Choosing an objective, while going through a large distance is not an easy thing to do.
There are many factors that can influence the decision: the photography style, the nature and the length of trip.
If you can afford it, you have to buy the best possible objective from the category you are interested in. This way you will have the possibility to improve the odds of taking the best traveling picture.
The focusing distance
Generally you can never know when exactly during the trip are you going to take the picture.
The subjects that may come in your attention can vary and if for example you are traveling in a big city and want to capture the architecture and the streets, you will need most likely a wide –angle objective. This type of objective will allow you to take pictures of the most buildings in optimal conditions.
In order to capture details, far away objects and people you need a focusing objective reaching 135 to 200 mm. The focus chosen depends on the personal style you use for the digital photography.
The ideal thing is to have around at least two wide-angle and one high focus objectives. Another option would be to choose an objective with a wide application span like Nikkor 18-135mm or Tamron 18-200mm.
The disadvantage would be that these objectives have usually a lower quality than the specialized ones.
The maximum opening of the diaphragm
The situations with low light are often met during traveling, in museum and art galleries (where the usage of the flash light is forbidden), the evening light photography, inside events etc. So the more light comes through the objective, the better.
Unfortunately these objectives are quite expensive. The investment is worth because it comes with amazing results. The best zoom objectives are Canon EF 70-200mm f/2.8L IS USM.
A large objective represents a problem during a trip because of its weight.
Even if you are carrying several objectives, in order to avoid the permanent transport of a photo bag full of items, of which you are about to use only a few , the better choice is to choose which of them is more useful before you leave.
Apart from this there is always the risk of losing the bag or getting it stolen so it’s better to be safe than sorry.
A large number of objectives requires time to change them and comes with the risk of missing the moment for the perfect picture.
You must always keep in mind that the main ingredient for the picture of a lifetime is the skill of the photographer and only after that the gear he uses.