Change zoom from dolly to "real" zoom

[Rainer Wahnsinn]

I'm trying to change the zoom effect in my application to be based on a change of FoV instead of just a z-translation which is called dolly. User input should be the basis so my current code looks as follows:

void setupMatrices()
{
	// Set ProjectionMatrix
	projectionMatrix = glm::perspective(90.0f, (GLfloat)width / (GLfloat) height, 0.1f, 1000.f);
	glBindBuffer(GL_UNIFORM_BUFFER, globalMatricesUBO);
	glBufferSubData(GL_UNIFORM_BUFFER, 0, sizeof(glm::mat4), glm::value_ptr(projectionMatrix));
	glBindBuffer(GL_UNIFORM_BUFFER, 0);
 
	// Set ModelViewMatrix
	glm::mat4 identity        = glm::mat4(1.0); // Start with the identity as the transformation matrix
	glm::mat4 pointTranslateZ = glm::translate(identity, glm::vec3(0.0f, 0.0f, -translate_z)); // Zoom in or out by translating in z-direction based on user input 
	glm::mat4 viewRotateX     = glm::rotate(pointTranslateZ, rotate_x, glm::vec3(1.0f, 0.0f, 0.0f)); // Rotate the whole szene in x-direction based on user input
	glm::mat4 viewRotateY     = glm::rotate(viewRotateX,  rotate_y, glm::vec3(0.0f, 1.0f, 0.0f)); // Rotate the whole szene in y-direction based on user input
	glm::mat4 pointRotateX    = glm::rotate(viewRotateY, -90.0f, glm::vec3(1.0f, 0.0f, 0.0f)); // Rotate the camera by 90 degrees in negative x-direction to get a frontal look on the szene
	glm::mat4 viewTranslate   = glm::translate(pointRotateX, glm::vec3(-dimensionX/2.0f, -dimensionY/2.0f, -dimensionZ/2.0f)); // Translate the origin to be the center of the cube
	modelViewMatrix = viewTranslate;
	glBindBuffer(GL_UNIFORM_BUFFER, globalMatricesUBO);
	glBufferSubData(GL_UNIFORM_BUFFER, sizeof(glm::mat4), sizeof(glm::mat4), glm::value_ptr(modelViewMatrix));
	glBindBuffer(GL_UNIFORM_BUFFER, 0);
}

I've tried a lot of things now but couldn't find out which of the code I need to change in order to achieve real zooming. I followed the instructions of a number of tutorials and looked at different example code snippets, but it just didn't worked out. Since I just started learning OpenGL, I hope that someone can tell me which instructions I need to change and how.
rotate_x, rotate_y and translate_z all come from mouse input

[Aleksandar]

Why don't you just change FOV parameter in a projection?

[Rainer Wahnsinn]

Yeah, that's what I tried, but I couldn't figure out how. Additionally, I probably have to change the modelview matrix as well, since the z translation doesn't make sense anymore. Could you give me a hint or even some sample code on how to change those things exactly? I do understand that I'll need glm::lookat / perspective, but I'm not sure about the parameters if user input is involved.

[imported_tonyo_au]

Have a read of this http://www.terathon.com/gdc07_lengyel.pdf

in particular

e = focal length = 1 / tan(FOV / 2)

As I understand with a telephoto lens what you are changing is the focal length.