Can anyone help to resolve the camera problem Nokia E90 phone?

This is not a big problem. The main problem is that your zoom button has become loose or totally damaged internally. That's why this type of problem is happening. You do not have to worry. It is a simple problem. As you have written in the question that you do want to fix this problem on your own, so I am telling you the method to do this on your own. First, if it is loose then you should open it and again fix it tightly and see whether it is working properly or not. If it is working properly then it is alright. Otherwise you have to buy a new button of this model and then remove the older button and fix this new button in the place of the older one properly and tightly so that it is not loose. Now, try to operate the camera of the Nokia e90 again. This time you will be able to open it correctly and I think or I must say that I am sure that you will not face any zooming problem.