What is the rotatable circle outside the lens of Canon 18- 135?

In fact, this lens can have two rotatable adjusting rings. The following picture shows the solid lens of Canon 18- 135.

Let's talk about the focus ring first. The so-called focusing means adjusting the focus. When you rotate this ring, as long as you focus, the image in the eyepiece will be very clear.

Only when MF manual focusing starts can this ring be rotated manually. When AF autofocus is selected, you can't rotate the circle manually because the electric focusing motor is working, which may damage the motor.

As the name implies, the zoom ring is used to change the focal length. The focal length of this lens is 18- 135 mm. If you rotate inby, you can change your business in this range.

Its essence is to change the angle of view of the lens, and arbitrarily choose the composition between the maximum of 74'20 "and 1 1'30", so that the size of the subject can be changed within the visual framework.

After the focus is completed, changing the zoom generally does not affect the focus.

So if you choose the manual shooting mode, you can focus first and then zoom to achieve the purpose of free composition.