Zoom on 3D Objects

[CasualDays]

Hi!
I've been following these tutorials, and I was wondering if it was possible to modify this tutorial to allow for zooming:

iPhone Development: OpenGL ES From the Ground Up, Part 3: Viewports in Perspective

I just want to zoom in and out using a slider.

I'm thinking by modifying this piece of code:

CGRect rect = view.bounds; 
GLfloat size = .01 * tanf(DEGREES_TO_RADIANS(45.0) / 2.0); 

glFrustumf(-size,                                           // Left
            size,                                           // Right
           -size / (rect.size.width / rect.size.height),    // Bottom
            size / (rect.size.width / rect.size.height),    // Top
            .01,                                          // Near
            1000.0);                                          // Far

I should be able to achieve what I want. But I need some guidance!

Thanks for your time!

[smasher]

This FAQ recommends multiplying everything except near and far by your zoom factor:

OpenGL FAQ / 8 Viewing and Camera Transforms, and Using gluLookAt()