SLR: The CAT did it

How to buy a used mirror lens

1-Call photo stores and ask for the Used Equipment department. Inquire about the availability of mirror lenses in the 450-600mm range.

2-If possible, go to the store with your camera body to examine lenses yourself. You can probably try lenses right there and maybe even shoot some test pictures at the store.

3-If an in-person visit isn't possible, ask the salesperson to describe lens by phone: indicate its size, weight, close-focusing capability, price, warranty, and shipping and handling costs. Check on the price of a T-mount for your camera. Make sure you can get a money-back guarantee.

4-When you receive the lens and T-mount, attach them together and then to your camera body. If lens scales are not at top, loosen three T-mount screws with a jeweler's screwdriver. Turn lens so scales are upright on camera, tighten screws.

5-Make a quick, simple lens test: On a bright day, load camera with ISO 400 print film. Find a set of buildings or a scenic with details at infinity that appear at the center and edges in your viewfinder. Place camera and lens on a suitable tripod or other support. Focus carefully on subject using camera focusing screen. Select aperture-priority autoexposure. Ask your photofinisher for an 8x12 enlargement. Examine sharpness at center and edges. If the sharpness pleases you, keep the lens.

6-Alternatively, make a more precise lens test: Load up with ISO 100-200 fine-grained slide film. Find a detailed building façade at least 25 feet away that is parallel to your tripod-mounted camera's film plane. Shoot as in 5. Now, change lens and mount one of your non-mirror lenses that you know to be sharp. Approach the building until you frame the same view as the one you shot with the mirror lens. Make an aperture-priority exposure at f/8. When slides are processed, compare results using a 10X or greater magnifier. Quality of mirror lens photo should approach picture made with your non-mirror lens. But don't expect the mirror lens picture to equal it.

Eight tips for using mirror lenses

1. At close range, stationary subjects are easiest to shoot. My bird remained perched long enough for me to focus carefully; the matador stood stock still waiting for the charge of the bull; the lion and I stared at each other for quite a while.

2. To handhold lens steady, rest lens in left palm. Hold camera to your eye. Draw arms and elbows into body as far as possible. Press shutter release very gently.

3. Buy a beanbag. When using a tripod is inconvenient (which it often is, since handholding a lens gives you so much more fun and freedom), try resting your lens on a beanbag if you can. Most camera stores have or can order them. Plastic pellet-filled bags are lightest and best. Figure on spending under $10.

4. While the old rule of thumb calls for you to use the reciprocal of the focal length as the slowest shutter speed (1/500 sec for a 500mm lens), if you practice, you'll be able to shoot at 1/125 or 1/250 sec. With bracing (see bird and lion photo captions), even 1/30 sec is possible.

5. For a fairly accurate check of your lens' light transmission, aim lens on camera at a clear blue sky, an evenly lit wall, or place directly on top of light table. Check the shutter speed reading using aperture-priority mode. Replace mirror lens with one of your regular lenses and make the same reading at f/8. The difference in the shutter speed indicates the approximate true light transmission loss. For instance, if your mirror lens gives you a 1/60 sec reading but your regular lens shows 1/125 sec, that's a loss of a full stop. So that mirror lens would have a light transmission close to T/11. Mirror lenses can vary from 1⁄2-to 1 1⁄2-stops of light loss.

6. While many mirror lenses may have light transmissions less than the f-stop markings, conversely the focal length is often greater than marked. While there is no easy, precise way to check true focal length, you can find if there is a large discrepancy between actual focal length and marked focal length. While in a camera store, ask to check out a 500mm tele or zoom set to your mirror lens' focal length. View the same focused area with both lenses. Note how much greater the mirror lens image probably is. Some cheap mirror lenses may indeed have shorter focal lengths than marked. This method lets you check it.

7. The focusing ring of your mirror lens will probably revolve further than the infinity or closest-focusing markings on the lens barrel. Worry not. This is merely an allowance for the lens barrel's expansion or contraction due to temperature fluctuations.

8. For shots within range, don't forget to use flash when possible to freeze action and minimize the effects of camera shake. But make sure your flash beam clears the barrel of the lens.