For those of you that like the short version, I'll state the question first. How would one calculate the magnification ratio of a non-macro zoom lens with a diopter attachment at any given focal length?

I have been experimenting with macro photography recently by using a Raynox DCR-150 on my DA 55-300 Zoom It is a diopter of 4.8 and seems to make some pretty good images.

I've been considering stepping up and buying one of the various prime macro lenses, either the Tamron AF90 or the Pentax D FA 100mm, both of which has a 1:1 ratio and a minimum focus distance of about 30cm.

All of this thinking brought a question to mind and I haven't been able to find any info about it, so I thought I'd throw it out here for some discussion.

I have read that you can calculate the minimum focus distance of a lens with a diopter adjustment by dividing 100mm by the diopter value. This seems to be the case for the Raynox because the diopter on that is 4.8 and everything wants to be at around 20cm to be in focus. So the question is how would one calculate the magnification ratio of the lens at each focal length of the zoom, say at 100mm, 200mm and 300mm? I suspect it is less than 1:1 especially at the long end, but I'd like to understand how to calculate it.

Someone here has to know, right?

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Last edited by GregK8; 03-16-2010 at 12:21 PM.
Reason: calrify language
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