By Jay Holben, December 21, 2008
Editor's Note: This is an expanded edition of the DV101 column that appears in the December edition of DV magazine.
Although relatively small-sized camcorders, especially of the HDV variety, have allowed more videographers entry into the HD arena, there are several trade-offs to smaller chip sizes. One of them is the wide-angle shot, as the smaller imagers of 1/3" (and smaller) cameras make it increasingly difficult to inexpensively manufacture quality lenses, especially with wide angles of view.
Angle of View
The angle of view (AOV), or field of view, describes the degree of view a particular lens is capable of. A 180-degree AOV would see everything perpendicular to the lens, including the edges of the lens itself!
Years ago, when I was first teaching myself photography, I sat on the floor of my mother’s living room with a 35mm SLR, a roll of masking tape, two plastic cups, a tape measure and a protractor. The AOVs I was reading about just didn’t seem right to me, and I wanted to see if they related to real-world optics. I set the camera down on the floor and carefully positioned each plastic cup at exactly the same distance from the lens and moved them to the very extreme of the horizontal view from the camera. I then taped a line with the masking tape from each cup to the lens, thereby creating a large “V” angle on the floor. Using my trusty protractor (left over from high school geometry), I measured the angle of the tape V and — lo and behold — it matched the published angle of view for that focal length precisely. And that went a long way toward solidifying various lens sizes in my mind. Years later, I could look at a scene, imagine a framing and request a specific prime lens to match that perspective — and get pretty close about 90% of the time. Experience and experiments like this one helped develop those skills.
Any lens has two angles of view: one for the horizontal plane and one for the vertical. For our purposes here, we’ll only look at horizontal AOVs.
MM is MM — Target Size Defines AOV
All focal lengths, no matter what the format, are optically the same. A 50mm lens for a 35mm motion picture camera has the same distance between the nodal point (roughly the center of the lens where the light rays converge) and the focal plane (film) as does a 50mm lens on a 1/6" consumer CCD camcorder. Modern zoom lenses, and primes with multiple elements, can cheat this physical distance to a degree, but the optical properties remain the same. In a 35mm motion picture camera, the diagonal of the frame aperture (Academy) is 1.07" — a pretty sizeable target on which to project a very wide angle of view. With a motion picture camera and a 12mm lens, which is very wide, you can have a field of view of 92.1 degrees without significant distortion. That same 12mm lens on a 1/3" CCD camera, which has a diagonal of .236 inches, will give you an angle of view of only 28 degrees! The smaller target for the lens gives a considerably narrower angle of view. This makes wide-angle shots on small-chip cameras very challenging.
Enter the Adapter
This is where lens adapters come into play. I’m not talking about adapters that allow film lenses to be used on digital cameras; I’m talking about optical adapters that attach to the camera’s lens to decrease the focal length, which, in turn, increases the angle of view. Adding the additional optical elements, especially aspherical elements, allows you to cheat the maximum angle of view to achieve wider shots.
There are limits to how wide you can go with any adapter before you begin to distort the image, of course. Rectilinear wide lenses (those in which real-world straight lines are represented as straight lines in the image) are difficult to manufacture. The wider the angle of view, the more likely it is to distort the image. Fisheye lenses, the antithesis of rectilinear, provide extreme wide angles of view but also extreme image distortion.
A 16x9 0.7x adapter.
Lens adapters are calibrated by their conversion factor. A 0.7x will mean you simply multiply your focal length by that conversion factor to see how wide your converted focal length will be. 16x9 Inc. is one company that manufactures high-quality lens adapters for small-chip cameras. For my Canon XL2, a 1/3" SD camera that has a 20x lens with a 72mm lens thread diameter, 16x9 Inc. offers two rectilinear wide-angle adapters: a 0.7x and a 0.75x. The XL2’s lens has a 5.4mm to 108mm focal range, and the 5.4mm has a field of view of 54.8 degrees. With the 0.7x adapter, that converts the lens to the equivalent of a 3.78mm, which increases the field of view to 73 degrees. That’s an increase of 25 percent! And it gives the same field of view, roughly, of an 18mm lens on a 35mm motion picture camera.
Here's an example of how adapters can alter the angle of view, using both a subject — some palm trees — and a test chart. The distance from the chart remained at 26". The distance from the trees remained the same as well.
View from a standard Canon XL 1/3" CCD at 5.4mm. AOV = 54.8° Distance from chart = 26"
View from a standard Canon XL 1/3" CCD at 5.4mm with 16x9 0.7x adapter. AOV = 73° Distance from chart = 26"
Beyond that, you start to get into image distortion and fisheye lenses. 16x9 also makes two fisheye attachments, with a lens conversion factor of 0.45x and 0.4x. The wider, the 0.4x, will turn the Canon’s 5.4mm lens into a 2.2mm! This gives you an angle of view of 103.6, but with significant distortion.
View from a standard Canon XL 1/3" CCD at 5.4mm with the 16x9 0.4x fisheye adapter. AOV = 103.6° Distance from chart = 26"
To Afocal or Not to Afocal
The 16x9 adapters that I mentioned above are afocal adapters, meaning that they create no specific focal point themselves, but merely resize the image by optically altering the lens’ focal length. A non-afocal adapter, such as the Red Eye adapters, create a new focal point for a given lens. The Red Eye does this by projecting the image in front of the lens, requiring a macro focus lens to work. By doing this, the Red Eye creates a wide angle of view with a single lens in a very lightweight package.
The Red Eye non-afocal adapter.
The disadvantage to a non-afocal adapter is that it will only work if the lens can focus on the new focal point (in this case, requiring a macro focus lens to focus on the image projected mere inches in front of the lens) and will only work on a fixed focal length. This makes the Red Eye adapter a non-zoom-through, whereas the 16x9 adapters will work for the entire focal range of a zoom lens.
It may seem like an odd technique to have a wide-angle adapter and then zoom through it, but there will be times when you’ll need to recompose an image quickly, and retaining your full range of zoom (albeit shortened by the adapter’s conversion factor) is very handy.
Afocal adapters are larger and considerably heavier than focal adapters. The 16x9 0.75x adapter adds nearly 1.5 lb. to the lens, which may not seem like much until you’re going handheld for an extended period of time.
Adapters will, in addition to altering the focal length, alter the depth of field of your shot. As you are converting to a wider angle of view, if you maintain the same camera position, aperture and distance to your subject, you will have an increased depth of field, so that’s something to be aware of. In addition, with the wider view and larger optics, many accessories — matte boxes, lens shades, etc. — will no longer fit on the lens, or may actually be seen in the shot.
In addition to 16x9 and Red Eye, Tiffen, Century Optics (a division of Schneider Optics) and others also manufacture high-quality adapters. Prices range from $400 to $800, depending on the conversion factor and manufacturing.
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