diode and resistor in parallel with resistor

how do you calculate V$_{R3}$ in the circuit below?

V$_{in}$, all the resistor values and the type of the diode are known.

By simulating this circuit one can see that V$_{D}$ is increasing with higher values for R$_{3}$. How is that even possible?

• Did you know there is a circuit editor? You can use it when you post questions. – Voltage Spike Apr 5 '16 at 21:42
• Yes, i saw the circuit edtor. But the picture was already made at this point. Thanks for the remark, next time i will try the editor. – drak0 Apr 7 '16 at 19:05

For rough calculation and relatively high voltage $V_{IN} (\frac{R_3}{R_1+R_3})$ compared to $V_F$ you may get a pretty accurate answer without any iterations.