Paul C - Once again, common-sense will usually prevail. No one wants to prise valuable, fragile items away from our passengers unless there's a valid safety reason for doing so.
I would guess the reason you are usually allowed on board (bearing in mind you've to get past the Check-In Hurdle, through the Security Minefield and negotiate the Boarding Gate Obstacle long before sinking into your aircraft seat!) is because you've taken all reasonable steps to make the photographic case as light as possible and been prepared to divide the contents if need be.
If you make your reservation by phone (becoming rarer these days), you can always ask the reservations agent to put a comment in your booking that you will be carrying photographic kit as cabin baggage so that the message will be displayed to the check-in agent (British Airways DCS/PRS based systems only) as soon as your Passenger Name Record is retrieved.
Hope I have been of some help - Good Luck!
