Determining the correct coffee grind size is crucial for successful coffee preparation. If your coffee tastes too watery, bitter or dry or alternatively tastes too intense, sour or empty this can mostly be attributed to an incorrect grind size.
As you have already learned, there are many differences between filter and espresso coffee. One main difference is the contact time of the ground coffee and the water. For espresso this is about 30 seconds and for filter this can be several minutes. The aim is to dissolve the soluble flavours of the coffee grounds optimally and thus produce a flavor-balanced brew. For those who want to know exactly: 18 to 22% of the soluble flavours should be extracted from the coffee. This can be measured with a TDS reader which measures the Total Dissolved Solids of each brew.