<PAGE>

                                                                      EXHIBIT 11
 
                            DUPONT PHOTOMASKS, INC.
 
                         EARNINGS PER SHARE COMPUTATION
 
                (DOLLARS IN THOUSANDS, EXCEPT PER SHARE AMOUNTS)
                                  (UNAUDITED)
<TABLE>
<CAPTION>
                                                                                              QUARTER ENDED 
                                                                                             DECEMBER 31, 1996
                                                                                        --------------------------
                                                                                           BASIC        DILUTED
                                                                                        ------------  ------------
<S>                                                                                     <C>           <C>
Weighted average shares outstanding...................................................    15,100,000    15,100,000
Dilutive effect of stock performance plans............................................                     439,182
                                                                                        ------------  ------------
                                                                                          15,100,000    15,539,182
                                                                                        ------------  ------------
                                                                                        ------------  ------------
Net income............................................................................  $      9,433  $      9,433
                                                                                        ------------  ------------
                                                                                        ------------  ------------
Earnings per share....................................................................  $       0.62  $       0.61
                                                                                        ------------  ------------
                                                                                        ------------  ------------
 
<CAPTION>
 
                                                                                               QUARTER ENDED 
                                                                                             DECEMBER 31, 1997
                                                                                        --------------------------
                                                                                           BASIC        DILUTED
                                                                                        ------------  ------------
<S>                                                                                     <C>           <C>
Weighted average shares outstanding...................................................    15,165,132    15,165,132
Dilutive effect of stock performance plans............................................                     440,227
                                                                                        ------------  ------------
                                                                                          15,165,132    15,605,359
                                                                                        ------------  ------------
                                                                                        ------------  ------------
Net income............................................................................  $      8,261  $      8,261
                                                                                        ------------  ------------
                                                                                        ------------  ------------
Earnings per share....................................................................  $       0.54  $       0.53
                                                                                        ------------  ------------
                                                                                        ------------  ------------
<CAPTION>
 
                                                                                              SIX MONTHS ENDED 
                                                                                             DECEMBER 31, 1996
                                                                                        --------------------------
                                                                                           BASIC        DILUTED
                                                                                        ------------  ------------
<S>                                                                                     <C>           <C>
Weighted average shares outstanding...................................................    15,100,000    15,100,000
Dilutive effect of stock performance plans............................................                     343,047
                                                                                        ------------  ------------
                                                                                          15,100,000    15,443,047
                                                                                        ------------  ------------
                                                                                        ------------  ------------
Net income............................................................................  $     18,448  $     18,448
                                                                                        ------------  ------------
                                                                                        ------------  ------------
Earnings per share....................................................................  $       1.22  $       1.19
                                                                                        ------------  ------------
                                                                                        ------------  ------------
<PAGE>
<CAPTION>
                                                                                              SIX MONTHS ENDED 
                                                                                             DECEMBER 31, 1997
                                                                                        --------------------------
                                                                                           BASIC        DILUTED
                                                                                        ------------  ------------
<S>                                                                                     <C>           <C>
Weighted average shares outstanding...................................................    15,165,132    15,165,132
Dilutive effect of stock performance plans............................................                     497,525
                                                                                        ------------  ------------
                                                                                          15,165,132    15,662,657
                                                                                        ------------  ------------
                                                                                        ------------  ------------
Net income............................................................................  $     17,464  $     17,464
                                                                                        ------------  ------------
                                                                                        ------------  ------------
Earnings per share....................................................................  $       1.15  $       1.12
                                                                                        ------------  ------------
                                                                                        ------------  ------------
</TABLE>