Spatial Resolution
Numerous measures can be used to assess the quality of still images the most discussed being the pixel count, presumed to correlate with spatial resolution. This is measured by the quantity of picture elements (pixels) in the image sensor, usually counted in millions and called “megapixels”. Measuring the resolution of both film and digital pictures involves numerous issues. For film, this issue depends upon the size of film used.
Similarly, digital cameras have a variable relationship of resolution to megapixel count; other factors are vital in digital camera resolution, such as the number of pixels used to resolve the image, the effect of the Bayer pattern or other sensor filters on the digital sensor and the image processing algorithm used to interpolate sensor pixels to image pixels. In addition, digital sensors are typically arranged in a oblong grid pattern, making images susceptible to rippled water pattern artefacts, whereas film is not affected by this because of the random orientation of grains.
Estimates Approximations of a photograph’s resolution taken with a 35 mm film camera vary. Greater information may be recorded if a finer grain film and or developer are used. Conversely, less resolution may be recorded with poor quality optics or with coarser-grained film. A 36 mm x 24 mm frame of ISO 100-speed film is estimated to contain the equivalent of 20 million pixels.
On the whole the commercial photographer would look to select a better quality film and would decide between medium and larger formats over the standard size to gain improved quality. Because of the size of the imaging area, they have higher resolution than the current top-of-the-range digital cameras. It is estimated that a medium format film image can record around 50 megapixels, while large format films can record around 200 megapixels (4 × 5 inch) which would equate to around 800 megapixels on the largest common film format, 8 × 10 inches, without taking into account lens sharpness. A medium format DSLRs provides from 42 to 50 megapixels, which is relatively similar quality to the medium format film quality.
When choosing between film and digital and between different types of camera, it is necessary to allow for the medium which will be used for display and the viewing distance. For example , if a photograph will only be viewed on a home tele or PC, (which can resolve only about 0.3 megapixels and 1-2 megapixels, respectively, as of 2008. HD sets of 1080p are around 2.07mp), then the resolution provided by a very low-end digital cameras may be sufficient. Print mediums work to far greater qualities of around 300 dots per inch (dpi).
Noise and Grain
Thermal noise is a condition that damages shadow areas of electronic images with random pixels of the wrong colour. Film grain becomes obvious in areas of even and delicate tone. Grain and film sensitivity are linked, with more sensitive films having more obvious grain. Likewise, when used at high sensitivity settings, digital camera photographs show increased noise than those made at lower sensitivities.
Available technology currently introduces random noise to the images taken by digital cameras, produced by heat and manufacturing defects. Nearly all digital cameras apply noise reduction to long exposure photographs to counteract this. For very long exposures it is necessary to operate the image sensor at low temperatures to avoid noise impacting the final image. Film grain for visible light is not affected by exposure time, although the apparent speed of the film does change with longer exposures, a circumstance known as reciprocity failure.
Dynamic Range
The topic of dynamic range (DR) is complicated. Comparisons between film and digital media should consider:
What film? For example, low-contrast print film has greater DR than slide film’s low DR and richer gradation in recorded tones.
What film format? Larger formats record larger images, so grain is less detectable at film’s limits of exposure and images require less enlargement for a given image size.
What size sensor? As with film, smaller sensors produce smaller images which require greater degrees of enlargement for a given image size. The more convenient pocket digicams use smaller sensors than professional-quality cameras.
What scanner? Variations in optics, sensor resolution, scanner DR and precision of the analogue to digital conversion circuit cause variations in image quality.
What counts as image and what is noise? This question defines the limits of DR within a single photograph, and may vary with subject matter.
One comparison cannot show that digital or film has a smaller or greater dynamic range. Some amateur authors have performed tests with inconclusive results. R. N. Clark, comparing a top end digital camera with 35 mm film, reached the conclusion that – Digital cameras, like the Canon 1D Mark II, show a huge dynamic range compared to either print or slide film, at least for the films compared.
Ken Rockwell comes to a different conclusion: “CCDs and the related capture electronics will need about ten times more dynamic range (three stops) than they have today to be able to simulate film’s shoulder….This is the biggest image defect in digital cameras today.”
Some CCDs like Fujifilm’s Super CCD combines photosites of different sizes to give increased dynamic range. Other manufacturers use in-camera software to prevent highlight overexposure. Nikon calls this feature D-Lighting.
Effects of Sensor Size
Most consumer digital compacts and some DSLRs have sensors that are smaller than a 36 mm x 24 mm frame of 35 mm film. This affects aspects of the captured photograph and the way the camera is used. These effects include:
Increased depth of field;
Decreased light sensitivity and increased pixel noise;
For digital SLRs, cropping of the field of view when using lenses designed for 35 mm camera;
Lenses may be smaller because they only need to project their image onto a smaller area;
Increased degree of enlargement of the final image.
The depth of field of a digital camera and lens set up increases as the imaging area decreases, for a given f-number. This may have benefits for point and shoot compact cameras since they are intended for taking snapshots. More of the image will be in focus than with a larger sensor and the autofocus system does not need to be as accurate to produce an acceptable image. Conversely, photographers often restrict the depth of field to create certain effects, such as isolating a subject from its background. Cameras with imaging areas smaller than 36 mm x 24 mm require a wider aperture on the lens to achieve the same degree of selective focusing.
Pixel size is related to both light sensitivity and pixel noise, which is in turn related to sensor size and resolution. As the resolution of sensors increase, the size of the individual pixels has to decrease. This smaller pixel size means that each one collects less light and the resulting signal is amplified more to produce the final value. With a smaller signal, the signal-to-noise ratio decreases. More noise is present in the image and the higher noise floor means that less useful information is extracted from the darker parts of the image.
