How do I calculate magnification with macro extension tubes?

[sygyzy]

Hi,

I just ordered a set of 3 Kenko extension tubes for my Canon 40D. I read something once where this person explained that with a 50mm lens (used as an example), and the x mm + y mm extensions, you could get a nearly 1:1 mag. I don't understand this math.

Could someone explain this to me?

 

[troytime]

magnification ratio = extension mm / focal length
so 50mm of exension will give you 1:1

basically by increasing the distance that the light travels between the lens and the sensor, the light spreads more and the sensor picks up a smaller portion of the original light array (i think)
but this also means that you essentially lose a few f-stops because the lens element and aperture blades are further away from the sensor

you can ALSO reverse that 50mm directly onto the camera OR reverse it on top of another lens (in which case the magnification ratio is main focal length / reverse stacked focal length)

Let us know how the extension tubes work out...i've been torn between the kenko tubes and the phoenix 100mm macro lens (and being poor)
i've tried reversing a 50mm and using closeup filters already...but want something better

 

[sygyzy]

Well I picked up the set b/c practically every recommends them. What they don't tell you is they are $169. I got them eBay though, from a seller in HK so $140 shipped. But still. It's not cheap!

 

[ghostman]

I got Pro Optic extension tubes. The construction is probably not as sturdy as Canon or Kenko tubes, but they work and I couldn't bring myself to shell out that much money on a handful of hollow tubes.

http://www.adorama.com/MCAETEO...l?sid=1217623646458299

Let me know what you think of the Kenko tubes. A slip of paper that came with my Pro Optic tubes indicated that I shouldn't use AF with all three tubes stacked. In reality, with my Tamron 28-75, I find AF with only two stacked tubes to be rather frustrating. Perhaps it's due to my lens, but I find myself using MF quite a bit.

 

[fuzzybabybunny]

Originally posted by: ghostman
I got Pro Optic extension tubes. The construction is probably not as sturdy as Canon or Kenko tubes, but they work and I couldn't bring myself to shell out that much money on a handful of hollow tubes.

http://www.adorama.com/MCAETEO...l?sid=1217623646458299

Let me know what you think of the Kenko tubes. A slip of paper that came with my Pro Optic tubes indicated that I shouldn't use AF with all three tubes stacked. In reality, with my Tamron 28-75, I find AF with only two stacked tubes to be rather frustrating. Perhaps it's due to my lens, but I find myself using MF quite a bit.

I have the ProOptics as well. The only thing that I don't like about them is that sometimes there's too much play between the tubes, meaning that sometimes the electronic contacts don't touch fully and the camera throws an error message back to you. I've only experienced this on long, heavier lenses.

As for auto focus, I wouldn't worry about it too much. The DOF is so shallow that I find moving the entire camera in or out is the best way to focus.