|T-mount lenses let you explore extreme focal lengths without breaking the bank. Notice the out-of-focus donut shapes the mirror lenses produce.|
The T-mount system first appeared as an early solution for the age-old problem facing photographers who wanted to use the same lens on two or more camera bodies with unlike mounts. T-mount lenses have a thread that's 42mm in diameter and a pitch of 0.75mm. Tamron, the originator of the system some 60-odd years ago, offered matching mounts for various cameras. That's where the "T" comes from.
The T-mount method isn't without flaws. It provides a solid mechanical and optical connection, but it fails to transfer aperture control, autofocus, exposure information or anything else that requires data exchange or moving parts. It allows the lens to be attached to the camera, period. Tamron later introduced the Adaptall series, which solved many of these limitations.
Today, the T-mount is very much alive despite the shortcomings. Mirror lenses, telescopes and certain other accessories are strictly manual focus by design and have only one fixed ƒ-stop. There are no automatic features to lose, so lack of aperture control and autofocus is of no consequence.
Mirror lenses deliver very long focal lengths, typically 300mm to 800mm, in very small packages. Factor in the focal-length multiplier for your DSLR, and you're quickly in the 1000mm to 1600mm extreme-telephoto range. While great for nature photographers, especially those desiring to get closer shots of distant wildlife, mirror lenses are inherently lower in contrast than their all-glass brethren. They also turn out-of-focus highlights into circular donut-shaped rings. These were once highly prized by photographers of yesteryear when mirror lenses were first introduced.
Mirror lenses also focus very close, closer than other telephotos, as a rule. The same camera-shake restrictions that apply to all-glass lenses apply equally to mirror lenses. Unless you're shooting at very high shutter speeds, use a tripod or at least a solid monopod.
The Pro-Optic 500mm ƒ/6.3 gives you a big telephoto reach in a small and inexpensive package. It's compact and lightweight, and consists of a reflex design that has a fixed aperture of ƒ/6.3. This Pro-Optic mirror lens is well suited to wildlife and sports photography. Its simple T-mount adapters screw on the lens to fit almost any SLR camera. It works with autofocus cameras in manual focus mode only, and a soft carrying pouch is included. For the 500mm reach that the lens gives you, it's quite affordable at $159.
Kenko, part of the Tokina-Hoya-Kenko group, offers a handsome 400mm ƒ/8 mirror lens that measures just 3.2 inches long and weighs less than 12 ounces. It's well suited for nature photography because of its compact dimensions. It's easy and fun to use, but requires the same precautions as any long telephoto lens—shoot at high ISO to maximize shutter speed and use a sturdy tripod or other adequate brace. Cameras like Sony and Pentax that have body-integral image stabilization offer a distinct advantage.
Using a T-mount to attach your DSLR to a telescope for terrestrial or celestial photography requires an adapter made specifically for your brand and model of scope, plus a T-mount adapter. The combination is simple to use. Connect the telescope adapter to the appropriate T-mount adapter. Remove the telescope eyepiece and replace it with the new hardware. Attach your camera body to the T-mount. Set your camera on Manual, and you're ready to go. Some companies, Celestron, for instance, offer adapters that are also tele-extenders. The Celestron Deluxe Tele-Extender is a good example.
A few words of caution: Be sure to carefully match the telescope adapter to the telescope brand. Also, be prepared for a whole new world of camera shake because of the extreme magnification. A tripod is an absolute necessity, and by all means, use a cable release or the self-timer to trip the shutter.