A method used for estimating the distance between objects in photographic images is the depth from focus (DFF) method. The method has reportedly been implemented many times using only image-side telecentric optical systems because a change in image magnification becomes a problem due to differences in focus positions. Therefore, there have been no examples of this method having been implemented using a general optical system camera, such as a smartphone. In this study, we implemented an image-distance estimation program on the basis of the DFF method using an Android smartphone camera. This paper describes the possibility of estimating the distance between objects in an image taken with the camera of a general smartphone by using the DFF method. This paper also describes how we plan to resolve current flaws with the method that were discovered from this study.