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Know Your Lens Barrel (1): Useful Marks, Rings, and Switches

How well do you really know your lens barrel? In Part 1 of this series, we explore some useful features found on most if not all lenses. Read on, and you might just discover functions you didn’t know existed!

1) Filter diameter
2) Closest focusing distance
3) Lens mount index
4) Lens hood attachment position mark
5) Zoom ring
6) Focusing ring
7) Control ring
8) Focus mode switch
9) Image stabiliser switch
10) Zoom ring lock lever
11) Focusing/control selector switch
12) Lens mount contacts


What are those numbers and symbols on my lenses?

1) Filter diameter

Also known as the filter size, this refers to the diameter of the filter mounting thread that allows you to attach a screw-in filter (e.g. UV protectors, polarising (PL) filters, and neutral density (ND) filters to your lens. For example, a 77mm filter would fit this lens.

Know this: Some lenses are unable to take screw-in lens filters due to the size or shape of their front lens element. You will have to either use them with drop-in filters or a filter holder system.


2) Closest focusing distance

The closest focusing distance gives you an idea of the nearest your subject can be to the image sensor before the lens becomes unable to focus. It is important especially for macro photography!

This distance is different for every lens. But don’t worry, you don’t need to memorise it. On recent lenses, it is conveniently indicated on the side of the lens barrel. The EF-M28mm f/3.5 Macro IS STM that we see here has a 0.093m (9.3cm) closest focusing distance.


3) Lens mount index

The lens mount index is a mark on the lens mount that helps you to attach the lens securely to the camera body. Align this mark with the identical one on the camera mount, and then turn the lens until you hear a click.

Know this: Different lenses have different lens mount index indicators

If you are using an APS-C format DSLR, you will see two different marks on the camera mount. This is because APS-C format EOS DSLR cameras can take EF lenses (designed for full-frame DSLR cameras) in addition to their native EF-S lenses, but the mounting positions are different. Align the mark on the lens with the corresponding mark on the camera mount.

EF lens: Red dot
RF lens: Red line
EF-S lens: White square
EF-M lens: White dot

The same also applies when you are using a mount adapter.

Learn more about the different mounts at:

Know this:
- When you attach an EF-S lens to the EOS R system via a mount adapter, the camera will automatically switch to 1.6x crop mode.
- When you attach an EF (full-frame) lens to an APS-C format DSLR, or to an EOS M series camera via a mount adapter, the angle of view will be cropped to 1.6x of the full-frame equivalent focal length.


4) Lens hood attachment position mark

The red dot at the tip of your lens barrel indicates where to mount your lens hood.

When attaching the lens hood to the lens, make sure that the red dot on the lens hood is aligned with the one on the lens. Then, turn the lens hood clockwise until you hear a click. The lens hood is now securely fastened onto your lens.

Also see:
3 Reasons Why You Should Start Using a Lens Hood


What do those rings and switches on my lens do?


Here are the rings you will find on most lenses:
(1) Zoom ring (zoom lenses)
(2) Focusing ring
(3) Control ring (RF lenses)

Note: The number of rings and their position on the barrel varies depending on the lens model. If unsure, it’s best to check your lens manual.


(1) Zoom ring

Present on zoom lenses, turning the zoom ring changes the focal length. The numbers on the zoom position index are commonly used focal lengths and help you to gauge your current angle of view.

You turn the zoom ring anti-clockwise to zoom in.

Also see: Zoom ring lock lever

Know this: When you turn the zoom ring during a long exposure, you can get rather cool effects. Learn more about the zoom burst technique here.


(2) Focusing ring

For certain scenes, using the MF mode helps you to focus more precisely to get the shot you want. Regardless of design, all lenses will have a focusing ring, which allows you to carry out manual focus (MF). Turning the focusing ring moves the focusing lens group inside the lens (either mechanically or electronically, depending on the lens design), which changes the focus.

Also see: Focus mode switch

Prime lenses like the EF-M32mm f/1.4 STM don’t need a zoom ring, but they will definitely have a focusing ring!

- If you are using a compatible mirrorless camera, you can use MF peaking and the Focus Guide to aid your manual focusing.
- On the EOS R system, you can customise the focus ring sensitivity of your RF lenses.

Know this: Full-time manual focusing

If your lens supports electronic manual focusing (check your lens manual), you can override the AF to make fine adjustments with manual focus even if you are still in AF mode. This is called ‘Full-time manual focusing’.

To enable full-time manual focusing, you will need to change your camera settings and enable one of the two options indicated by the arrows. Refer to your camera manual for more details. On most lenses, full-time manual focusing is supported only in One-Shot AF mode. However, some lenses such as the RF400mm f/2.8L IS USM and RF600mm f/4L IS USM also support it in Servo AF mode.

Learn about how manual focusing can achieve cool effects on a certain lens in:
RF24-105mm f/4-7.1 IS STM: The Surreal World of Centre Focus Macro


(3) Control ring

Control rings are unique to RF lenses. They can be assigned to control either aperture, shutter speed, ISO speed, or exposure compensation during Manual mode.

Some lenses such as the RF50mm f/1.8 STM combine the focus ring and control ring functions into one ring to simplify the lens design. You use the Focusing/control selector switch to switch between the functions.

All control rings have a unique diamond textured pattern that makes them feel different from the other rings on the lens.



1) Focus mode switch

To use autofocusing (AF), set this switch to ‘AF’. To shoot fully in manual focus (MF) mode, set it to ‘MF’. If your lens supports electronic manual focusing, setting your camera to enable full-time manual focusing lets you do manual focusing even if the switch is set to ‘AF’.

If your lens doesn’t have this switch, you will need to use your camera to switch between AF and MF.

2) Image stabiliser switch

This lets you turn the Optical IS on and off. Turning the lens IS off when you don’t need it helps you to save battery power, and may also give you better results while using a tripod.

Telephoto zoom and super telephoto zoom lenses have a modified image stabilizer switch, which we shall explore in another article.

3) Zoom ring lock lever

When using a long zoom lens, have you ever experienced zoom creep, where the lens barrel extends on its own when the camera is tilted downwards? This switch helps to ‘lock’ the lens barrel in place when the zoom ring is set to the wide-angle end.

4) Focusing/control selector switch

This is found on RF lenses that have a combined focusing/control ring. To use the ring as a focusing ring, set the switch to “Focus”. To use the ring as a control ring, set the switch to “Control”.


Last but not least: Lens mount contacts

From exposure control and AF to image stabilisation, real-time lens aberration correction and more, there is a lot of communication between your lens and camera body when you shoot, and all that communication happens via the lens mount contacts. These contacts are sensitive, and lens function might be affected if they get scratched or dirty—even with fingerprints! Clean them with a soft cloth if they get soiled, and protect them with the dust cap when the lens is not attached to the camera.

Know this: RF lenses have 12 connection pins; EF lenses have 8

More connection pins, together with improved transmission protocols, enables faster, higher-capacity communication between an RF lens and EOS R system camera body. This increases possibilities in camera and lens performance.



The features that we introduced above are the most common—some special classes of lenses have unique features that cater to their characteristics or the scenarios they are frequently used in. In the next article, we will explore features and functions you can find on telephoto and macro lenses.

Fascinated by lens trivia? Check out:
Lens FAQ: What Does a Lens Name Mean and Why are Some Lenses White?
In Focus: Lenses FAQs

Or learn more about the different types of lenses and lens techniques in:
In Focus: Lens Basics

You can also learn more about the different parts of your camera here


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