A comparison of
the Photographic Process for conventional film photography with digital
imaging. For simplicity, the images illustrating the film process are shown in
black and white. A modern black and white emulsion, despite the absence of
colour in the final print, still contains a mixture of silver halide crystals
that are sensitive to the three colours blue, green and red. The blue-sensitive
crystals arise from silver halides' inherent sensitivity to blue and
ultra-violet light; green and red sensitivity has to be introduced into the
emulsion by the use of sensitising dyes, to produce a panchromatic (sensitive to all colours) emulsion. I will
try to draw comparisons for the stages in the process between film and digital,
but there will inevitably be problems at certain stages, and some parallels
will not necessarily stand up to close scrutiny.
The Original Scene.
The Original Scene.
 |
| Pic du Midi d'Ossau, French Pyrenees |
The process
from the perspective of film.
LIGHT from the
subject passes through the camera lens and strikes the film producing a negative LATENT IMAGE. (Latent meaning hidden, or invisible). The concentration of
latent image sites within the film emulsion is proportional to the amount of
light energy received at each point, and therefore proportional to the
brightness of the associated region in the original scene. The greater the quantity of light that strikes the film, the greater the concentration of latent image 'specks', and therefore the greater the concentration of developed silver in the final image. This means that a bright (or white) area in the original scene will equate to a dark (or black) area in the image, hence the opposite (or negative) of the original scene.
The film is
bathed, first in a DEVELOPER which reveals the hidden image and makes it
visible, second in a STOP BATH which halts the development, and third in a
FIXER which fixes the image, ie. makes it permanent. The film is then washed in
clean water, possibly treated with a wetting agent to aid uniform drying, then
allowed to dry. The resulting NEGATIVE is then exposed, via an enlarger, onto
photographic printing paper
producing a positive LATENT IMAGE. The paper then undergoes a similar development sequence
to produce the FINAL PRINT.The process from the perspective of digital imaging.
Light from the subject
passes through the camera lens, then through a coloured mosaic known
as a “Bayer Mask”. (The function of the Bayer Mask is to provide colour
sensitivity, and is analogous to the presence of the blue-sensitive, green-sensitised
and red-sensitised silver halide crystals in the film emulsion).
Having passed through the colour filter layer, the light then
strikes an array of light-sensitive capacitors. These are analagous to the
silver halide crystals in film (a capacitor is a device which can accumulate
electrical charge). The size of the electrical charge is proportional
to the amount of light striking the capacitor. The pattern of electrical
charges across the image sensor can be seen as the equivalent of the latent
image in film. The image is demonstrably present, but requires further processing in
order to make it visible (development equivalent) and permanent (equivalent to
fixing). The next two operations (transfer to the buffer, then saving to the
memory card) are very loosely the development/fixation equivalents, and the result is a digital negative. I am using the term here in a very
imprecise way, and it should not be confused with the various RAW file formats
that are referred to as digital negatives.
A control circuit transfers the voltages of the
individual capacitors (ie. the records of light intensity and colour for that
picture element, or “pixel”) into a memory buffer. The storage capacity of the
buffer, coupled with the processing speed of the camera control system, will
govern how long the data remains in the buffer, and therefore how long the
photographer must wait before being able to make another exposure.
From the buffer, the image data is
transferred into a memory storage device. This is commonly some form of memory
card, a solid-state computer chip. The number of images that can be stored on
the card will depend upon the capacity of the card and the degree of
compression applied to the image data before storage.
When the memory card is full, it becomes
necessary to copy the image data to some other form of data storage device,
then erasing the data from the card, thus rendering it ready for re-use. Image
data from memory cards is commonly copied directly onto a computer hard-drive,
or onto a CD or DVD depending upon the facilities available.
