Understanding Shims in Cinema Lenses

Over the past decade, the world of cinema optics has opened up. Lens manufacturers, responding to the growing demand for flexibility, have embraced modularity—giving end-users the option to swap mounts, service mechanical components, and yes, even adjust their own back focus. This accessibility is a win for the working cinematographer, but with it comes a flood of assumptions and, frankly, some dangerous misinformation. Grab a cup of coffee and read on for a more in-depth understanding of this commonly misrepresented feature.

One of the most persistent myths: that each lens mount—EF, PL, E, etc.—has a standard shim stack. A sort of golden number. Users will say, “This EF mount uses a 0.010” shim stack,” as if every EF-compatible lens exists in a vacuum of dimensional perfection. But this isn’t how optics work. Each cinema lens is a precisely tuned assembly of metal, glass, and mechanics. The shim stack is not a universal spec—it’s the result of careful calibration, accounting for individual variances in manufacturing, wear, and even thermal expansion. Shimming is about tuning, not copying.

Let’s demystify the role of shims in cinema lenses, explain why mount swaps aren’t as plug-and-play as you’ve been led to believe, and walk through the proper tools and methods for getting your back focus spot on.

What Are Shims and Why Are They Used?

Shims are thin, precision-cut spacers—often stainless steel, brass, or plastic—placed between a lens mount and the rear-most housing of the lens assembly. They exist for one simple reason: to ensure the correct flange focal distance (FFD), or more accurately, the optical back focus. This is the precise distance from the lens mounting flange to the focal plane, typically the camera sensor.

Now, why is this so critical? Because cinema lenses are not parfocal by accident. Every element—from the front element to the rear—is designed around maintaining consistent image plane alignment. If the lens sits even a few microns too close or too far from the sensor, your focus calibration is off. That means your witness marks are lies, your focus puller is guessing, and your 135mm at infinity might as well be a pinhole.

A properly shimmed lens ensures:
• Accurate witness marks across the focus scale
• Parfocal performance in zooms and some primes with floating elements
• Infinity focus lands exactly where it should
• The lens behaves consistently across camera bodies (assuming the body is calibrated)

But that consistency is only possible if the camera flange is equally well-calibrated. These days, many cameras ship with user-swappable mounts which introduce their own variable into the back focus chain. If the camera’s flange depth is even slightly off, the lens will never hit its marks, no matter how precisely it’s been shimmed.

And here’s the part many manufacturers don’t emphasize enough: just because a mount is swappable doesn’t mean the shims are one-size-fits-all. Changing a PL mount to EF doesn’t mean you can reuse the same shim stack across all your primes. Each lens has its own optical path, its own rear element depth, and therefore, its own ideal back focus distance.

The Myth of “Standard” Shim Stacks

This topic is actually what sparked this article—and truthfully, it’s the reason I started this entire blog: to curb the spread of misinformation and help educate the industry, especially those just beginning to explore the world of precision cinema lenses. Spend enough time lurking around online forums or in comment sections, and you’ll inevitably come across it: “DZOFilm Vespid Prime EF mounts need a 0.020” shim stack” or “Just use the same stack from your other lens, it’ll be close enough.” These kinds of blanket statements get passed around like gospel, but they’re rooted more in anecdotal convenience than optical precision—and they often lead users further from true focus accuracy, not closer.

The idea that mounts have a default shim thickness—PL = X, EF = Y, E-mount = Z—is a gross oversimplification. Here’s why:

Every cinema lens is built within a tolerance window. We’re talking about thousandths of an inch—microns, really—where mechanical stack-up, thermal expansion, and optical centering all play a role. The rear optical group might sit a hair deeper in one lens compared to another of the exact same model. The internal helicoid might have worn just enough to shift your focus slightly. Even the torque on mount screws can influence flange distance when tolerances are this tight.

Now add to that the lens mount itself. Most user-swappable mounts are machined independently of the lens housing, and their flange thicknesses can vary ever so slightly. Even mounts from the same manufacturer batch can have microscopic differences that affect back focus. This means there is no such thing as a universal shim stack for a given mount type.

