I don't know how to calculate it either but why don't you just measure it?
Stick a ruler in the field of view and see how much you can see when it's in
focus. According to the eSIF am OM-1 will give you 97% of the image in the
viewfinder. When you can see (97% of 36mm) 35mm across the field of view
you're
at 1:1.
ps: I've gotten more pragmatic with increasing age. I'm reminded of a
conversation I had with an automotive engineer shortly after taking a drafting
course in high school. I commented that it must be extremely difficult to
compute the actual length of (for example) a copper tube fuel line running from
the fuel tank to the engine with only the drawing of a truck chassis to work
from. He smiled and said; "No, no, we don't do that. We wait until the first
chassis is built and then go down to the assembly line and install the fuel
line. Then we remove it, streth it out and measure it." That was one my first
head slapping experiences. I've had many more since.
Chuck Norcutt
Andrew Dacey wrote:
> Wow, an actual on-topic post that's not digital to boot, will wonders
> never cease?
>
> 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?
>
>>From my math, the 90 goes to 1:2 so that's 45mm of extension. 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. So does that mean I have a
> total extension of 78mm for a total reproduction ratio of 1:1.76?
>
> 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.
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