Hi JiMo.
I will answer the simple Q first: the 50mm
lens and Kenko tubes will work.
It is one way - and relatively cheap - to do some macro photography.
For the Q in your 2nd post, I'd say yes, a 90mm macro
lens would be a "generally" better solution to macro photography.
Now that means that it is a wide field and there are many solutions that can work - depending on what you want to do.
Firstly, I would point out that "macro photography" usually refers to recording images of objects at around 1:1. Basically, it means
getting a
lens to focus when it is held fairly close to the subject. Extension tubes "push" the
lens further away from the sensor so that
it can get closer to the subject, and a reversing ring allows a
lens to project an image onto the sensor from about the usual focusing distance
"behind" it, ie approximately from its focal plane.
Many
lenses, however, have a "macro" label on them but only allow image reproduction of about 1/2 or 1/3 (sometimes even less) of subject size.
Dedicated macro
lenses are (almost always) capable of 1:1 or better. They usually cost more than the combination you mentioned above.
In practical terms in the field, the use of extension tubes and reversing rings - THOUGH NOT BAD in themselves - can make for some awkward
photographic conditions, like getting enough
light onto your subject, giving only a very shallow
DOF (depth-of-field), and getting so close as to
affect your subject in other ways. Dedicated macro
lenses allow for some reasonable distance to the subject.
Now is that 55/3.5
lens a prime
lens? Ie, not a zoom? In the short term, you could potentially get some good practice with it and the cheaper
methods described.
Finally, the longer the focal length of a "macro
lens", the further it allows you to be from the subject, while still yielding (typically) 1:1 reproduction.
I hope this is not too confusing.
Am.