Shim stacks must be custom-fit—not just to the mount, but to the lens it’s being installed on. You may find that your Cooke 50mm wants a 0.008” stack, while your 75mm in the same series needs 0.006”. And once you account for wear, environmental expansion, or a camera body with a slightly shallow flange, all bets are off.

Unfortunately, the flexibility of swappable mounts has been marketed without a corresponding push for education. Manufacturers tout the ease of swapping EF for PL and back again in the field, but they rarely emphasize the need to re-collimate. The result? Cinematographers and camera operators are chasing their tails trying to figure out why their lens “won’t hit infinity,” or why their AC is always pulling soft. Swapping a mount without checking or adjusting the shim stack is like swapping tires on a car without re-balancing them.

The Real Method: How Back Focus is Tuned

When it comes to setting back focus correctly, there’s often a gap between what’s ideal and what’s practical. The gold standard is an autocollimator—an optical test device that produces a collimated image at infinity, allowing precise alignment of a lens’s optical axis. It creates an artificial infinity in a zero-parallax environment, eliminating camera variables, environmental inconsistencies, and subjective chart interpretation. For lens technicians, prep techs, and owner-operators who truly know and love their optics, the autocollimator becomes indispensable. It’s by far the most accurate and efficient method for evaluating and adjusting back focus.

This tool allows trained techs to take a quick, reliable reading of any back focus discrepancy and fine-tune accordingly. But here’s the thing—most end users, and even many rental houses, don’t have access to one. That’s perfectly fine, if you know what you’re doing. Most back focus tuning happens on the camera itself, and with the right chart, proper distance, and a calibrated setup, it can be done effectively.

The On-Camera Method: Practical and Precise (If Done Right)
You don’t need a collimator to test back focus accurately. What you do need is a controlled setup, a good test chart, and a properly calibrated camera. Here’s the basic setup:

• A focus chart—personally I recommend a Duclos Lenses focus chart—mounted as far away as possible (the longer your focal length, the farther you need to be from the chart).
• A tripod-mounted camera with known flange accuracy. If your camera has a swappable mount, verify its flange depth using a known-good reference lens.
• A high-resolution monitor or EVF, critically sharp and peaking-enabled.

Steps:

1. Set the lens to its marked infinity position.
2. Aim the camera at the chart and check focus. The image should be critically sharp without adjusting the focus ring.
3. If the image is soft:
   • If sharpness improves by rotating toward minimum focus → the lens is too far from the sensor → remove shim(s).
   • If sharpness improves by rotating past infinity → the lens is too close → add shim(s).
4. Adjust shim stack in small increments (0.001” or 0.025mm at a time), then repeat.

A good on-camera test can get you within microns of perfect, assuming you’re methodical and your camera’s flange is accurate. I won’t dive into the details of how to properly calibrate your camera. That’s an article for another day. For zooms or lenses with floating elements, you’ll also want to check for parfocality—zoom in, set focus, zoom out, and see if it holds. If it doesn’t, your back focus is still off.

Mount Swaps: When and How to Shim

The rise of user-swappable mounts has been one of the biggest shifts in modern cinema lens design. On paper, it’s a dream: the ability to take your lens set from an Alexa Mini to a RED Komodo to a Sony FX9—all without sending anything to service. And mechanically speaking, it is simple: remove a few screws, pop off the EF, drop in the PL, and you’re good to go. But optically? That’s where things get messy.

Mount swaps don’t just change the interface between the lens and the camera—they alter the mechanical spacing between the rear optical group and the sensor plane. And because that spacing has to land within microns of the designed flange focal distance, even small differences in mount machining—or the shim stack under that mount—can throw your back focus off just enough to matter.

Why One Shim Stack Doesn’t Fit All – Lens manufacturers often include a shim kit with their interchangeable mounts. But those shims are not pre-calibrated. They’re simply a range of thicknesses to give you the tools to dial in the correct spacing. Yet users often interpret this as, “PL mount = 0.008” stack,” or worse, they assume the shim stack from one lens can be reused on another.

That’s not how it works.

