Processing ZoomOut

Question

I am currently developing a simple 2D map with terrain and other stuff, but I have encountered a problem relative to the zoom. In fact I can zoom in without any problem, but when I try to zoom out the zoom doesn't simply work and stays still.

int cellScale = 4;
int mapWidth, mapHeight;
int gridWidth, gridHeight;
map theMap;
float zoom, cameraX, cameraY;

void setup(){
  size(800,800,P3D);
  mapWidth = 1*width;
  mapHeight = 1*height;
  gridWidth = mapWidth/cellScale;
  gridHeight = mapHeight/cellScale;
  theMap = new map();
  cameraX = width/2.0;
  cameraY = height/2.0;
  zoom = (height/2.0) / tan(PI*30.0/180.0);

}
void draw(){
  theMap.display();
  keyInput();
}

void keyInput(){
  if(keyPressed){
    if(keyCode ==  LEFT){
      cameraX-=5;
    }
    if(keyCode ==  RIGHT){
      cameraX+=5;
    }
    if(keyCode ==  UP){
      cameraY-=5;
    }
    if(keyCode ==  DOWN){
      cameraY+=5;
    }
    camera(cameraX, cameraY, zoom,
           cameraX, cameraY, 0,
           0,1,0);
  }
}

void mouseWheel(MouseEvent event){
  float e = event.getCount();
  zoom = max(1.0, zoom + 10*e);
  camera(cameraX, cameraY, zoom,
         cameraX, cameraY, 0,
         0,1,0);
}

I have noticed that after I do the zoom out with the mouse wheel, with the screen staying the same, when I zoom in I have to "re beat" the zooming out. I mean if I zoom out let's say 50, with always the screen that doesn't change, when I want to zoom in of 20 I have to zoom in of 70 because I have before zoomed out without succes of 50.