Extension tube in Macro

<1>

The main issue with using extension tubes with a normal lens is that you are using them just to get to a 1:1 magnification. Most standard lenses are a 1:4 or even 1:5 max magnification. Where as most zooms that have the word "macro" attached to their designation or a 1:3.5 or 1:2 at best. Where as a true macro lens is a 1:1 ratio(meaning life size in the frame).

What extension tubes due is allow the lens to focus closer to the subject hence allowing more magnification and/or filling the frame with the subject.

The best choice is to use extension tubes + macro lens to achieve greater then 1:1 magnification ratios. Or you can use them with zoom macros to achieve a true 1:1 ratio. They work best with primes however. And with a non-macro prime your benefit would be more of the subject filling the frame. The only cost of using them is light. For example, if using them you will need more light for a proper exposure then without them. I forget the exact amount of loss they cause off hand, sorry.

I always have at least 2 sets of them on hand to use. They come in handy, even with a true 1:1 macro lens, in order to get smaller stones to fill the frame better. I use them with both lenses and even with my bellows.

Here is my old setup when I had my Canon 20D, Novoflex long bellows, Novoflex macro rail slide, Stroboframe bracket + adjustable flash mount + Sigma DG 500 Super flash + 68mm stacked extension tubes + Pentax Takumar 135mm f/3.5 lens:
http://www.pbase.com/digitaldevo/image/58582504

The following shots taken with that setup:
This one shows piece of tumble polished agate with battery for size reference and then the top half is the magnified 100% crop
http://www.pbase.com/digitaldevo/image/68030641

This is a simple grain of sea salt used in a salt shaker for food, lol
http://www.pbase.com/digitaldevo/image/61457417

I finally got about 90% of my equipment unpacked now from the move and almost have the studio finished as well. Held up currently by bad weather which makes my RSD /CRPS flare up and keeps me bed ridden, sadly. But as soon as this crap passes I will gladly take some images with and without ETs(extension tubes) to show the differences if you like?

They are the cheapest method of gaining magnification without causing image degradation.

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<2>

Panasonic solution for FZ35:  Panasonic extension tube DMW-LA5 + DMW-LC55 close-up lens.  May get more magnification but quality is probably worse than close up lens attached to (directly or through a thin adapter) the main lens.