This is a good question! As you can read in the previous posts everyone has made their own, often very different, experiences with carry on luggage.
I happen to work in a place where almost all of our guests bring heavy camera gear and this is what I can tell you:
1: Bringing underwater housings, domes, lenses, cameras is not a problem. Security might wan't to have an extra look at it because they don't know what it is. I have never heard of anyone being denied bringing their UW camera as a carry on because of having the items deemed "dangerous" or such.
2: More and more airlines have started weighing carry-ons. In fact today I had guests arrive, coming from the US with Singapore Airlines, they had to talk to the local airline manager to be allowed to bring the 13kg of carry on camera gear that they had with them. Max weight allowed was 7kg, but in the past they would never weigh the carry on.
It would be quite interesting to start a thread about which airlines weigh carry-on luggage on which stretches!
Greetings from Indonesia,
- Eyematey likes this