Need some advise. I have a Tamron 90mm SP Macro lens on my D70 and I would like to have higher magification. Will adding an extension tube work well ? If yes, what kind of extension tube should I get ? Estimated price ?
Yes, that is about right. The problem is that if you add 90mm tube to a 90mm lens to get 1:1, or 180mm to get 2:1, 270mm to get 3:1 and so on, you will loose a lot of light. 270mm tube is a LOT OF TUBE! I believe the only way to do it is by using bellows. So, while a 90mm may be good for 1:1 magnifications, for larger magnifications a shorter lens, like 50mm is better because you will need less tubes and loose less light. For large magnifications, best is to use specific bellows lens and a macro bellows.
Here is a link to a post regarding loss of light that you can read if interested.
There is no IQ loss due to tubes, only due to the TC. The tube (and the TC) causes some light loss, but 25mm tube with that lens is just a 1.3 time loss. You have to add the TC also to get the total light loss.
BTW, 2:1 is at the near end, not at infinity. Right? The TC multiplies the focal length and at the same time keeps the nearest point that you can focus on.