Which key do I use to zoom with Minecraft Optifine?

Default key: Ctrl (Although in some versions it's c)

If that doesn't work look for the option under Options > Controls > Zoom. Now you can see what key is bound for zooming. If you want to rebind it, click on the key next to the "Zoom" and press the key you want to rebind it to.

Hold down the "Control" key. Profit.

Actually, for most new optifine users, in 1.8 the default key for running is in fact Ctrl and optifine therefore makes the default key for zoom C. This is just if CTRL is not working :).

If you are running the optifine forge version, the key you're looking for is ctrl, if you are using vanilla optifine, it is C.

Its c I have it I changed it to c! How to do this go to options, control, zoom and then change by pressing the letter you want on the keyboard!!

It's just C, or if it dosen't, work try Ctrl. If it still doesn't work, go to control settings and then to miscellaneous. Find zoom and change it to the key you want.

C is the zoom in key, but you have to have Optifine.

You use the C key, and if that isn't working, try Ctrl, if all else fails, go to settings, controls, and find the zoom setting, also, if your spring key is set as C or Left Ctrl they might not either work because their fighting each other for the control. So make sure the two controls aren't overlapping each other, you can tell because the button will be red in the control settings.

It depends on what version of Optifine you installed. If it's lower than 1.8, it will probably be Left-CTRL. But if it's installation was based on a certain version and you play on another, your keys might be messed up. The best option is to go to Settings, and Miscellaneous. Go to the Zoom option, and look at the key it's based on. If it's red, set it to another, as it means that it's also controlling another option.