Isolated closing contact other than relay?
Let's assume this is for a Canon camera (other types are available).

The auto focus is the first switch (pre-trigger), the shutter is the second. Both are operated by taking the signal to ground. This can be done by a push to make switch, a relay contact or more simply by a transistor. Depending on the particular model you will require a 2.5mm jack plug or an N3 connector. A typical interface circuit using opto isolators is shown below that can be manually operated (Sw1,2) or controlled by 2 * 5V gpio lines.