# How do objectives magnify so much over such short distances

According to the thin lens equation $$\frac{1}{f} = \frac{1}{a} + \frac{1}{b}$$ and magnification is given as $$M = \frac{b}{a}$$ where b is image and a is object distance. So in order to get $M > 1$ we need $b>a$ and $a>f$ (for convex lens) to get a real image. My question is how do modern objectives achieve high magnifications of 20x and more on such small distance? Is it just lining up enough lenses to get a small enough effective $f$ or is there another principle behind it that makes it possible?