Wednesday, February 20, 2008

Extension Tubes verses Close-up Filters

A frequent question that seems to pop up a lot is “What’s better; extension tubes or close-up filters?” and the answer isn’t as simple as some of you might think…

The general theory is that an extension tube is going to give you a sharper image because it’s an air gap and, unlike a close-up filter, there is no additional glass. An extension tube magnifies an image by increasing the size of the image circle that a lens projects onto the sensor, so the only negative aspect is a decrease in light reaching the sensor and a dimmer image in the view finder. On the surface that seems like a logical answer, but it’s just not that simple…

Non-macro lenses are designed to be sharp when focused toward infinity, and not when focused at the minimum focusing distance of the lens. Adding an extension tube to a non-macro lens just makes the problem worse since any image softness at the minimum focusing distance is just getting magnified.

But a close-up filter, like the 500D, changes the way that a non-macro lens focuses and it will actually cause a lens to be just as sharp at minimum focus as it normally is at infinity –something that an extension tube won’t do. The effect is so profound that you can see the difference in image quality without having to view a photo at 100% pixels. So the answer to the “What’s better?” question really depends on what lens you’re using…

If you’re shooting with a macro lens then you could argue that the extension tube is better, since a macro lens is designed to be sharp at minimum focus and the tube is just air. But as long as it’s a high quality, dual element, close-up lens like the 500D the image quality is just as good as using a tube.

There is one other advantage to using a close-up filter over an extension tube: The filter magnifies the light before it enters the lens, so there is no light loss and having a brighter image in the view finder makes focusing the scene a lot easier…
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