When you're comparing ND filters for an action camera, it is easy to focus on the obvious things: ND strength, optical quality, price and how quickly you can get the filter on and off.
The way the filter attaches to the camera can matter just as much.
Slip-on filters have some obvious advantages. They are usually cheaper, they are quick to install, and you don't have to remove the camera's original lens cover. In normal lighting, a decent slip-on filter can produce perfectly good footage.
But a slip-on filter and a replacement-style twist-on filter are not optically or mechanically equivalent. The mounting method changes what is sitting in front of the camera's lens, how accurately it is positioned, and how securely it stays there.
Some of those differences are a lot more important than they first appear.
What is the difference between a slip-on and a twist-on ND filter?
A slip-on filter fits over the camera's existing protective lens cover. Depending on the design, it may use friction, clips or a flexible frame to hold itself in place. The original clear glass stays on the camera and the ND filter sits in front of it.
A replacement or twist-on filter removes the camera's original clear lens cover and takes its place, using the same mounting interface provided by the camera manufacturer. On cameras designed with removable lens covers, this is exactly what that mount is there for: the cover is meant to come off easily and be replaced.
That distinction sounds mechanical, but it also changes the optical system.
The biggest optical difference: another piece of glass
With a slip-on filter, the camera's original protective glass remains in place and another flat piece of glass is added directly in front of it. On many action cameras, the two pieces of glass are large, close together and almost perfectly parallel.
That can be a problem when bright light enters the camera.
Every air-to-glass surface reflects some light. Anti-reflective coatings can reduce those reflections dramatically, but they cannot make an additional optical surface cease to exist. Adding another piece of glass adds another two air-to-glass interfaces and creates additional paths for light to reflect between surfaces.
This is basic optical engineering. Edmund Optics explains that an uncoated air-to-glass interface can reflect about 4% of the light that reaches it, which is why anti-reflective coatings are so important in any optical system containing multiple transmissive elements.
In a camera, those reflections can show up as ghost images, flare, washed-out areas and reduced contrast. A bright light source does not necessarily have to be directly in the middle of the shot. Light just outside the frame can be enough to create problems.
ZEISS has a good technical demonstration of what internal reflections do to photographic images, including examples where reflected light appears as obvious ghosts and others where it simply spreads across the image and reduces contrast.
This matters especially on action cameras because their extremely wide field of view exposes the filter to light from many more angles, making flare and internal reflections more likely to affect a large part of the image.
We go into this in more detail in our guide to reducing glare in action-camera footage.
Good coatings help, but the extra surfaces are still there
This does not mean every slip-on filter will produce terrible flare. A well-made filter with effective anti-reflective coatings can control reflections very well. In favourable lighting, you may not see a meaningful difference at all.
Lighting is what tends to expose the difference.
Put a bright source in or near the frame and those extra surfaces have more work to do. The quality of the coatings becomes important, the spacing between the pieces of glass becomes important, and the angle at which light reaches them becomes important.
A replacement filter avoids one part of that problem completely. Instead of adding another optical element in front of the factory cover, it takes the factory cover's place.
You still need good glass and good coatings. You just aren't asking them to deal with an unnecessary second window in front of the lens.
Alignment matters more than you might expect
There is another difference that is easy to miss until you have actually used slip-on filters for a while.
They don't always get pushed on perfectly.
A slip-on filter can look installed while one side is sitting slightly farther out than the other. Depending on the mounting design, it may also shift after installation. When that happens, the filter glass is no longer sitting square to the camera.
That isn't simply a cosmetic issue.
A tilted piece of parallel glass changes the path of light passing through it. Optical engineers account for this when designing imaging systems because a tilted optical window can cause image displacement and introduce aberrations such as astigmatism. The effect depends on the thickness of the glass, its refractive index, the amount of tilt and the optical system behind it.
This is a known problem in machine-vision optical design, where even the tilt of a sensor's cover glass has to be considered because of the dramatic effect it can have on image quality.
On an action camera with an extremely wide lens, a visibly crooked filter is therefore not something we would dismiss. It can change the image, and the difference isn't always subtle. Reflections can change, and image quality can become less consistent across the frame.
A replacement filter has a defined installed position. Once it is properly locked into the camera's mount, the glass sits where the filter was designed to sit.
Retention is a different kind of problem
Slip-on filters normally depend on friction or clips to stay on the camera. A good design can hold very securely, but the fit is ultimately dependent on the dimensions and flexibility of the filter mount and the part of the camera it grips.
That matters more on an action camera than it does on a camera sitting on a tripod.
Action cameras vibrate. They get mounted to motorcycles, mountain bikes, cars and drones. They get knocked against things. They get handled with gloves, dropped, stuffed into pockets and repeatedly exposed to temperature changes.
A twist-on replacement filter uses the camera's mechanical lens-cover mount. Once it is properly installed, it has a defined locked position rather than relying purely on friction around the outside of the existing lens cover.
That doesn't make a twist-on filter impossible to lose, and it still needs to be installed correctly. It does give the filter a much more repeatable mechanical connection to the camera.
Slip-on filters usually need more room around the lens
A slip-on filter has to fit around something that is already on the camera. That usually means its frame extends farther around the lens than a replacement filter that uses the camera's existing mounting interface.
That can matter with tight cages, underwater cases, protective frames and other accessories. On ultra-wide cameras, a poorly designed or oversized filter mount can also get close enough to the optical field of view to cause dark corners (vignetting) or other interference.
What about waterproofing?
A slip-on filter leaves the camera's original waterproof lens cover in place, but the space between the filter and that cover is usually not sealed.
