Multi-focus image fusion is a process of combining two or more partially defocused images into a new image with all interested objects sharply imaged. In this paper, after reviewing the multi-focus image fusion techniques, a wavelet based fusion scheme with new image activity level measurement is presented. The proposed multi-resolution image fusion technique includes three steps: First, multi-resolution discrete wavelet transform (DWT) is applied to obtain the wavelet coefficients of the source images. Then, applying proposed coefficients fusion scheme to the obtained coefficients, the wavelet coefficients of the fusion image are reconstructed. Finally, the final fusion image is generated by applying inversed wavelet transform. In experiments, artificial defocused images (by applying low-pass filter to the specified regions) are utilized to investigate the performance of proposed scheme and to select suitable wavelet family and wavelet decomposition scales. Then we apply the proposed image fusion scheme to optical microscopy domain to solve the short depth of focus characteristic of optical microscope. The experiments results verify the validity of the proposed multi-focus image fusion scheme.