What's the magnification ratio?

Say I have a Nikon 105mm micro that goes to 1:1, and I attach a Raynox DCR-250 (8-diopter) to the front.

 

What the mag ratio if my lens is at infinity? And what is it when I'm focused at 1:1?

 

And how did you figure out the answer?

 

Many thanks!

At infinity focus the combo magnification (m) ~= 105 / (1000/8) ~= 105/125 ~= 0.84x

 

At 1:1 focus heicoid extension, the Nikon 105 macro is a bit more complicated. It uses internal focus which reduces

the lens' focal length as well as increases its extension. Best is measure the magnification directly. Anything else is

just an guess. My guess is ~= 1+ 0.75 ~= 1.75x.

At full extension the 105mm Micro Nikkor becomes approximately 95mm FL.This makes it equivalent to about ten diopters. Combining that with an eight diopter lens gives you eighteen diopters on a 95mm extension. This will give you an almost 2:1 reproduction ratio! I wouldn't expect much in the way of image quality, however.

You get 1:1 magnification when a lens is at twice its focal length from the film, so the total extension of the

105 is 210mm at 1:1, and presumably that's all the extension that is available. We'll use that number in a

minute... However, if your lens is an internal focus design, it might be changing the focal length so the

formula might change (I'll continue anyway with the calculations for a non-IF 105mm.)<P>

 

====================================<P>

 

A 105mm lens is a 9.52 diopter lens by itself. Add 8 diopters to a 105mm lens, and the combination is 17.52

diopters, or about 57mm. So the magnification with the lens at "infinity" will be <B>(105-57)/57 = .84X</B><P>

 

At maximum extension (210mm) the magnification of the 105mm + DCR-250 will be <B>(210-57)/57 = 2.68X</B><P>

 

====================================<P>

 

The diopter power of a lens is equal to 1000 divided by the focal length. Diopter values can be added directly,

then divided into 1000 to get the FL of the combination.<P>

 

Magnification is equal to <B>(Total Extension - Focal Length) divided by Focal Length.</B> For a combination of

lenses, use the focal length of the combination in the calculation.<P>

 

As I noted before, if your internal focus lens is actually changing the focal length of the lens, you'd need to

know that change and plug in the new diopter value of the lens when calculating at maximum magnification.<P>

magnifiction ratio. none of the above.

 

1:1 means that that the size of the subject(object), actually measured on the film or sensor with a measuring device(ruler, in inches mm or ?) is equal to the actual physical size of the subject measured on the same same scale. if you have a picture of a dime on film or sensor, and you actually measure the physical size of the image and it it is physically 1/2 the size of the real dime then you have a 1:2 magnification ratio.

Alan D., thank you. I think you nailed it.

You're welcome. The numbers never lie. Sometimes I get lucky and use the right ones....