Coverage w/ Process Lenses

[david_f._stein]

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

[michael_briggs2]

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.

[pete_andrews]

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.

[struan_gray]

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.

 

<p>

 

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.

 

<p>

 

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.

 

<p>

 

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.