[Tex/LaTex] Subscript spacing in equation

math-modespacingsubscripts

I have a document, where I want to use $\tau_{ff}$. If this is rendered it looks like a space is inserted between the two f's. $\tau_{rl}$ looks fine …

Where does this additional space come from and how can I avoid it being inserted?

(Miktex 2.9 on windows, if this is relevant).

$\tau_{ff}$ $\tau_{f\!f}$ $\tau_{\mathit{ff}}$

If the meaning of your subscript is some kind of invisible product of two f then use one of the first two, or some other negative space other than \! to taste. If on the other hand it is a multi-letter identifier with name ff then it is more appropriate to use a text font, as in the third example where the text italic font is used in math mode via \mathit and there ff produces a single ff ligature glyph.