Nikon Zoom Lens Internal Workings

It’s difficult to appreciate just how complex modern zoom lenses are. In this article, I’ll try to show some of the amazing technology and complexity that is built into a typical Nikon zoom lens.

I disassembled the Nikkor 55-300mm f/4.5-5.6G AF-S ED VR DX lens into its basic components. This lens has very complex glass elements inside it, and these elements have to shift in a very complicated way relative to each other while zooming.

There are also some electrical aspects to zooming, which are there to provide feedback (saved into the photograph EXIF data) about the focal length.

Nikkor 55-300 mounted on Nikon Z9 with FTZ-II adapter

The 55-300 lens elements, courtesy of Nikon

This lens has 17 different lens elements. For zooming purposes, the elements are housed into 4 different groups. You might think that just the front group of glass moves during zooming, but that’s not even close to what actually happens.

The front of the lens, which ‘telescopes’ during zooming

The shot above shows the front section of the lens, which moves away from the camera when zooming to longer focal lengths.

4 main groups of optics

Note that the optics group that’s second from the right above. This group also contains the lens aperture (on its left end) and the vibration-control mechanism (on its right end).

The (left side above) front group has 3 lens elements. The next group behind the front group also has 3 lens elements. The group that includes the VR control and aperture has 7 elements, and the group nearest the lens mount has 4 elements.

As you’ll see, these 4 groups are moved separately from each other during zooming, which alters the relative spacing of each group.

The lens front spiral guides

Just inside of the lens front is a cylinder with spiral ridges and grooves. The interior portion of the lens front has cams that move along these grooves when twisting the zoom ring. The first 3 lens elements, which include the exterior front lens element, move together when the lens front moves along these grooves.

The spiral ridges keep the lens front stable and wobble-free, while maintaining a minimum of friction with only a small area of actual surface contact. These ridges are coated in a high-quality grease to further reduce friction.

Note the second group of optics is visible at the top of this module. This optics group movement is controlled separately from the front-most optics group.

The lens front spiral guides viewed from behind

Lens front spiral guides separated from ‘middle’ zoom sleeve.

Shown above, the front spiral guide sleeve has been turned upside-down and separated from the ‘middle’ zoom sleeve. The middle zoom sleeve shows both a white roller-cam and a black roller-cam in its grooves. These white and black cams move along very complicated grooves, which adjust two separate sets of optical groups.

The shapes of these grooves accommodate the very complex relationship between the optics groups that control the focal length. The spacing between these optics changes at a very un-even pace while zooming. There are actually 3 separate sets of grooves with their independent path shapes. The grooves are machined in triplicate around the sleeve, to provide an even load distribution.

The optics shown on the left-hand side above represent the third optical group. This group is mounted inside the vibration-control module. This same group has the aperture blades mounted at the opposite end.

Cams at right-hand side of the grooves

At the zoom position shown above, the optics are at their deepest inside the inner zoom sleeve. The inner zoom sleeve has straight slots in it, and the optics are sitting at the bottom of these slots. This would be the 300mm zoom position.

Cams at left-hand side of the grooves

At the zoom position shown above, the optics are at their farthest outside of the inner zoom sleeve, at the top of the inner sleeve’s straight slots. This would be the 55mm zoom position.

Side view at mid-zoom position

Note in the shot above that the cams move along straight grooves on the inner sleeve. This moves the optics while keeping them parallel to the camera sensor. The sleeves are coated in grease to reduce the sliding friction.

Side view, near the 55mm zoom position

Zoom sleeves with optics removed

Middle zoom sleeve by itself

Imagine the insane mathematics that goes into these path shapes. Modern optics design is incredibly complicated.

Inner zoom sleeve with its straight slots

The cams slide in these slots to push the optics groups forward and backward to adjust the focal length. It’s all coated in messy grease. Who would have expected so much grease inside a lens?

Zoom sleeves with optics and cams

Focus

SWM stepping motor for autofocus

Just below the SWM motor, with its green teeth, is the gear train that the motor spins to adjust focus. This little motor is only 12 millimeters in diameter, yet it has the power and accuracy to obtain and track focus with very little battery drain.

Only the 7-element group of optics gets moved to obtain focus. The front and rear optics groups don't move during focusing. This is what they mean by an "internal focusing" lens.

Focal Length Detection

Electrical conductors for focal length detection

The picture above shows 5 rows of gold conductors. 4 of these rows are segmented into an uneven pattern. These rows are the mechanism for how to determine the “Focal Length” setting, which gets saved into the photograph EXIF data.

When you zoom, little conductive ‘fingers’ get dragged along these rows. Depending upon the zoom setting, some fingers will complete a circuit, and some fingers won’t. The 5th (bottom) row is just an electrical ground to complete the circuit.

The vertical positioning of the gaps in the top 4 rows represents bits. With one set of fingers dragging across these rows, you would then get 4-bit data. With 4 bits of data available, this lens can indicate up to 16 different focal lengths.

By the way, there’s a similar circuit for determining the focus distance, but there are 4 rows of gold conductors instead of 5 rows. This gives 3-bit data, or 8 different focus distances saved into the EXIF data. The last row, similar to the focal length circuit, is just an electrical ground.

Summary

There’s a lot more going on inside a zoom lens than most people could appreciate. I hope this little tour helped clear up some of mystery of those inner workings.