Each lens—yes, even within the same series—requires its own back focus tuning. Different mechanical tolerances, rear group depths, and optical path lengths mean the optimal shim stack is unique. You can’t just carry a mount between lenses without rechecking focus accuracy.

Swapping a Mount the Right Way – If you’re going to swap a mount yourself, here’s what a proper process looks like:

1. Remove the Mount
   • Use high-quality screwdrivers to avoid cam-out or burrs. Nobody likes a stripped screw.
   • Keep screws and shims organized. I highly recommend a Service Tray mini or Service Tray PRO.
2. Install the New Mount
   • Start with the previous stack as a baseline (not a final setting).
   • Torque screws evenly. Inconsistent torque can tilt the mount and affect flange distance.
3. Test Back Focus Immediately
   • Use the on-camera method with a chart at distance (see previous section).
   • Adjust shims incrementally based on focus discrepancy.
4. Verify Witness Marks
   • Infinity is critical, but so is consistency across the scale.
   • If witness marks drift at mid or close focus, your back focus may still be off.
5. Final Torque and Thread Locker (Optional)
   • If the mount is staying on for a while, some mounts benefit from light thread locker, nothing too strong.
   • Follow manufacturer torque specs when provided.

Mount Material and Thermal Expansion – One often overlooked detail: different mounts are made from different alloys. Anodized aluminum, stainless steel, and titanium all expand at different rates. While negligible in most shooting environments, temperature shifts can subtly influence flange depth. Many of my long-time readers will know my distaste for nickel-plated brass. IMO, stainless steel or go home!

Bottom Line – A mount swap is not just a mechanical change—it’s an optical adjustment. If you’re not checking back focus after every swap, you’re not getting the performance your lens was designed to deliver.

Environmental and Mechanical Factors

Even if you’ve nailed the shim stack. Even if you’ve swapped mounts with care, used the right torque, and tuned your back focus to a crisp collimated Siemens star. Time still wins. The physical world—temperature, gravity, vibration, and simple mechanical wear—will eventually take its toll. Understanding the forces that act on flange depth is the first step toward maintaining long-term optical precision.

Temperature: Expansion is Real – All materials expand and contract with temperature—and cinema lenses are no exception. Aluminum, brass, stainless steel, and magnesium alloys each have different coefficients of thermal expansion. If your lens is collimated in a 68°F (20°C) service bay but you’re shooting in a 105°F desert, the expansion of the barrel, helicoid, and even the mount itself can subtly shift the optical path.

It’s usually minor—fractions of a thousandth of an inch—but with wide angle lenses or tight tolerances, that’s enough to throw off infinity focus or soften critical shots. Some lenses are thermally compensated by design (modern zooms especially), but many are not. In critical environments—macro, telephoto, or VFX plate work—it’s worth rechecking back focus once your gear acclimates to ambient temperature.

Mechanical Wear: Tolerances Don’t Stay Perfect Forever – Every time you mount and unmount a lens, torque is applied to the flange interface. Over time, screw threads can loosen, mounts can develop micro-play, and helicoids can wear down. That wear isn’t necessarily visible, but it shifts the relationship between the focus scale, the optics, and flange. Vintage prime lenses can be particularly susceptible due to their simple design and age as well as zooms given their complex internal movement and higher mechanical stress.

Camera Mount Degradation – We tend to blame lenses when focus is off, but the camera mount is just as likely to be the culprit—especially in user-swappable systems. A worn PL or EF mount, even slightly bowed or with uneven flange contact, can introduce tilt or focal plane error. If you’re seeing inconsistent focus across the frame, don’t just check the lens—check the camera body with a known-good reference lens.

Environmental Stress and Impact – Rough transport, repeated hot/cold cycling, or even gravity (over time) can pull on lens elements—particularly in older or longer focal length glass or particularly complex optical designs with heavy glass assemblies. This can slightly decenter elements or pull the rear group out of ideal collimation, especially if the lens is carried upright for months on end in a hot truck or improperly supported on long shoots.

