### Lens coverage angle

greenspun.com : LUSENET : Large format photography : One Thread

Im interested in purchashing a wide angle lens for 4x5 negatives. Schneider shows the coverage angle as being 105 degrees for the 65mm Super Angelon, 105 degrees for the 75mm Super Angelon and 100 degrees for the 90mm Super Angelon and 100 degrees for the 165mm Super Angelon. Is it correct to conclude that "angle of view" is measured as a factor of the image circle rather than as a factor of the negative size. As an example if angle of view is determined by image circle then the 75mm Super Angelon with an image circle of 198mm is twice as wide when used with a 4x5 negative as the 165mm Super Angelon with an image circle of 395mm. It appears that lenses of a common design will have a similiar "coverage angle".

So the question is how much wider is a 65mm Super Angelon compared to the 90mm Super Angelon in terms of image actually recorded on a 4x5 negative? It appears that for lenses of a common design, the shortest focal length that produces an image circle that will cover the negative will produce the widest image on the negative.

Is this correct?

-- Dennis Felty (dennis.felty@verizon.net), July 01, 2001

I'm sure all this has been covered before - many times - but here we go again.
The angle of coverage will give you the diameter of the image circle, with the use of just a little bit of geometry. Tan(half the angle)*the focal length of the lens = radius of coverage circle.
BUT the angle of coverage given is usually the maximum angle that the lens is capable of, and you shouldn't assume that that coverage is available at all apertures. It's far better to get the figure for the coverage circle itself, usually given at both maximum aperture, and at f/22, by most lens makers.

The actual angle of view of a lens is dependent ONLY on the focal length of the lens and the format it's used with; provided it'll cover that format, of course.
Tan(half the angle of view)=half the film width/focal length.
In other words: AoV=2*(arctan(format/2*Fl)). Where 'format' is the height, width or diagonal of the film frame; depending on whether you want to know the vertical, horizontal, or diagonal angle-of-view.

-- Pete Andrews (p.l.andrews@bham.ac.uk), July 02, 2001.

Dennis:

I would opt for the 90mm unless you have a preference for rollfilm holders, extreme wide angle shots, or architectural work where shorter focal lengths are often required. If you select the 65mm, be aware that you may also need a center filter and, depending on your camera, a recessed lensboard. Also, if you buy a used lens, remember that many older 65mm Super Angulons do not cover 4x5. They were originally designed for 6x9. Hope this helps.

...........................................

-- Dave Willison (Dwillisart@aol.com), July 02, 2001.