice the clothes that are on the figure in the underdrawing below the final layer of paint that are revealed by the specially filtered images.

2. A digital image can always be created that has the necessary information.

If finer resolution is required for digitizing an artwork, it is quite easy to take images of small sections of the artwork and build a mosaic of the images using computer software. If the lighting was particularly poor in a digital image, you can use software to increase the contrast, and thus the usefulness of the photo, with very little loss of information.

The key feature of this point is that computers and their associated software have given us the tools to easily manipulate digital photographs in the same way that film photographs can be subtly altered with different development techniques. The purpose of both methods is to enhance the information available in the image. The nice thing about digital photo manipulation is that it can be much quicker than adjusting the development of film photos. Digital manipulation, however, is capable of so much more than could ever be accomplished in a darkroom, which leads into the third point.

3. Digital images can be analyzed and manipulated in ways that are just not possible with film photographs.

Film development techniques are available to false color images, improve contrast, and so on. But what if you wanted to identify and false color all the small cracks and losses in an old painting? Standard film development could not accomplish this. However there is software that exists that can automatically do shape detection and coloring on a digital image in about the time it takes you to read this sentence. This potential of digital images is truly what makes the forensic analysis of artwork from digital images possible.

A composite x-radiography of Picasso's The Tragedy that has been altered by software to highlight the tortured horse and bullring arch that was painted previously on the same canvas.

The power of digital imaging is in the computational force that can be brought to bear on an image to reveal and highlight details normally lost in a simple visual examination of an artwork or photo of an artwork. While creating a digital image inherently loses some information, the quality of information that we can potentially generate from the digital image is increased by orders of magnitudes over a simple film photo. It is this information quality, and the techniques for enhancing the digital information, that we will look at in the digital processing techniques section.