What length of extention tubes do i need...?

J

James Greed

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...in order to turn the sigma 17-70 macro into a 1:1 ratio at 70 mm? is there some sort of calculation i can do to work it out?
 
Not really. But you can just hold the lens in front of the camera body and look through it. If your sensor was say 20mm wide and 20mm on a ruler filled the full width of the viewfinder you will have 1:1. Then look to see how far away the back of the lens is from the body :)
 
I think you'd need your extentions tubes mm to be the same as your focal length to get a 1:1 magnification focused at infinity.
 
It is possible to calculate the length of extension tubes required. It's a bit late now but if you let me know the exact model of your 17-70 then I'll try to give you some idea tomorrow.
The basics are the internal focus distance of the lens and the focal length. The calculation will only be valid to get 1:1 at the minimum focus distance.
Bob
 
I've not got the lens yet, but am thinking about getting the sigma 17-70. Minimum focus distance is 20 cm so as standard it allows a ratio of something like 1:2.3.
 
Okay, my perception may be flawed and the info on Sigma's website may have rounding errors but I believe that you'll need 41mm of tube behind the lens.
Be aware the you'll lose the ability to focus to infinity and that this value is for the lens at full zoom (70mm)

Bob
 
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