A Polariser, How To Use It
Feb 10, 2010 Photography
What is a polariser? And how to use it? A polariser is an essential addition to any photographer’s kit bag. The filter may look like a simple piece of grey glass, but its effect can be far more impressive, adding extra punch and colour saturation to your images. The classic use for a polariser is darkening blue skies, but they can also reduce reflections and increase the overall colour saturation of your shots. The effect of a polariser varies as you rotate the filter, so most versions are circular and come in a mount that allows you to move the filter once it is fitted to the lens. Lee Filters produces a square polariser for its filter system, but the more useful type is a screw-in version that attaches to the front of the Lee Filters holder using a 105mm adapter ring.
Although almost all polarisers are circular in shape, they actually come in two types that are confusingly called linear and circular. This name describes the way that the filters work, rather than their shape, and which type you need depends on your camera. If your camera has autofocus or spot (or multi-segment) metering you need to use a circular polariser. For older manual models you can use the cheaper linear type, although you can also use the circular type and the effect is the same. So, if you use a digital SLR make sure you’re using a circular polariser to ensure that your camera will work properly!
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Tags: camera lens, lens, polariser
Choosing the Right Lens for Photography
May 27, 2009 Photography
Slowly, but surely, the worth of the fixed focal length lens (or prime lens) to the non-professional photographer is being eroded by the increasing perfection of zoom lens technology. The days when you packed your camera bag with trusty 28mm, 50mm, 85mm, 105mm, and even 180mm lenses are rapidly disappearing. Why bother when you can simply reach for something like Nikon’s all-purpose Nikkor 18-200mm DX VR zoom lens and know that you have both flexibility and optical speed at your fingertips?
The truth is that, while the range of focal lengths may be increasing within a single zoom lens, no all-purpose lens is ever likely to perform at the level of eqivalent prime lenses for its full range of focal lengths.
Zooms are engineered with a good deal more glass than is found in a prime lens, and because of this they “swallow” more light than the corresponding lens of the same focal length. Unless you are photographing under very bright light, the required increased shutter duration time of a zoom may make the difference between a good image, and one that is ruined by camera lens wobble.
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Tags: cameras, Digital Camera, lens
Photography - Understanding How Image Stabilizing Lenses Work
Sep 22, 2008 Photography
If you have not yet purchased an image stabilizing lens, you might nonetheless have heard the term “image stabilization” or “vibration reduction” bandied about in reference to various point-and-shoot cameras and SLR lenses. In that case you probably understand that IS or VR technology supposedly results in sharper images. But does the technology really work, and how well? What actually happens to make it work? In what situations does it not work at all? And most important of all, do you really need it?
The answer to the first question is yes, image stabilization technology not only works, it works well. If it did not, the remainder of this article would be entirely unnecessary. But it is useful to understand exactly what image stabilization is, and when it can be used to improve your photography before you shell out the extra few hundred dollars for a lens that has IS built into it.
Note: While the term “image stabilization” is used more frequently by writers when discussing this topic, “vibration reduction” is the term that Nikon uses to describe the same feature in its lenses, and it also happens to better describe what is going on. You should consider the two terms as interchangeable.
To understand how vibration reduction works, let’s consider for a moment the effect that it corrects for. When a camera is hand-held, and the shutter is open for a duration longer than the camera can be pointed in one direction without moving, then the resulting image is seen to be blurred. What happens is that as the camera lens tilts upwards, sideways, or a combination of the two, the focused image shifts position on the image sensor. A focused point of light ends up being rendered as a line. The accumulation of all the shifted points of light that make up an image results in a blurred image.
The obvious correction therefore would be to shift the image sensor by the same amount that each point of light is shifted, so the image and the image sensor move together in relative lock-step. Some camera manufacturers implement this exact solution, and float the image sensor so that it can track the image if the camera shifts during exposure. But this is the exception, rather than the norm. Generally the solution is achieved by adding the vibration reduction to the lens, rather than the camera body.
So how does this work? Actually it is fairly simple to understand. Instead of a floating image sensor, the lens body contains floating lens elements. The position of these lens elements is controlled by motors that are coupled to sensors which detect how much the lens body is rotated vertically or horizontally during exposure.
If the lens body rotates upward, the sensors compensate by telling the motors to drop the floating lens elements down a little to maintain the optical path of light traveling through the lens elements. If the lens body is rotated to the left, the lens elements shift to the right to compensate. The end result is that the image maintains its position on the image sensor during the exposure, creating a much sharper image than if the vibration reduction system had been deactivated (which it can be).
It might seem remarkable that the system can track camera shake so well. In fact, it is remarkable, but it all works because camera motion can be monitored about every 1/1000th of a second, while the camera wobble occurs on a time scale of, say, 1/30th, 1/15th, or even 1/4th of a second.
Vendors of these image stabilizing lenses claim that the technology allows you to gain about 3 to 4 stops on your exposures. This means that if, for a given photographic situation, the slowest hand-held shutter speed that consistently results in an OK image is 1/125th of a second, then by adding vibration reduction you ought to be able to shoot at 1/15th or perhaps even 1/8th of a second. This is a big deal if you are forced to shoot into shadow, or the light is fading, or you need to close down the aperture to improve depth of field.
But image stabilization only proves itself in a range of shutter speeds that are neither too great, nor too small. If you try to hand-hold a 1 second exposure it’s unlikely your VR compensation will be able to keep up with the erratic motions you supply it. Likewise, if you are shooting at 1/500th of a second or higher, the camera won’t have a chance to experience any wobbling, so the VR system will not add any improvement.
Another thing to remember about vibration reduction is that it has to do with compensating for camera motion during exposure. It has nothing to do with the speed of the object you are trying to photograph. So that fast-moving baseball will still be rendered as a blur unless you capture it at 1/500th of a second.
Do you need IS or VR lenses to improve your photography? This one is fairly easy to answer. If you regularly find yourself shooting at speeds less than 1/250th of a second then there is a very good chance you can rid yourself of a substantial number of those blurred, or unsharp, shots by investing in a good VR lens. Better yet, when you buy your next digital camera, simply go for a point-and-shoot with vibration reduction built in, or if you purchase a digital SLR, try to grab a model with vibration reduction built into the supplied kit lens.
Author: Stephen Carter
Tags: lens, Photography