Water can get trapped between the two pieces of glass, leaving droplets or a film directly in front of the lens and potentially ruining the footage. That partially enclosed air gap can also make fogging more likely when temperature or humidity changes.
A properly designed replacement filter avoids that extra cavity and takes the place of the original cover while maintaining the camera's waterproof seal.
For example, our DJI Osmo Action 6 ND filters, DJI Osmo Action 4/5 ND filters, GoPro Hero 9–13 ND filters and GoPro Mission 1 / Mission 1 Pro ND filters all replace the original clear cover while maintaining the camera's waterproof seal.
What about durability?
Slip-on filters are usually designed to be inexpensive and thin enough to fit over the camera's existing lens cover. That often means plastic frames and conventional, thinner optical glass.
Camera Butter replacement-style filters use Corning Gorilla Glass and aluminum frames, so the filter also provides serious physical protection for the camera.
Our Shock Mod ND filters take that idea even farther, with raised aluminum impact guards that extend beyond the glass to help take impacts before they reach the optical surface.
Slip-on filters do have real advantages
There is a reason people buy them.
- They are usually less expensive.
- They can be extremely fast to put on and take off.
- The factory lens cover stays on the camera.
For occasional use, especially when you aren't shooting into difficult light and the filter fits securely and squarely, those advantages may matter more to you than the disadvantages.
Price matters too. A replacement-style filter is more complicated to manufacture. Instead of making a frame that simply grips the outside of the camera, the manufacturer has to reproduce the camera's mating geometry accurately, control the installed position of the glass and, where required, deal with the sealing interface.
That costs more to make.
The important thing is understanding what you are getting for the difference in price.
Twist-on vs. slip-on ND filters at a glance
| Slip-on / clip-on filter | Replacement / twist-on filter | |
|---|---|---|
| Factory clear lens cover | Stays on camera | Removed and replaced by filter |
| Glass in front of lens | Factory cover plus filter | Filter replaces factory cover |
| Additional reflection surfaces | Yes | No additional window beyond the replacement filter |
| Alignment | Depends on filter being fully and evenly seated | Defined by camera's mounting interface when properly installed |
| Retention | Usually friction or clips | Mechanical twist-lock, bayonet or threaded mount |
| Space around lens | Usually requires more clearance | Can remain close to original lens-cover dimensions |
| Changing filters | Usually fastest | Usually takes a few seconds longer |
| Factory waterproof seal | Original cover stays in place | Replacement filter must be designed to maintain it |
| Typical cost | Lower | Higher |
| Water between filter and lens cover | Can become trapped in the gap | No additional cavity |
So which type should you buy?
A good slip-on filter can make sense if low cost and very fast filter changes are your main priorities. Plenty of people use them successfully.
If image quality matters, a replacement design has a significant optical advantage: it does not put another large, nearly parallel piece of glass in front of the one that is already there.
It also gives the filter a defined mechanical position on the camera, avoids depending entirely on a friction fit, and usually takes up less space around the lens.
Those are the main reasons Camera Butter uses replacement-style mounts wherever the camera gives us a practical way to do it.
For our current GoPro ND filters, DJI Action 4/5 filters and DJI Action 6 filters, the filter replaces the original clear lens cover instead of stacking another piece of glass in front of it.
We then worry about the other half of the equation: whether the filter itself is actually high quality optically.
That means neutral spectral transmission, good optical coatings, accurate ND strength, low reflections, strong materials and coatings that are easy to keep clean. We measure those things rather than relying on terms like "premium glass."
You can see the results and our testing methods in our ND filter quality comparison.
If you're still wondering what an ND filter actually changes in your footage, our guide to what ND filters do, and what they don't do covers that separately.
Twist-on vs. slip-on ND filter FAQ
Are slip-on ND filters bad?
No. A well-made slip-on filter can produce excellent footage and they are inexpensive and convenient. The tradeoff is that they leave the original protective glass in place and add another optical element in front of it, along with another mounting interface that has to stay square and secure.
Why can slip-on filters cause more glare?
A slip-on filter normally adds another piece of glass in front of the camera's existing protective cover. That creates two additional air-to-glass surfaces and more opportunities for light to reflect between optical surfaces, which can increase flare, ghosting and loss of contrast in difficult lighting.
Can a crooked slip-on filter affect image quality?
Yes. If the filter is not seated evenly, the glass sits at an angle to the camera's optical system. Tilted optical windows can change the path of light and introduce image displacement and optical aberrations. The amount of visible change depends on the filter, lens and degree of misalignment.
Are twist-on ND filters harder to change?
Usually slightly. A slip-on filter can often be pulled off and replaced immediately, while a replacement filter has to be unlocked and twisted off. On cameras designed for replaceable lens covers, the difference is normally only a few seconds.
Does a twist-on ND filter keep an action camera waterproof?
Only if the filter is designed to reproduce the camera's original sealing interface correctly. Camera Butter replacement filters that are specified as maintaining the waterproof seal are designed for that purpose. Check the specifications for your particular camera and filter.
Does the mounting style determine optical quality?
No. Glass quality, spectral neutrality, coatings, flatness and manufacturing quality still matter. The advantage of a replacement-style mount is that it avoids adding an unnecessary second optical window and gives the filter a controlled mounting position. The filter itself still has to be good.
Can water get trapped behind a slip-on ND filter?
Yes. The space between a slip-on filter and the camera's original lens cover is usually not sealed, so water can become trapped between the two pieces of glass. That can obscure the image and can also make fogging more likely.