Airlines usually weigh your cabin luggage at the check-in/baggage drop-off and do not allow more than 7-8 kilos per person. How to successfully board a flight if the total weight of the gear you are taking with you exceeds the maximum allowed for carry-on luggage?
The gear is, obviously, too precious and fragile to be transported in checked baggage, so the cabin luggage is the only option.
As many comments below are more concerned with avoiding the situation in question instead of overcoming it, Let's say the issue has no other ways of resolution, i.e. you have to board the plane with the gear you have in your carry-on and your bag exceeds the weight limit. It also not country specific and encompasses all of the world, including less developed countries. It means that:
- Taking less gear is out of question, all the gear is essential and you can't downsize or substitute anything for anything smaller/lighter (a good example is a large format film camera setup). What gear to take with you and how to take less of it is a completely different topic that deserves a discussion of it's own, but it's out of the scope of this question.
- Renting/borrowing the gear at destination is not an option as you could have gear that can't be easily, if at all, found to rent or it is financially prohibitive. There is also a concept of your own tool which essentially prohibits many photographers using other's gear, because it either doesn't feel the same or may be not as reliable as your own that you look after and which quirks (if any) you know well. For many the issue of being liable for any damage is an added stress factor that gets in the way.
- Sending your gear by mail/courier is impossible due to financial and logistics reasons (too expensive, many flights per trip, no trustworthy service etc.)
- Packing all the gear into a securely locked foam-padded hard shell case and checking it in is also not acceptable as it is not secure (someone can still cut the lock and steal the contents), mandates a payment for excess baggage (it's implied you already have one checked-in bag) and airport/airline security or customs inspection have the right to open your case without your presence if they suspect there is something dangerous/prohibited inside. For inspection they will cut any locks they can't open and secure the case afterwards with nylon zip-ties.
[optics]tag at Physics.SE, but that doesn't stop them from being on-topic here as well, considering how important lens optics are to photography.