Context : Difference imaging has proven to be a powerful technique for detecting and monitoring the variability of unresolved stellar sources in M 31 .
Using this technique in surveys of galaxies outside the Local Group could have many interesting applications .

Aims : The goal of this paper is to test difference imaging photometry on Centaurus A , the nearest giant elliptical galaxy , at a distance of 4 Mpc .

Methods : We obtained deep photometric data with the Wide Field Imager at the ESO/MPG 2.2m at La Silla spread over almost two months .
Applying the difference imaging photometry package DIFIMPHOT , we produced high-quality difference images and detected variable sources .
The sensitivity of the current observational setup was determined through artificial residual tests .

Results : In the resulting high-quality difference images , we detect 271 variable stars .
We find a difference flux detection limit corresponding to m _ { R } \simeq 24.5 .
Based on a simple model of the halo of Centaurus A , we estimate that a ground-based microlensing survey would detect in the order of 4 microlensing events per year due to lenses in the halo .

Conclusions : Difference imaging photometry works very well at the distance of Centaurus A and promises to be a useful tool for detecting and studying variable stars in galaxies outside the local group .
For microlensing surveys , a higher sensitivity is needed than achieved here , which would be possible with a large ground-based telescope or space observatory with wide-field imaging capabilities .