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I have a Nikon 4T close up lens to attach to a Nikon 120mm large format macro lens. The 4T is 2.9 diopter / magnification factor 0.11.

The usual equation is $${1\over \text{focal length}} = {1\over \text{object distance}} + {1\over \text{image distance}}$$

How does this calculation change with the addition of the magnifying lens?

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We mount a close-up lens in order to image closer than close focus of the camera allows. If a close-up lens is mounted, and the camera set to infinity, the plane of sharp focus will be focal length of the close-up lens. In this case, the close-up lens has a power of 2.9d. converting to millimeters, 1/2.9 x 1000 = 345mm. In other words, if the camera is set to infinity focus, objects 345mm from the front surface of the close-up lens will image in sharp focus.

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I stumbled over the answer:

First, convert lens focal length to diopter:

1000/120 = 8.33

Then add in the diopter of the magnifier - resulting in 11.23.

Now convert diopter back to focal length:

1000/11.23 = 89.02
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