Kenko extension tubes + sigma 70-200 f/2.8 = ???

[DocFrankenstein]

My math/physics is not very good. I can't calculate this on my own.

Sigma minimum focusing distance is 1.6 meters with any focal length.
12+20+36= 68mm of extension tubes

What magnification will I get if I stack up all of the kenko tubes on it?
What would be the min/max focusing distance on the lens?

Thank you.

[steven]

Magnification=(Extension/Focal Length of lens)
As for min/max focusing it think that depends on which lens, what it's non-extention focusing min is.

[Jon]

614 mm at 200 mm zoom, 139 mm at 70 mm using the standard optical formula 1/fl=1/i+1/o
where fl=focal length, i=image distance (total lens extension) o=object distance. That's about 1/2 life size at 200; life-size at 70.

Strictly speaking, i and o are measured from the lenses' back and forward nodal points; your lenses' focussing scale usually shows distance from the film plane; as you start working close in, or with retrofocus or telephoto lenses these will be markedly different. For calculation, I assumed the simple case where o for the lens without extension tubes was 1.6 m. In reality, o will be somewhat less (possibly, but not necessarily by the focal length plus actual displacement of the front element of the lens), so i will be somewhat larger and magnification will be somewhat larger.

[TomC]

614 mm at 200 mm zoom, 139 mm at 70 mm using the standard optical formula 1/fl=1i+1/o
where fl=focal length, i=image distance (total lens extension) o=object distance. That's about 1/2 life size at 200; life-size at 70.

.
Holy Crap! Wouldn't it be easier to just buy a longer lens? :D

[Scottes]

Holy Crap! Wouldn't it be easier to just buy a longer lens?
Yeah, I just stick them on and go. Jon's math made my head hurt. :)

[Jon]

Yeah, well, Masters in Geology with a Photointerpretation thesis, modifying the viewing system's optics as I went, doing technical photography for the department and multispectral aerial work for assorted clients to pay my way . . . Some things you never forget (like hanging out the window of a Cessna 172 at 750'/85 KIAS over Charlestown Pond in February - Brrrrrr!!, or the various lens formulae).

[Mogwyth]

Having just got a set of extension tubes for Christmas and already having the Sigma, this is useful info. Thanks

That is if I can get my head round your maths as my tubes are slightly shorter 65mm:confused:

[Jon]

Start by figuring the lens extension at closest focussing distance without the tubes. Then you can add the tube length in and reverse the calculation to get new closest focus. You could also set up a system of two equations (second equation in Doc's case would be: i+o=1700; i and o as above) to find out the real, actual i and o values to use. Or, since you've got the tubes and the lens you could just put them together and measure. :{)#

[Mogwyth]

Thanks for the help.

[eric1]

not sure about all that math, but with the two largest Kenko tubes attached, my 70-200 will close focus at 3 1/2 inches (70mm).

[leony]

I'm not sure what all the math is about. It only makes sense if you're doing chrome repro work for archives or evidence or cartography. If you're shooting digital, just look through your view finder. The digital image can be reproduced in any size you need. If you need to match sizes exactly, include a ruller in your shot.

Either that, or I'm totally lost.

[Jon]

The math will be useful in two cases:

You're trying to decide what tubes to get, given that everyone has their own idea about how long each tube in a set should be
You want to build a table so you can choose the optimal lens/tube combination for your macro work. Store it on your PDA for quick reference in the field.
If you've got a set of tubes, and you know you'll always use them with the one lens, then cut and try is the easiest. But if you do a lot of close-up work, having a set of pre-computed tables can be very handy. If you do go that route, include a column of calculations with your teleconverter in place, too.