Varifocal vs Fixed Lens Cameras

What is the practical difference between fixed and varifocal?

A fixed lens camera, commonly built around a 2.8 mm or 4 mm lens on a 1/2.8 inch sensor, is set once at the factory and cannot be adjusted afterward beyond digital zoom, which adds no pixels. A varifocal camera houses a lens that physically moves, letting the same body cover anywhere from a wide entryway shot to a narrower, longer range view.

Motorised varifocal lenses, sometimes marketed as an optical zoom feature, can be adjusted from an app after the camera is already mounted, which matters when the exact distance to a target was not measured before installation.

Some manufacturers also sell a motorised varifocal lens as a separate accessory that can be swapped onto certain camera bodies, blurring the line further between what counts as a fixed camera and what counts as a varifocal one. The underlying physics stays the same regardless of how the adjustment mechanism is packaged: a moving lens element changes focal length, and only a change in focal length changes field of view and DORI distance.

How does focal length change field of view?

On a common 1/2.8 inch sensor, 5.12 mm wide, a 2.8 mm lens produces an 84.9 degree horizontal field of view, wide enough to cover a driveway and both side yards from a single porch mount. Stepping up to a 4 mm lens narrows that to about 65.3 degrees, and an 8 mm lens narrows it further to about 35.5 degrees.

That narrowing is not a downside by itself, it is the mechanism that buys distance. The same sensor and resolution reach much farther down a narrow field of view than a wide one, because the available horizontal pixels are spread across a smaller slice of the scene.

The relationship is not linear with focal length itself. Field of view narrows quickly at first as focal length increases from a very wide setting, then more slowly as focal length keeps climbing, because the geometry runs through an arctangent rather than a straight ratio. Doubling focal length from 2.8 mm to 5.6 mm cuts field of view by more than half, while doubling again from 5.6 mm to 11.2 mm cuts it by a smaller fraction of what remains.

How much farther does a longer lens actually reach?

On an 8MP (4K) sensor, a 2.8 mm lens identifies a face to about 27.6 ft and detects a person out to about 275.6 ft. Step to a 4 mm lens on the same sensor and those numbers grow to about 39.4 ft identify and 393.7 ft detect. Step further to an 8 mm lens and identify distance reaches about 78.7 ft with detect out to about 787.4 ft.

The table below lays out all three focal lengths side by side on the same 4K sensor, and the pattern holds at any resolution: a longer lens multiplies DORI distance by the ratio of the two lenses' tangent of half their field of view, not by focal length directly.

The same scaling applies to observe and recognise distances at each focal length, not just detect and identify. A 4K camera on a 2.8 mm lens reaches roughly 110 ft for observe and 55 ft for recognise, and those numbers scale by the same ratios as identify and detect when the lens changes, since all four DORI levels sit on the same underlying pixels-per-metre curve.

What is the difference between optical zoom and digital zoom?

Optical zoom, whether from a manually adjusted varifocal lens or a motorised one, physically changes the focal length and genuinely increases pixels per metre at distance. Digital zoom crops and enlarges pixels the sensor already captured and adds nothing new, so it leaves the pixels-per-metre value at any real world distance exactly where it was.

A varifocal camera's real advantage over a fixed lens with digital zoom is entirely in that physical adjustment, not in any software feature layered on top of either one.

This is also why pairing a wide lens with digital zoom is sometimes marketed as equivalent to a longer physical lens. It is not: a digitally zoomed crop from a wide lens has the same total pixels spread over the same real world area as the uncropped frame, it has simply thrown away the surrounding pixels rather than gathering new ones, so the pixels-per-metre value on the remaining subject stays exactly what it was before the zoom.

When does a fixed lens make more sense than a varifocal one?

A fixed lens is simpler, usually cheaper, and has one less moving part to fail over years outdoors. For a porch, doorbell or close entry point where the distance to the target rarely changes, a correctly chosen fixed lens covers the job permanently with nothing to adjust or drift out of focus.

A fixed lens also tends to hold its focus setting more reliably over years of temperature swings and vibration than a varifocal lens with a motor and moving optical elements, one more reason installers favor fixed lenses on straightforward, unchanging positions.

• Close range entry points where the target distance is already known
• Budget builds where every camera covers a similar short distance
• Locations where the field of view rarely needs to change after install

When is a varifocal or multi-lens camera worth the extra cost?

A long driveway, a gate at the far end of a property, or any spot where the right focal length was not obvious until after the camera was mounted are the strongest cases for a varifocal lens. Being able to nudge the field of view from an app avoids a second trip up a ladder.

A dual lens camera that pairs a wide fixed lens with a longer telephoto lens on the same body, rather than one lens that physically moves, is a related option: the wide lens keeps overall awareness of the area while the telephoto lens covers the specific distance that needs identification quality resolution.

Dual lens cameras do add one practical complication: two separate video streams, sometimes viewed side by side and sometimes switched between, which can roughly double the bitrate and storage draw of a single lens camera at the same resolution on each lens. That tradeoff is worth weighing against a single varifocal lens covering the same spot with one stream.

How do you choose the right focal length for a specific spot?

Measure the actual distance from the mounting point to the farthest point that needs to be identified, not just detected, then work the lens field of view and DORI distance calculators backward from that number rather than picking a lens by habit.

A wide 2.8 mm lens is the right default for a porch or doorway. Anything past about 30 to 40 ft that needs a usable face, a license plate or a package thief identified rather than merely noticed, needs a longer fixed lens or a varifocal camera set accordingly.

Where the exact distance genuinely cannot be measured in advance, such as a lot line that has not been finalized or a gate location still under discussion, a varifocal or dual lens camera removes the risk of guessing wrong on a fixed lens and having to remount or replace the camera once the real distance is known.