Several digital SLRs use lens mounts originally designed for film cameras. If the camera has a smaller imaging area than the lens’ intended film frame, its field of view is cropped. This crop factor is often called a “focal length multiplier” because the effect can be calculated by multiplying the focal length of the lens. For lenses that are not designed for a smaller imaging area whilst using the 35 mm-compatible lens mount, this has the beneficial side effect of only using the centre part of the lens, where the image quality is in some aspects higher.
More often expensive digital SLRs have full-frame sensors that are 36mm × 24 mm, which eliminate depth of field and crop factor problems when compared to 35 mm film cameras.
The smaller sensor size of digital compact means that prints are extreme enlargements of the original picture and that the lens must perform well in order to provide enough clarity to match the tiny pixels on the sensor. Most digital compacts have sensors that exceed the maximum resolution that the lens is capable of delivering. Increased sensor resolution may even have a negative effect on the overall resolution because of increased noise reduction and in camera processing.
Cleanliness
The cameras available to the portrait photographer address the problems of sensor dust and marks better than consumer cameras.
DSLR cameras are especially prone to dust problems because the sensor remains in place, where a film advances through the camera for each exposure . There is a risk of debris in the camera, such as dust or sand, scratching the film; a single grain of sand can damage a whole roll of film. As film cameras age, they can develop wear in their rollers. With a digital SLR, specs are difficult to avoid, but easy to rectify using a computer with photo editing software available. Some digital SLRs have systems that remove dust from the sensor by vibrating or knocking it, sometimes in conjunction with software that remembers where dust is located and removes dust-affected pixels from photography .
Compact digital cameras are fitted with fixed lenses; dust does not enter the imaging area. Similar film cameras are often only light tight and not environmentally sealed. Some modern DSLRs, most notably are the high end Nikon models, incorporate extensive dust and weather seals to avoid this problem.
Integrity
Film produces an original , which contains only the information admitted through the aperture of the camera.
Film images are very difficult to falsify , thus in police investigation and in cases where the authenticity of an image is critical, like passport or visa photographs, film provides greater security over most digital cameras as digital files may have been modified using a computer. If someone modifies an authenticated image, it can be determined with special software.
SanDisk claims to have developed a write once memory stick for cameras and that the images once written cannot be altered.
Archiving
Many types of digital media are not capable of storing data for prolonged periods of time. Magnetic disks and tapes may lose their data after twenty years, flash memory cards even less. Good quality optical media may be the most durable storage media for digital data.
It is important to consider the future readability of storage media. Assuming the storage media can continue to hold data for prolonged periods of time, the short lifespan of digital technologies often causes the drives to read media to become unavailable. For example, the first 5¼-inch Floppy disks were first made available in 1976. However, the drives to read them are already extremely rare 30 years later.
The ability to decode the data is important. Digital cameras save photographs in JPEG format, that has existed for approximately 15 years. Because the instructions on how to decode this format are publicly known, it is unlikely that this files will be unreadable in the future.
Most professional cameras can save in a RAW image format, the future of which is less certain. Some of these formats contain proprietary data which is protected by patents, and could be abandoned by their makers at any time for economic reasons. This could make it difficult to read these ‘raw’ files in the future, unless the camera companies were to release information on the file formats.
However, digital archives have several methods of overcoming such obstacles. In order to counteract the file format problems, many organizations prefer to choose an open and popular file format. Doing so increases the chance that software will exist to decode the file in the future.
Additionally many organisations take an active approach to archiving rather than relying on formats being readable decades later. This takes advantage of the ability to make perfect copies of digital media. For example, rather than leaving data on a format which may potentially become unreadable or unsupported, the information can typically be copied to newer media without loss of quality. This is only possible with digital media. Digital images may be printed and stored like traditional photographs.
Convenience and flexibility
With digital imaging, images may be conveniently stored on a personal computer for modification. Professional-grade digital cameras can store pictures in a raw image format which stores the output from the sensor directly rather than processing it immediately to an image. When edited in suitable software, such as Adobe Photoshop or the GNU program GIMP (which uses dcraw to read raw files), the user may manipulate certain parameters of the image, such as contrast, sharpness or colour balance, before producing a final image. Alternatively, users may retouch the content of recorded JPEG images; software for this purpose may be provided with consumer-grade cameras.
Digital photography allows the collection of large amounts of archival documents in a short period of time which has many benefits for the researcher including convenience, saving money and an increased flexibility in using the documents.
Price
Film and digital imaging systems have different cost structures . With digital photography, cameras tend to be significantly more expensive than film equivalents. With digital cameras, taking pictures is effectively cost-free. The price of digital cameras continues to fall and using film may be seen as more expensive than digital.
High quality film cameras are less complicated and therefore less expensive. The major expenses are ongoing film and processing costs. The commercial photographer will only identify unsuitable images after developing and printing have been paid for.
Film offers the photographer more control over the depth-of-field than a DSLR with an APS sensor, and the cost of full-frame sensor cameras may be very high. 35 mm single-lens reflex cameras may be bought for a fraction of the price of a full-frame DSLR. Some lenses are interchangeable between digital and film cameras; film can be an attractive introduction to photography because of this.
The costs associated with digital photography are specialist batteries, memory cards, paper, printer ink cartridges and long-term storage.
With many photographers switching to digital, film cameras and lenses are now available on the second-hand market at often much-reduced prices, allowing for semi-professional and even professional film cameras to be owned by people who would once never have been able to afford them.
