Allen, E., Triantaphillidou, S. , 2011, ‘Introduction to the imaging process’

Allen, E., Triantaphillidou, S., 2011, ‘Introduction to the imaging process’, in Manual of photography, 2011, Focal Press, Oxford, pp. 1-18.

Notes move to digital (discrete packets of data) as opposed to analogue (continuous data) for traditional film/chemical (silver halide)  based photography.  2

Core science for both digital and traditional analogue is much the same – remains important.  2

Photography def. ‘to draw with light’ (literal translation) –

Both film and digital photography rely on light hitting a light sensitive medium (film or digital sensor) – to render a permanent image of what has been recorded.

Definitions

Quantization – a pixel’s data is recorded with discrete integer value – which later defines its pixel value.

CCD – Charge coupling device – type of digital sensor at focal plane of camera – charge is recorded when light hits the sensor – which is then transferred to an analogue-digital converter.    

CMOS – Complementary metal oxide semi-conductor – image sensor which performs the digitization within the chip and outputs digital values.

Tone reproduction – how the range of intensities in the original scene is mapped to those in the final image.

The process

  • Light passes through lens (or pinhole e.g.) and is refracted by it (refraction of rays changes light velocity and direction as light moves through materials of different densities)
  • Hits image plane (film or sensor)
  • Amount of light hitting image plane controlled by a combination of aperture (size of hole) and shutter speed (total time plane is exposed to light).  Recopricity equation – H=Et where E is the illuminance in lux, t is the time of exposure and H is the exposure in lux-seconds.  (Different combinations of the variables will produce the same result at each exposure level.) 3
  • Dif combinations of exposure and aperture produce varied depth of field and sharpness/motion blur.
  • 1 stop = a single increment in the scale of possible values for both shutter and aperture (there are now ½ stops in many cameras).  Each stage of a single stop halves or doubles the amount of light falling on the sensor.  3
  • Image is formed when the sensor or film changes or produces a response in areas where exposed – proportional to the amount of radiation falling on it.
  • For film and other trad means – there is then a fixing process required to fix the latent image – a chemical process to do with changing of e.g. silver ions to silver atoms.  Silver specks are formed (for B&W) on final image which show up as black.  The basic process forms a negative which is then reprocessed onto photographic paper to produce a positive final image (which may be enlarged or same size(contact prints).  See table on p. 3 of book.
  • Digital processing depends on type of sensor – for Charge-coupling devices – CCD – the charge is transferred from the sensor, amplified and sent to an analogue to digital converter.  Sampled at discrete intervals corresponding to individual pixels and quantized.
  • Digital images are not continuous – because they are discrete pixels – so, image appearance of continuity depends on number of pixels and how magnified they are – (where as silver halide photography is only discrete at an atomic level with a smooth and continuous gradation to the eye even at high levels of magnification).
  • Image perpetuation – image is rendered permanent.  Film – is stopped, fixed etc.  Digital – saved as a unique digital image file which can be output e.g. to screen, printer for viewing.  (figure 1.2 – p. 4)
  • Image shape and size – is determined by the camera used – size of the sensing area.
    • Large format cameras – technical or view cameras, 5×4″ –’ designed to allow camera movements, physical manipulation of the two planes containing lens and imaging sensor separately, enabling the photographer to change the size, magnification and perspective of elements on the plane of sharp focus .’  More in chapter 11.
    • Image shape is also controlled by focal length of the lens – ‘in a simple positive lens the focal length is the distance from the lens to the rear principal focus, defined as the point on the optical axis at which the lens brings a distant object to a sharp focus.  Focal length determined by curvature, thickness and refractive index of the optical components, and this in turn defines the angle by which light rays are deviated (refracted) as they pass through them.  This determines the field angle of view‘.  5 also fig. 1.3 p. 5
  • Depth of field – the amount of the scene/image that is in focus acceptably – shallow – only one plane is focuses accurately – large – a large amount of image from near the camera to the distance is focused.  Zone of focus.  Depth of field depends on:
    • Lens focal length
    • Distance to focused object
    • Lens aperture.  5
  • Tone and contrast – tone – the intensity of the light reflecting from an object.  Contrast – the ratio between the brightest and darkest tones in the scenes and the range of possible intensity levels in between.  Tone and contrast may be manipulated in a variety of ways.
  • Lighting control in the original scene  – light sources can change tone and contrast – e.g. with a flash fill to brighten foreground of a full sun portrait.
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