“Back focus isn’t a “set it and forget it” adjustment. It’s a living measurement—one that should be revisited whenever the mechanical or environmental context changes. In the high-stakes world of cinema optics, a micron is a mile.” – Matthew Duclos

When to Leave It to the Pros

There’s a fine line between capability and hubris. While user-swappable mounts and on-camera tuning have made back focus more accessible than ever, not every situation should be handled at your workbench—or worse, on set with an angry AD asking why you’re not rolling. There are times when precision demands experience, and when opening up a lens without the right tools, skills, or environment risks doing more harm than good. At Duclos Lenses, we’ve spent decades building the infrastructure, tools, and most importantly, the expertise to handle this work. Whether it’s a simple mount swap or a full overhaul on a cinema zoom our technicians work to factory—or tighter—standards, backed by a reputation that we’re very proud of and have worked very hard to achieve.

Floating Elements and Internal Optics: Leave Them Alone – Many modern lenses, particularly mid-to-high-end primes and zooms, incorporate floating elements—internal optical groups that shift independently of the main focus group. These are designed to maintain sharpness across the focus range, correct spherical aberration, and reduce breathing. But they’re precisely timed and collimated at the factory. If you adjust back focus without understanding how it interacts with these internal groups, you can throw the entire optical system off. You might get the lens to hit infinity, but close focus will fall apart, or witness marks will become nonlinear. Worse, you’ll never know it’s off until it’s too late.

Zoom Lenses Demand More – Zooms, in particular, are a different beast. Maintaining parfocality requires that multiple internal groups move in harmony. The shim stack affects this delicate balance directly, and even a small misadjustment can cause focus shift when zooming. While it’s possible to perform a basic check at home or in the field, dialing in a zoom’s back focus with precision really does benefit from the proper tools—collimators, factory charts, and trained hands.

When Things Just Feel Off – If a lens has taken a hit, has drifted over time, or just isn’t lining up the way it used to, you don’t need to chase the problem on your own. It might be as simple as a mount re-seat, or it might involve decentered optics or internal wear that no amount of shimming can resolve. There’s no shame in tapping out when a problem feels outside your comfort zone. A professional lens technician won’t just fix the issue—they’ll give you a full optical and mechanical evaluation, often catching issues before they become major problems.

Duclos Lenses: When It Needs to be Right – This is the part where I’d be remiss not to plug my own operation—Duclos Lenses. We’ve built our reputation by working with everyone from individual owner-operators to some of the largest rental houses in the industry. Whether it’s a straightforward mount swap and collimation, or a full teardown and recalibration, our mission is the same: return your lens performing at or above its factory spec.

We’ve invested in the right tools—autocollimators, OEM jigs, test projectors—and more importantly, the right people, to do the job right the first time. If you’re ever in doubt, or just want peace of mind that your lens is performing at its best, we’re here for that. You’re not giving up control—you’re choosing precision. Back focus isn’t difficult to grasp, but it demands respect. Know your limits. Use the proper tools. And when the job calls for it, don’t hesitate to hand it off to someone who does this day in and day out. If that time comes, Duclos Lenses is here to help.

In Conclusion

Back focus adjustment isn’t glamorous. It doesn’t trend on forums or rack up likes on social media the way vintage coatings or swirly bokeh tests do. But if you care about sharp, consistent, production-grade imagery, it’s one of the most critical—and often overlooked—aspects of lens performance.

It’s easy to assume that mount swaps are plug-and-play, or that shim values are standardized. But optics don’t work like that. Every lens is a finely tuned system, and every mount interface, every optical group, plays a role in achieving proper flange depth and maintaining image fidelity.

Learning how to shim and check back focus on your own gear is valuable—and we encourage it. That’s why we wrote this article. But knowing your limits is just as important. For many users, there will come a point where the tools on your bench aren’t enough. And when that happens, there’s no shame in calling in the pros.

At Duclos Lenses, we specialize in exactly this kind of work. Whether you’re due for routine maintenance, need help dialing in a tricky zoom, or just want peace of mind that your gear is performing at its best, we’re here to help. You can contact us  for a service quote, or visit our Logistics page to get started with sending in your lens.