Photography
Longer Lenses for Fewer Dollars
By Ron Spomer
Getting good wildlife photos usually requires a long telephoto lens (see elsewhere on this site.) But there are inexpensive and innovative ways to “cheat the system.”
The traditional method is to extend the reach of a shorter lens by adding a tele-converter behind it. Converters, also called extenders (which aren’t too be confused with glassless extension tubes) are essentially elaborate magnifying glasses that enlarge the magnifying power of the lenses in front of them. A 2X teleconverter, for instance, behind a 300mm lens would boost it to the power of a 600mm, or from a magnifying factor of 6X to a whopping 12X. A 1.4X converter would multiply that same 300mm by a factor of 1.4, pushing its power to 420mm.
Whoo hooo! Free lunch!
Well, not quite free. A good teleconverter, meaning one specifically matched to your lens, will run around $500. Canon’s 2X EF Extender III for it L series of super-telephoto lenses is selling on discount for $474. As of this writing (12-26-2011,) its 1.4 Extender III goes for $469 at B&H Photo in New York. That’s not chump change, but it’s a lot less than a new 600mm lens!
Something else working against the teleconverter “free lunch” is light loss. Because these converters add additional glass and length to the light path, they reduce brightness the same amount they increase magnification. So the 2X converter knocks two f-stops or two shutter speeds off your exposure. The decreased light also compromises autofocus systems.
These days most camera bodies won’t autofocus with maximum apertures smaller than f/8. So unless you want to focus manually, you’d better not add a 2x teleconverter to any lens slower than f/4. Starting with an f/2.8 lens is an even better idea.
Here’s one more factor working against the free lunch: camera shake. When you magnify the view with a big telephoto, you simultaneously magnify any shaking going on. The old standard rule says that you can’t hand-hold any lens at a slower shutter speed than the reciprocal of its focal length without getting a blurry image due to camera motion. In other words, if your shutter speed is 1/250, your lens had better not be longer than 250mm. A 500mm lens should be hand held at 1/500 minimum. A few careful, steady shooters can shoot a speed slower, but many need at least one speed faster. At any rate, if you stick a 2X teleconverter behind your 300mm, you’ll need to increase your shutter speed to 600mm – unless you have another of technology’s wonderful advancements – image stabilization.
Many of today’s top cameras and/or lenses include image stabilization or shake/vibration reduction technology. Either A. a floating lens element within the lens is moved via electromagnets to counter shake or B. the camera sensor itself is moved. Both systems have the potential to let you handhold from 1 to 4 shutter speeds slower than normal. The advantages of a lens-based stabilization system are that autofocus works more accurately in low light and you see a stabilized image in the viewfinder of DSLRs. The disadvantage is that you have to buy it in each lens you want stabilized. With in-camera, sensor-stabilized cameras, images from all your regular lenses are stabilized. And if sensor stabilization technology improves, you only have to buy one new body instead of all new lenses. The disadvantages are that sensor stabilization doesn’t work as well with super telephotos. The viewfinder image jiggles and shakes along with your lens, too, but this has been the standard with non-stabilized lenses for decades, so who cares.
One sneaky way to get a “free” telephoto is to shoot with a high quality shorter lens using exceptionally good technique (to maximize sharpness and focus) and then enlarge the portion of the frame you want. You’re in effect “blowing up” the tiny critter in the middle. This works fairly well with minimum enlargement, but you do not gain the advantages of out-of-focus backgrounds and narrower fields of view. You may notice behind your subject more distracting items and elements than you would with a real
super telephoto.
Related to this enlarging technique is the use of a smaller than fullsize (35mm equivalent) sensor camera. Professional SLR cameras sport a sensor that is the same size (35mm corner to corner) as the old 35mm film (think Kodachrome slides.) Semi-pro and amateur cameras use slightly to significantly smaller sensors, some only 1/3 as large as a full-size sensor. These smaller sensor sizes are commonly referred to as crop factors. Because there is less sensor area on which a lens is projecting its image, some of the image edge is lost. You’re essentially seeing/recording only the center of the lens image. How much of the center depends on the crop factor. Nikon semi-pro camera sensors have a crop factor of 1.5. Canon’s are 1.6. This means a 300mm lens on a 1.5 crop factor sensor would “enlarge” or produce an image equivalent to that of a 450mm lens on a full-size sensor. On a 1.6 sensor it would equate to a 480mm lens. Just multiply lens focal length by the crop factor.
An additional advantage to a smaller sensor accrues because the center of most lenses projets the highest optical quality. You’re pulling the prime image from the center of the lens and “throwing away” the softer
edges.
Visual noise (the appearance of grain-like distortions in the image) increases with smaller sensors, but depth-of-field increases in a given lens size. These are just a few of the technical trade-offs that concern professionals. If you’re just a serious amateur and not competing for the cover of National Geographic, you should be more than happy with the results from semi-pro cameras and smaller sensors. I sell images made with 1.6 crop factor sensors all the time.
Now for the final tally: Start with a relatively short 300mm f/4 Canon lens with a stabilizer. Attach it to a EOS 7D body with its 1.6 crop factor. You end up with a 480 equivalent focal length, or nearly 10X magnification. Stick a 1.4X Extender behind that 300mm and it becomes a 420mm lens. Add in the 1.6X crop factor and it’s a 672mm super telephoto. Add a 2X Extender instead of the 1.4X and you’ll have the incredible reach of a 840mm equivalent super zoom.
Such power will take precise control and steady shooting techniques, but by using the image stabilization and fairly high ISO speeds, you should be able to get some great results. Work with a sturdy tripod for effective slow shutter speed shooting in low light.
In our next installment we’ll investigate methods for making a telephoto focus closer than normal, turning it into a unique macro lens.