Ultrawides: Focal Length vs Angle of View

[David Cerbone]

Perhaps there's an easy answer to my question, but I'm feeling a bit confused about the ultrawide LF offerings. My understanding going into doing some research was that there's a pretty tight correlation between the focal length of a lens and its angle of view. However, that does not seem to be the case with the ultrawides I've been looking at. Specifically, the Super Angulon 47mm/5.6 is listed as having an angle of view of 105 degrees, while the Grandagon APO 45mm/4.5 has 110 degrees (pretty close, but still seems like more of a difference than 2mm of focal length would make). The real anomaly, though, is the Super Angulon 47mm/5.6 XL, which is listed as having a whopping 120 degree angle of view, which is as much as the Super Angulon 38mm XL. (When I say "listed," I'm working with charts that I downloaded from this site, but I've also cross-checked with other sources as much as possible.)

A further fun fact: the Fujinon 65/5.6 SWD is listed as having a 105 degree angle of view, the same as the Super Angulon 47mm/5.6 (non-XL). I had been thinking of adding the 47mm to my outfit to get a bit wider than my Fujinon 65 would allow, but now I see that the only real difference would come with the 47mm XL (a far 'spendier' option).

What's going on here? Where should I go to get clear about this?

Thanks for any insights.

[Archphoto]

The focal length determines what you see on your focussing screen when the lens is dead center.
The angle of view dermines the image circle and with that the posibilities for shift for a given format.

On the other hand if you have a 8x10" camera and you put a 4x5" lens on it you can use that lens as a 6x17 lens at times.

I hope I've helped a bit.
Peter

[Darren Kruger]

Perhaps there's an easy answer to my question, but I'm feeling a bit confused about the ultrawide LF offerings. My understanding going into doing some research was that there's a pretty tight correlation between the focal length of a lens and its angle of view. However, that does not seem to be the case with the ultrawides I've been looking at.

There's the angle of view in front of the lens and the angle of coverage for behind the lens. The angle in front is determined by focal length and film format used. The angle in back determines the image circle and is usually the one published by lens manufacturers.

-Darren

[Ernest Purdum]

Angle of view and focal length are independent of each other. For coverage, both factors apply. A 75mm lens working at 105 degrees will have a larger image circle that a 47mm lens workikng at 105 degrees. There are several other factors involved in the lens design. A designer wanting to produce a relatively compact, light, and inexpensive lens may decide on a moderate angle. Someone else intending to produce the ultimate lens in its category might decide on a large aperture lens with a very wide angle of view.

As Darren points out, we don't normally put all of what the lens sees onto our film.

[Nathan Potter]

Ernest is on the right track about your question - I think. Another way of answering your question is to say that an optics engineer can design two identical 75 mm focal length lenses and each can have a different angle of view, or vice versa. The trick to obtaining larger angles of view with a given focal length is that ever more sophisticated optical corrections are required to obtain high resolution and field flatness at the image plane and the more expensive is the manufacturing cost. So there is not a consistent relationship between angle of view and focal length. It may be that the cost differentials between "wide" and "superwide" would track more accurately. :)

[Bruce Watson]

This seems to be an Angle of Coverage vs. Angle of View (http://en.wikipedia.org/wiki/Angle_of_view) question.

The angle of coverage is a lens design parameter which establishes the angle of projection for light coming from the lens. This constrains the diameter of the circle of coverage (aka image circle).

Angle of view on the other hand describes the angle the film can see through the lens when the lens is focused at infinity. It is directly related to focal length:

angle-of-view = (2)(arctan (film-dimension / (2)(focal length)))

and is not directly related to the angle of coverage. If the angle of view is greater than the angle of coverage however vignetting will occur at infinity focus and perhaps closer.

It is not unusual for a family of lenses to share the same design and therefore the same angle of coverage but have different focal lengths.