Coverage w/ Process Lensesgreenspun.com : LUSENET : Large format photography : One Thread
Given a process lens designed to cover a certain area at 1:1, will the coverage increase or decrease at greater subject distances. Will the given f/stop be a factor? Thank
-- David Stein (DFStein@aol.com), March 29, 2000
Coverage is smaller when the lens is focused at farther distances. The easiest way to understand this is to view the lens as projecting a cone of light. When the lens is focused on infinity, it is one focal length away from the film and the cone illuminates the film to a certain diameter. As the lens is focused closer, it is moved farther from the film and the cone (of constant angle) illuminates a larger circle. The circle of coverage at 1:1 has twice the diameter of the circle of coverage of 1:infinity because when focused to project a lifesize image, the lens is positioned at twice its focal length from the film.
-- Michael Briggs (MichaelBriggs@earthlink.net), March 29, 2000.
Most process lenses have a manufacturer's stated coverage angle of 45 to 48 degrees. However, the specification for a process lens is a lot tighter than for general photography. It must have excellent geometry and almost no chromatic abberation, together with good resolution. If these rigid requirements are relaxed, then process lenses can be used over a wider field.
-- Pete Andrews (email@example.com), March 29, 2000.
The angular coverage is constant with object distance (give or take a few aberrations) so the film-plane coverage gets less as you go from 1:1 to infinity because the lens moves closer to the film.
Stopping down often improves aberrations like coma and spherical aberrations (though it won't do much for colour errors). This effectively increases the coverage. Lenses which are marginal for a format wide open can often work perfectly well when stopped down.
Modern process lenses are often fitted with 'field stops': a posh name for a baffle that stops light outside the formal spec for angular coverage from ever leaving the lens. Older process lenses don't have these, and depending on the model, can be used sucessfully on larger formats than their spec would imply since the tolerances for many types of pictorial photography are often more relaxed than those for the platemaking and colour seperation work for which these lenses were designed.
As an extreme example, I recently bought a Wray 18" APO process lens, which nominally has a coverage at infinity of 300 mm. It actually illuminates 20x24 without obvious vingetting and although I haven't tested it in any real sense, it is more than adequate for colour work intended to be contact printed on my 12x16 - an image circle of 500 mm.
-- Struan Gray (firstname.lastname@example.org), March 29, 2000.