public static SPUser GetUser(this SPListItem item, string displayName)
{
    //get field and cast to User Field type
    var field = item.Fields.GetField(displayName) as SPFieldUser;

    if (null == field)
    {
        throw new ApplicationException("This field is not a User!");
    }

    var fieldValue = field.GetFieldValue(item[field.Id].ToString()) as SPFieldUserValue;

    SPUser user = null;
    if (null != fieldValue) 
    {
        user = fieldValue.User;
    }

    return user;
}

public static SPUser GetUser(this SPListItem item, string displayName)
{
    //get field and cast to User Field type
    var field = item.Fields.GetField(displayName) as SPFieldUser;

    if (null == field)
    {
        throw new ApplicationException("This field is not a User!");
    }

    var fieldValue = field.GetFieldValue(item[field.Id].ToString()) as SPFieldUserValue;

    if (null == fieldValue) {
        return null;
    }

    return fieldValue.User;
}

Extension method to get SPUser object from SPListItem person field

C#