Which extension tube for what level of magnification?

[dima1109]

I currently have a Sigma 105mm macro lens, and I was looking at getting some cheap tubes (Kenko I guess). I don't quite have the money for a full set, so I may have to buy just one or two, look out for good deals. My question is: how can I calculate the magnification level depending on the length of the tube?

[troypiggo]

Get as long as you can afford to give the biggest magnification. 68mm (full set of 12+20+36mm) with your lens will give around 2:1 mag. I believe the calculation is crop factor times length of tubes divided by focal length of lens. What's your budget? Kenko full set is only around US$130 new I think.

[brecklundin]

I found this thread using the search terms "calculate magnification tubes":

http://photography-on-the.net/forum/showthread.php?t=594936&highlight=calculate+magnification+tubes

[troypiggo]

Aah, yes. That's the one I was looking for but couldn't find it.

[LordV]

Magnification for a macro lens with tubes = existing mag + length of tubes/focal length of lens.
so for a 105mm with a full set of tubes it should be 1 + 68/105.

There is a large error in this calculation mainly caused by the focal length of macro lenses shortening at min focus, so a 105mm lens behaves more like a 75mm lens and you actually get nearly 2:1 mag.

Troy - magnification calculation is nothing to do with crop factor - it's a property of the lens not the camera it's attached to. :)
Brian V.

[troypiggo]

Sorry if I posted mis-information. Got it all jumbled relying on my memory when I couldn't find the formulae brecklundin found.

[LordV]

Sorry if I posted mis-information. Got it all jumbled relying on my memory when I couldn't find the formulae brecklundin found.

No probs- I always think the magnification definition used is mis-leading anyway - would be much more informative to know what the print magnification is.

eg a P&S camera with a 0.3:1 lens actually gives the same print mag as a 1.6 crop DSLR with a 1:1 lens.

Brian V.

[brecklundin]

Magnification for a macro lens with tubes = existing mag + length of tubes/focal length of lens.
so for a 105mm with a full set of tubes it should be 1 + 68/105.

There is a large error in this calculation mainly caused by the focal length of macro lenses shortening at min focus, so a 105mm lens behaves more like a 75mm lens and you actually get nearly 2:1 mag.

Troy - magnification calculation is nothing to do with crop factor - it's a property of the lens not the camera it's attached to. :)
Brian V.

Cool Brian,

that means my 180mm w/my 65mm of tubes (el cheapo generic non-electronic $12 set) has a mag factor of 1 + 65/180 = 1.36x....yes? Of course assuming I have the lens set to 1:1 at the time. Maybe a better way to say it is that I would add 0.36 to whatever the current mag setting might be?

I thought I read somewhere some internal focus macro lenses actually change focal length even though they don't change length when focusing? Or is that just IF zooms with the pseudo-macro mode?

[LordV]

Cool Brian,

that means my 180mm w/my 65mm of tubes (el cheapo generic non-electronic $12 set) has a mag factor of 1 + 65/180 = 1.36x....yes? Of course assuming I have the lens set to 1:1 at the time. Maybe a better way to say it is that I would add 0.36 to whatever the current mag setting might be?

I thought I read somewhere some internal focus macro lenses actually change focal length even though they don't change length when focusing? Or is that just IF zooms with the pseudo-macro mode?


You will actually get a bit more than that - the 180 at min focus has a focal length around 120mm so 1 + 65/120
and that will be a slight underestimate. The focal length change at min focus seems to happen no matter whether the lens is internally focusing or not. The focal length at min focus approximates to the min focus distance/4

Brian v.

[dima1109]

Thanks for the replies :)

I'm assuming with a reasonable amount of extension tubing, getting beyond 2:1 would be pretty cumbersome.

[LordV]

Thanks for the replies :)

I'm assuming with a reasonable amount of extension tubing, getting beyond 2:1 would be pretty cumbersome.

Might be worth thinking about a canon 250D diopter filter - that would give you about 1.7:1 without adding much weight and easy to put on as well.
Brian v.

[dima1109]

Might be worth thinking about a canon 250D diopter filter - that would give you about 1.7:1 without adding much weight and easy to put on as well.
Brian v.

Would that be the same as getting a Raynox DCR-250? It's almost half the price of the Canon lens.

[LordV]

Would that be the same as getting a Raynox DCR-250? It's almost half the price of the Canon lens.

Sort of but I suspect you would get vignetting with the Raynox.
Brian v.

[brecklundin]

this sure turned out to be a very informative thread...thanks...I know I am adding an ef-s 60mm next spring and now know for sure I want to add an mp-e 65 later next year and not worry about using tubes to try and get more than 1:1...for now the 1.3x to 1.5x-ish on my I get with the 65mm of tubes will serve me well enough when I am in that mood.

Thanks so much to the OP for starting this thread...

[r.morales]

Thanks - mainly I posted to watch this thread .