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Zoom lenses are an aspect of photography that even casual photographers are familiar with; though they originally lagged behind their fixed focal length counterparts, technology has progressed to the point that they are at the least very close to, if not quite even with, fixed length lenses. As the technology has increased, zoom lenses of increasingly large magnification (the ratio of maximum to minimum focal length) have become popular- Olympus offers a DSLR lens with a 140-600mm range in 35mm equivalence, and Nikon’s Coolpix P80 prosumer digital camera offers a 18x zoom factor! Despite their versatility, these lenses do suffer from a number of associated complications, though many of these can be mitigated with a little knowledge and caution.
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As you zoom in towards the subject, the maximum aperture decreases; while this decrease gives a commensurate increase in depth of field, depth of field also happens to decrease with increasing focal length. (The reason for depth of field, in short, is that cameras can only focus at a single distance. Outside of that distance, points are actually small circles, but, as long as they are smaller than the acceptable circle of confusion, the human eye is unable to tell the difference between a point and a circle. The result is a range of distances at which a photograph appears to be in focus.) While an increase in depth of field can be useful, for instance, when you're trying to compensate for that loss due to zoom or get as much of your picture crisp as possible, the flipside is that the reduction in aperture decreases the amount of light reaching the sensor.
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This narrowing of aperture has an additional result- as a rule, long zooms will be more restricted by lighting than a wide-angle lens or even a “short zoom”. This becomes a serious concern when using a long zoom lens at high magnification, especially when doing so without a tripod. The end result is that the photographer is forced to use slower shutter speeds than might be desirable, resulting in blurring of any action (and sh ake-induced blurring, mentioned below). Flash may be an option under some circumstances, but in others (like landscape photography, for instance), it unfortunately isn’t.
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Trying to hold your camera perfectly steady can be trying, to say the very least. Many manufacturers now include image stabilization as a standard feature, but even this technology has trouble compensating at high magnifications, since a little simple trigonometry can show that a slight movement significantly alters the center point of a photograph when at extremely high magnification. This would usually be compensated for via the use of higher shutter speeds, but the already narrow aperture makes light too precious of a commodity to rely on high shutter speeds to always eliminate vibration-induced blurring.
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Thankfully, there’s an (occasionally inconvenient) method to counter these issues: use a monopod or tripod! Some models are very cheap (though you generally get what you pay for), and monopods are easier to get around with. They’re certainly worth the cost and trouble of having a picture ruined by insufficient exposure or camera shake! If you're lacking one of these, you can always resort to bracing against any object in your vicinity. Taking your photos in RAW can help too, as that allows for greater image post-processing to counteract exposure problems. Also, be sure to investigate your image stabilization options- if you can get it, it's definitely worth it, and that goes double for long range shooting!