If you are a raw beginner starting in the absolute beginning of photography i.e. with black and white emulsions (film), there have been several generations of film types over the years. The following is a breakdown of what was used in the past, and what is available today for you to use today.
ORTHOCHROMATIC MATERIALS
These film types were used in the early days of photography but orthochromatic film is still in use today. It is used in the graphic art industry as well as a niche film for photographic artists. However, its use has diminished greatly with the surge in digital photography. Similar results can now be achieved with Photoshop filters.
Blue Sensitive (actinic)
The first photographic emulsions were only sensitive to the blue spectrum. All silver halides in this emulsion responded to the longer wavelengths such as ultra-violet, violet, blue and green.
Isochromatic
The next emulsion included and additional dye claiming to respond to all colours. The claim was an exaggeration as the film did not respond to red at all, and the response to the remaining colours was not equal.
Orthochromatic
An improved emulsion was developed and termed orthochromatic, meaning correct colour. Again this was an exaggeration and the emulsion did not respond well to all colours of the spectrum.
The term orthochromatic is now applied to all emulsions that are not sensitive to red. These are the films that photographers and graphic artists to use today to create certain effects.
PANCHROMATIC MATERIALS
These materials are sensitised to the red region of the spectrum as well as blue and green, so therefore sensitive to the entire spectrum and termed panchromatic ie. sensitive to all colours. Panchromatic materials though do not respond equally to all colours and do not match the human eye's response to colour. They will record violet and particularly red, lighter than they appear to the eye and greens darker. Full matching can only be achieved with colour filters.
INFRA-RED MATERIALS
Infra red sensitising dyes were discovered early last century, but were not widely used until the 1930's. Infra-red monochrome materials are used with a filter to prevent any visible light or ultra-violet radiation entering the camera. This film is also somewhat sensitive to the red and blue region of the spectrum. This film is widely used in scientific photography and also by creative photographers, using the effect to produce ethereal photographs. The infra red effect can now be achieved by special purpose cameras built for scientific photography and the effect can be simulated with Photoshop.
COLOUR SENSITIVITY
Black and white emulsions reproduce colour as white, varying tones of grey and black. This is dependent on the spectral sensitivity of the B/W emulsion. We cannot judge whether these tones are correct without analysing and understanding the spectral sensitivity of our eyes.
THE EYE
Our eyes receive light rays through cone receptors that are of three mixed types, one particularly sensitive to blue and another to green and a third to red. The green receptors appear to be the most sensitive, closely followed by red receptors. Since all receptor activity overlaps, yellow-green is seen as the brightest colour of the spectrum in human visual perception. Shorter and longer wave lengths appear increasingly darker.
Under low levels of illumination maximum eye sensitivity shifts from yellow-green to blue-green. In addition to these response problems all humans respond to colour differently.
Black and white photography is not as straight forward as it may seem. An understanding of the emulsion types is first required and then a look into how we perceive is necessary. Once this is established it's on to the chemistry side of this fascinating area of photography.
No comments:
Post a Comment