English: A macro bellows, even when fully retraced like in this photo, will introduce a lot of extension (in the case of this particular one, over 40 mm), resulting in a loss of tons of working distance while introducing a lot in terms of magnification.

If all you want to do is to give your M42 lens just a slight bump on its working distance/magnification, you can use a T-adapter in place of an extension ring.

Of course, a T-adapter costs a lot more (around $15) than a cheap set of 3 extension rings so it doesn't make sense to purchase one just for this purpose. But, if you're like me and into astrophotagraphy, there is a 99% chance that you already own a T-adapter, so this is a good thing to know.

An added bonus is that with a T-adapter, you get to have the threaded end, to which you'll screw your M42 lens, and your camera's native bayonet mount on the other end, making the attaching/releasing of the lens a breeze, compared to a straight-up M42 extension ring used in combination with an M42 adapter (especially in the case of the useful, but clunky, Pentax K mount M42 adapters).