This is a simple Minecraft Forge mod that adds a configurable zoom key. (This is a forge port of Logical Zoom)
It stacks it's zoom with the spyglass too for maximum zooooooom!
By default the C key is mapped for zooming (as with Optifine) but you can change this in the Controls settings of Minecraft. If you already have the C key bound then you'll need to remap it before it'll work