I think some people just take a set of ext tubes to use if they are going out with say just a long length focal lens,and do not want to take a second lens,eg macro.I do have a set of the Kenko tubes,have only used once,with a 100-400.Not bad,they did auto focus quite well.You can take a macro type shot further away from your subject,eg butterfly,thereby less likley to scare the subject away.
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