Perhaps an example of what you are doing would help. The default display form for machine precision numbers is 6 places, but the remaining digits are still there. If you are doing a computation with both high precision numbers and machine precision numbers then the result will generally be machine precision unless there are in fact no machine precision numbers... and even then, depending on the calculation, the resulting precision may be less than that of the input numbers. E.g. (note the use of FullForm to show how many digints are actually there.
In[4]:= N[Pi, 30] + N[Pi] // FullForm
Out[4]//FullForm=6.283185307179586`
In spite of using a 30 digit precisin, when combined with a machine precision number the result is Machine precision
Without the FullForm only 6 digits are shown
In[5]:= N[Pi, 30] + N[Pi]
Out[5]= 6.28319