<PAGE>
                                   EXHIBIT 11
 
                            DUPONT PHOTOMASKS, INC.
 
                         EARNINGS PER SHARE COMPUTATION
 
                (DOLLARS IN THOUSANDS, EXCEPT PER SHARE AMOUNTS)
 
                                  (UNAUDITED)
 
<TABLE>
<CAPTION>
                                                                                         QUARTER ENDED SEPTEMBER
                                                                                                 30, 1996
                                                                                        --------------------------
                                                                                                         FULLY
                                                                                          PRIMARY       DILUTED
                                                                                        ------------  ------------
<S>                                                                                     <C>           <C>
Weighted average shares outstanding...................................................    15,100,000   15,100,000
Dilutive effect of stock performance plans............................................       246,912      345,651
                                                                                        ------------  ------------
                                                                                          15,346,912   15,445,651
                                                                                        ------------  ------------
                                                                                        ------------  ------------
Net income............................................................................  $      9,015   $    9,015
                                                                                        ------------  ------------
                                                                                        ------------  ------------
Earnings per share....................................................................  $       0.58   $     0.58
                                                                                        ------------  ------------
                                                                                        ------------  ------------
</TABLE>
 
<TABLE>
<CAPTION>
                                                                                         QUARTER ENDED SEPTEMBER
                                                                                                 30, 1997
                                                                                        --------------------------
                                                                                                         FULLY
                                                                                          PRIMARY       DILUTED
                                                                                        ------------  ------------
<S>                                                                                     <C>           <C>
Weighted average shares outstanding...................................................    15,129,611   15,129,611
Dilutive effect of stock performance plans............................................       555,379      656,158
                                                                                        ------------  ------------
                                                                                          15,684,990   15,785,769
                                                                                        ------------  ------------
                                                                                        ------------  ------------
Net income............................................................................  $      9,203   $    9,203
                                                                                        ------------  ------------
                                                                                        ------------  ------------
Earnings per share....................................................................  $       0.59   $     0.58
                                                                                        ------------  ------------
                                                                                        ------------  ------------
</TABLE>
 
         The accompanying notes are an integral part of this statement.