I understand the general concept of using them. (Closer focus, suitable for macro) Anyways, i was wondering if anyone has used them and if they are worth purchasing. I found these on amazon. I was thinking of buying them but first i have some questions. I understand they let you focus close, but will it slow the lens down in the process. For example, i was planning on using my 20mm Panasonic with it and the fastest f stop is 1.7. What would the equivalent be with the extension tubes? Thank you.