I have a question about a LF lens term. "angle of coverage" is defined in an old Calumet catalog as "the angle formed lines connecting the image nodal point to opposite sides of the image circle." This measurement is often prominently featured is LF brochures and what not; so my question is: what is its significance? The diameter of the image circle. focal length, aperture range, etc. seem far more important.

Thanks.

-- John Hennessy (northbay@directcon.net), February 09, 2001

Answers

The image circle is very specific. The figure given by the lens maker is usually measured with the lens focused on infinty.
Angle of coverage, on the other hand, can be used to find the radius of the image circle at any lens extension, using the tangent of the half-angle.

-- Pete Andrews (p.l.andrews@bham.ac.uk), February 09, 2001.

When you are trying to cover 16x20" with a 4" lens (not that I am), AOC is everything.

I believe the formula you want to use to find the minimum angle of coverage for a given format is:

2*(arctan(((x² + y²)/2)/F),
where x is the horizontal dimension of film, y is the vertical dimension of film and F is the lens focal length. Note that these angles are the minimums required to cover the given formats and do not account for movements.

-- Chad Jarvis (cjarvis@nas.edu), February 09, 2001.

Sorry...forgot the sqare root symbol was graphic:

2*(arctan(( (x² + y²)/2)/F)

-- Chad Jarvis (cjarvis@nas.edu), February 09, 2001.

Angle of coverage of course determines the image circle, so the two terms are directly related. As Pete mentioned, the image circle specified is when focused at infinity. As you focus closer, the image circle gets larger, since the angle of coverage remains constant.

-- Ron Shaw (shaw9@llnl.gov), February 09, 2001.

Angle of coverage is also useful when talking about a "family" of lenses of different focal lengths. For example, Artars of all different focal lengths will have about the same angle of coverage, but their image circles will, of course, vary with focal length. Knowing the angle of coverage of a lens family tells you whether it is best used as a wide, normal or long lens and allows you to extrapolate image circles from one focal length to another.

-- Chris Patti (cmpatti@aol.com), February 09, 2001.