Angle of Coverage

[john_hennessy]

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.

 

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Thanks.

[pete_andrews]

The image circle is very specific. The figure given by the lens maker

is usually measured with the lens focused on infinty.<br>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.

[chad_jarvis1]

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

AOC is everything.<br><br>

I believe the formula you want to use to find the minimum angle of

coverage for a given format is: <br><br>

2*(arctan((<img src="sqrt.gif">(x² +

y²)/2)/F),<br> where <b>x</b> is the horizontal dimension

of film,

<b>y</b> is the vertical dimension of film and <b>F</b> 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_jarvis1]

Sorry...forgot the sqare root symbol was graphic:<br><br>

2*(arctan((<img src="http://www.redhillphoto.com/sqrt.gif">

(x² + y²)/2)/F)

[ron_shaw]

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.

[chris_patti1]

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.