How can I use extension tubes properly (tutorial anybody) ?

[davidbridges]

Hi all,

Im sure this is going to be a relevant topic with Jessops selling the Canon tubes for a great price of £29.99 but if you look around TP £22.

Anyway, in the set of 3 you get one that says 31mm another with 13mm and the other with 21mm and i am not too sure what these numbers mean. I have grasped the concept that if i stack em up i can get really close and the 13 mm not as close as the 21mm and the 21mm not as close at the 31mm ect ect.

Is there any rules to using these tubes effectively to actually know what the focus distance will be with each one on ect ect.

Help will be greatly appreciated, and stick any other questions in here !!

Dave

 

[malla1962]

This might help.
http://www.toledo-bend.us/index.shtml?ExtTube

 

[Canon Bob]

David,

There are equations to calculate mag and MFD for a given extension. The first part requires knowing the built in extension of the lens and this is often not published in the specs and takes some finding.
For you current gear I can give you the following;
50/1.8 0.15x at 450mm without tubes
0.39x with 12mm extension
0.68x withh 25mm extension

18-55 0.28x max at 920mm without tubes
0.81x with 12mm extension
0.92x with 25mm extension

Your 90-300 doesn't take tubes very well as it probably has very little internal extension. Native mag is 0.25x max and this will increase to 0.36x with 25mm of tubes.

Sorry that it's a quick reply...off to work shortly.

Bob