Focal Length versus Magnificatio Ratio

Started Dec 16, 2009 | Discussions thread
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JasonOdell Senior Member • Posts: 2,282
Focal Length versus Magnificatio Ratio

All this hubbub over "loss of focal length" got me thinking about the actual matter, which is maximum reproduction ratio (magnification).

Consider Nikon's line of 200mm lenses and their reproduction ratios:

200/2 Ai-S-- 1:9.5
200/2 AFS VR-- 1:8.1
200/4 Ai-S--- 1: 7.4
200/4 D Micro-- 1:1

So, here are four different "200mm" lenses, each with a different magnification ratio at its closest focus. Based on the stuff I've been reading here lately, you'd think that each had a different "focal length" at MFD! Does the 200/2 Ai-S stop being a 200mm lens because it only offers 1:9.5 instead of the 1:8.1 of the 200/2 VR?

Given that a 60mm micro Nikkor can focus to 1:1 and a 200mm micro Nikkor can focus to 1:1, but each has a different MFD, is it any wonder why focal length of a lens is calculated at infinity and not at MFD?

So maybe instead of ranting about "loss of focal length," we can just realize that there are differences in maximum reproduction ratio between different lenses and leave it at that.


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