You could try imfindcircles(), but with only a small portion of the perimeter actually showing, it might not find a circle. Since there is no iris recognition/detection function built in to MATLAB, you can use one of the published algorithms that are listed here http://iris.usc.edu/Vision-Notes/bibliography/contentspeople.html#Face%20Recognition,%20Detection,%20Tracking,%20Gesture%20Recognition,%20Fingerprints,%20Biometrics See section 21.5.3:
21.5.3 Biometrics, Iris Recognition Systems
21.5.3.1 Iris Recognition Systems, Wavelet Based
21.5.3.2 Iris Detection, Segmentation and Localization Systems
21.5.3.3 Iris Recognition Systems, Systems, Evaluation, Comparison, Surveys
There's bound to be something in there to help you. It's an active area of research - even here, people ask several times a month about it (check the tags).
If you don't want to use a successful, published algorithm, then you might try your own by finding the whites, getting the edge with the iris and then using the FAQ to create a circle from the edge data.
