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Old Jun 9, 2006, 10:47 PM   #11
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Interesting article, good read.

You are right, another area of confusion brought by all the different sizes in digital.
Someday someone will get it all cleared up, I hope.

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Old Jun 9, 2006, 11:35 PM   #12
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This is from a site www.cameraontheroad.com

The basic rule for calculating reproduction ratio is that with the lens focused at infinity:

Reproduction Ratio=mm of Extension/Focal length

To show how this works, assume that you have a 50mm lens. In order to achieve a reproduction ratio of 1/2X life size, you need 25mm of extension, because 25mm of extension divided by 50mm of focal length equals 1/2X reproduction ratio.

If you want to achieve life-size magnification, or 1X, with the same lens, you need 50mm of extension. 50mm divided by 50mm equals 1X. This, again, is with the lens focused at infinity.

Lets take a more complicated example. The Nikon macro lens that we used for years was a 55mm lens that could focus to 1/2X magnification at its closest focusing distance. This means that the lens had built-in extension (notice that we've rearranged the equation algebraically) of:

mm of Extension
=(Rep. Ratio)(Focal length)



If we want to use this lens to achieve life-size magnification, how much extension do we have to add? We know that we need 55mm of extension to reach 1 to 1 (life-size, or 1X) with a 55mm lens. But the lens already has 27.5mm built-in. So we need to add another

(55mm - 27.5mm)=27.5mm of extension

How do you add extension? You use a device called an extension tube, which is little more than a hollow tube that mounts between the lens and the camera body. It pushes the lens away from the body, and allows the lens to focus closer. Nikon manufacturers an extension tube that is 27.5mm long, and it is called the PK-13.

With the PK-13 installed between the 55mm macro lens and the camera, and the focusing ring of the lens set at infinity, the reproduction ratio will be

Rep. Ratio
=(0mm from lens + 27.5mm from tube)
/ 55mm focal length

=1/2X life-size

If we then focus the lens to its closest focusing distance, we get

Rep. Ratio
=(27.5mm from lens + 27.5mm from tube)
/ 55mm focal length

=1X life-size

Any Nikon tube can be used on any Nikon lens to make it focus closer. Likewise, any Canon tube, or any Canon-compatible tube, can be used on any Canon lens to make it focus closer, and the same is true for any other brand of equipment. The entire secret is that when the lens is moved farther from the camera body, the lens will focus closer, and the reproduction ratio will increase.
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