Zoom lenses on broadcast and professional video camcorders are wonderful tools for the videographer, allowing quick and easy focal length change. But ultimately, a zoom lens is limited on both the wide and telephoto end of its zoom range.
On higher-end camcorders there is generally a choice of lenses, and thus zoom range, when the camera is purchased. But purchasing several zoom lenses would be an expensive option, and interchanging those lenses in the field would be cumbersome.
Many of the new DV and HDV camcorders have factory specified and permanently mounted zooms, which limit the zoom range to the specifications of the manufacturer-supplied lens. But thanks to add-on lenses, which can be quickly fit to camcorder lenses, video camerapersons can enjoy convenient lens-changing advantages, without breaking the bank.
High quality add-on lenses alter the properties of a video camera lens without affecting lens performance. Such add-on lenses can expand the angle of view at the wide end of a zoom, tighten the angle of view at the telephoto end of the lens, or reduce the minimum focus distance of the lens.
Lens add-on accessories are designed with either a screw-on, clamp-on or bayonet-mount. Attaching a lens accessory via a clamp-on or bayonet-mount is quicker and eliminates wear and tear on the lens’ mount.
Zoom lens makers typically make bragging points out of the telephoto properties of their lenses. However, a videographer may only use the lens on full telephoto a few times a week, where he probably can’t count on both hands how many times a day he’s pressing his back against a wall, trying to get just a bit wider field of view to capture the entire scene.
The answer for videographers are front-mounted Wide Angle Adapters and Converters, A Wide Angle Adapter is usually mounted to the lens front for a specific shot as it limits full zooming, where an add-on Converter maintains focus through the entire zoom range of the lens. High quality Converters are heavier and more expensive than Adapters but they offer the convenience of being able to leave them on the lens regardless of focal length being used.
For a wider view with little or no distortion, a videographer has a range of options. A .7X Converter offers 30 percent more field of view, and maintains focus throughout the entire range of the zoom. For a wider field of view, a .6X Adapter increases a lens’ field of view by 40 percent, however, full zoom capability is minimized.
For an even wider field of view, there are fisheye add-on adapters. The classic Fisheye Adapter features the characteristic fisheye barrel distortion, popular in today’s action sports coverage. A more profound fisheye effect is available by using the .3X Ultra Fisheye Adapter, which provides a 125-degree (horizontal) angle of view. For the widest perspective available there’s the 160-degree (horizontal) Century Xtreme Fisheye that is popular for shooting action sports, underwater photography, music videos, or that dramatic impact shot in tight environments where capturing the field of view is otherwise impossible.
More Telephoto Reach
At the tight end of a zoom lens, there are situations where a cameraperson wants more telephoto performance in order to see the subject better, or to crop out unwanted surroundings. The answer here is the Tele-Converter, an add-on lens accessory that increases the focal length of the lens throughout its zoom range with no measurable light loss.
A 1.6X Tele-Converter can make a considerable increase to the focal length of a lens’ zoom range. As an example, the 1.6X converts a typical HDV camcorder lens’ zoom range from 4.5-54mm to 43-86.4mm.
For maximum focal length increase to a zoom range, a 2.0X Tele-Converter on that same HDV camcorder lens converts the zoom range from 4.5-54mm to 54mm- 108mm, reducing the final angle of view from 5.3 degrees to 2.58 degrees.
For the videographer using a camcorder with a removable zoom lens, a 2X Tele-Extender can be mounted between the camera and lens to double the lens focal length. For example, a 2X Tele-Extender would turn a 9-144mm zoom, into an 18-288mm. 2X Tele-Extenders require 2-stop exposure compensation.)
Many zoom lenses offer a macro function for extreme close-up work. However, in the macro mode a lens is relatively limited as to the image size, and does not provide for zooming. A better solution for extreme close-ups, such as work with copy stands and flat fieldwork, are high quality Achromatic Diopters. These Achromatic Diopters are available in a range to allow various minimal focal distances.
For example, on a typical camcorder with a 4.5-54mm 1/3” format zoom lens, the minimum focus distance is about one meter or 3’3”. However, that same lens with a +3.5X Achromatic Diopter attached has a close focus working distance of 8.75”. With the lens set at close-focus, the image size at telephoto is less than 1” horizontal. When zoomed to wide angle the image size becomes 9.5” horizontal while maintaining full zoom capability.
Achromatic Diopters are significantly different than single-element close-up diopters. Where use of a single-element diopter for close-focusing sacrifices image clarity as a pin cushioning effect occurs, Achromatic Diopters, feature two sandwiched, highly corrected glass elements to minimize chromatic aberration and distortion, creating an image with edge-to-edge sharpness. Achromatic Diopters can be stacked one in front of the other, which results in a summing of their reduction of the minimum focus distance of the lens.
Video camcorder zoom lens makers go to great pains to design and manufacture lenses with high performance characteristics. They employ special glasses and coatings to produce lenses that are distortion-free from corner to corner, which hold focus throughout the zoom range and deliver the maximum amount of light to the camcorder’s imager.
A high-quality add-on lens accessory is designed and manufactured with the same amount of care, to maintain the same level of quality as the lens itself. Any camera person tempted to purchase an inexpensive add-on accessory should do a simple comparison with a similar add-on accessory of higher quality. How much does the quality degrade between an image taken with the naked lens and one with the cheap accessory? Is there vignetting in the corners? Will that image difference be acceptable to your audience and/or client?
Add-on accessory customers often ask why high-quality manufacturers have such a large family of these lens tools, sometimes with different models of adapters, converters, tele-extenders and diopters for each of the various camcorders and lenses. Couldn’t two similar zoom lenses, from different manufacturers with the same sized bayonet or threaded ring mount, use the same Wide Angle Adapter?
For the maximum optical performance, the answer is that this is not always possible. Wide Angle Adapters, Tele-Converters and Achromatic Diopters have to be carefully matched to not only the characteristics of the zoom lens, but to the performance of the camera itself. Maintaining focus throughout the range of the zoom as well as contrast and other parameters is critically important.
The coming of the 16:9 frame of the high definition image, which has taxed lens makers to deliver high quality to the very edges of the lens, has similarly put even more pressure on add-on lens accessory makers. High-quality manufacturers have risen to the task, providing products that yield uniform and distortion-free images across the entire 16:9 frame.
A videographer who owns a kitbag filled with high-quality add-on accessories such as wide-angle adapters, tele-converters, achromatic diopters and other lens tools, can greatly expand their creative options while maintaining the ability to work fast and deliver a high quality product.