Andrew Dacey wrote:
>Wow, an actual on-topic post that's not digital to boot, will wonders
>never cease?
>
>
Probably not.
>I'm wondering about how to go about calculating the reproduction ratio
>when combining a macro lens with a teleconverter.
>
>Specifically, I have a Tamron 90/2.5 macro (the 2nd version) which
>goes to 1:2 on it's own. If I attach my Vivitar macro focusing
>teleconverter that would take me to 1:1 (it's a 2x converter) right?
>But, then I can also focus the converter to get extra extension. How
>do I calculate what the total ratio would be? Do I calculate the total
>extension and then multiply by 2 or is there another way to work this
>out?
>
>
I don't think one can work this out with any great accuracy without
having the optical formulae of both lenses and a high powered optical
calculation program.
>>From my math, the 90 goes to 1:2 so that's 45mm of extension.
>
Should be. Just for fun, I just measured the 1:1 converter extension
tube made specifically for the SP 90/2.5 and it provides 43.4mm of
extension. The formulas for simple lenses aren't exact for more complex
ones.
But with the telextender, it isn't just simple extension. There is some
extension and there are additional optics and focusing is happening
through movement of the front set of optics relative to the rear ones,
not through simple extension of the whole optical system.
>The converter goes to 1:1.5 with a 50mm at infinity so that means you get 33mm
>of extension if my math is right.
>
The total physical extension is 17.4mm, but it is between the
telextender and the lens, so who knows what it means?
>So does that mean I have a total extension of 78mm for a total reproduction
>ratio of 1:1.76?
>
>
Almost certainly not. First, you have the ratio wrong. 1:1.76 is not as
close as 1:1, which the lens/MFCT combo already doess without the
extention in the MFCT. The instructions for the Viv MFCT provide a few
examples. One is for their 28-90 zoom. At 90 mm, closest focus and full
MFCT extention, the repro ratio is given as 1:1.7. Since that lens at 90
mm certainly doesn't focus as close as the 90/2.5 without converter, we
can be sure your combo will be better than that. At a wild guess, I'd
say it might go to 1:.7. As Chuck proposed, you can get a pretty good
idea by simple observation.
>PS, yes I know you're not supposed to use decimals in a ratio but it's
>more clear than saying the ratio is 25:44.
>
>
Lens manufacturers do it all the time.
Moose
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