What does the F stands for in my lens?

I need to send in my warranty for the lens of my Nikon D3100, but it asks for: F/, and i don't know what that is!

If it is the 18-55 mm, the f/stop is from f/3.5 to f/5.6. That should be clearly listed on your camera.

The "f" stands for "focal." It's asking you for the focal length of the lens that is under warranty.

Look at the lens itself… Printed right on the lens will be a range of numbers like f4.5-f5.6

The F is what's called the "f stop." On your lens it says f/ then a number. That number basically is how open or closed the "aperture" of the lens is. It should say the f stop right on the side or the front of the lens

Since you likely have the standard 18-55mm zoom lens its an f3.5-5.6 which is clearly shown on the lens, not the camera. So on your Warranty Card just write 18-55mm f3.5-5.6 and send it in. They'll probably want the serial number as well. Its also shown on the lens, usually around the lens mount.