1 INTRODUCTION  
2 DATA  
2.1 The STEP survey  
2.2 Building the star cluster sample  
3 METHOD  
3.1 Star cluster centre estimation  
3.2 Surface brightness determination  
4 FITTING PROCEDURE  
4.1 EFF model  
4.2 Comparison with literature  
5 ANALYSIS OF THE CMD  
6 RESULTS  
7 DISCUSSION  
8 CONCLUSIONS