Tandemfahren
Hello everyone!
A quick question for the maths experts:
How do I calculate the extension factor if I fit a macro ring BETWEEN the lens and the teleconverter?
To explain: the 2x extender for the RB (naturally) increases the minimum focusing distance. Because it has bellows focusing – i.e. behind the lens and, if applicable, also behind the extender. Inserting a 45 mm ring in between sorts everything out, but how am I now supposed to calculate the exposure – scratching my head...
I could, of course, ‘simulate’ the whole thing with a DSLR, but then I still wouldn’t have understood it, and that’s what I’m after.
By the way: it’s a proper beast; it wouldn’t look out of place on a tank! RB with bellows extended + extender + ring + 360mm telephoto + lens hood!
Cheers in advance
Frank
cfb_de
Hello Frank,
The magnification factor is calculated from the extension and focal length. In your case, the extension is 45 mm and the focal length is 720 mm. If my calculations are correct, the magnification factor works out to be 0.004.
I rather doubt that you can adjust the aperture on RB lenses in 0.004-step increments...
Best regards,
Franz
P.S.: My calculation applies to the assembly sequence: lens, extender, extension tube, camera.
Tandemfahren
Hi Franz,
Good point...
But I’m familiar with the formula. The question was, does anything change if I swap the order around?
The viewfinder image is quite dark. With both combinations.
Admittedly, with the 360, the extension doesn’t really matter, but I’d still like to get behind it. It’s a bit of a defiant reaction.
But thanks for now!
Frank
cfb_de
Hi Frank,
I haven’t really given much thought yet to using an extension ring between the lens and the teleconverter.
Best regards,
Franz
Tandemfahren
Hi Franz,
Not that it matters, but you actually didn’t do the maths quite right there. In the formula, ‘extract’ naturally refers to the total extract, not just its extension. That’s the only way you get a factor of 1 for the infinite series and a factor of 4 for the 1:1 ratio.
It doesn’t help me much, though.
Best regards,
Frank
Wolfgg
Hello Frank,
but the following approach might work: you calculate the total magnification factor in several steps:
1) Consider just the lens and close-up tube; this gives the magnification factor V1.
2) The converter always has a fixed extension factor V2; after all, it only magnifies the image behind the lens, regardless of whether a close-up tube is used or not.
3) The total extension factor is V1*V2.
Regards, Wolfgang
Tandemfahren
Hi Wolfgang,
That sounds logical, so I'll give it a go.
There's also an 82mm tube, which makes things even more interesting...
Thanks
Frank
