How many extension tubes needed?
I have a Canon 12 mm extension tube (EF II) and a Canon 25 mm EF II on order. I like to shoot macro of coins and bugs. I've noticed an online classified ad in my area for another 12 and 25 (same tubes) for an amazing price. If I am going to be working with a 50mm lens, should I add to the 37mm tube length that I'll have? I could buy either the 12 or the 25. This deal won't last.
The longer the extension, the higher the magnification, but also the shorter the free working distance. At a certain point, you reach the highest magnification as the subject hits the glass. Even earlier than this, it becomes difficult to illuminate the subject. There is also adverse impact of extension on image quality, but it tends to be worst at wide apertures, which are less used for macro due to thin DoF.
You could probably use another 25mm with a 50mm lens. Bellows are more convenient to operate than long stacks of tubes though. If you want to play with high magnifications, you should get a reversal adapter. Extension also increases magnification with a reversed lens; at higher magnifications this tends to produce a bit more working distance and better quality. For higher magnifications, you could consider reversing a lens with a shorter FL.