Lens selection determines the field of view, coverage distance, and low-light sensitivity of each explosion-proof camera. A 4mm lens provides wide coverage at 20-40 meters; a 12mm lens covers 80-150 meters with a narrower angle. Varifocal lenses allow angle adjustment during commissioning without changing the camera. For low-light hazardous area applications, an F1.2 or F1.4 lens captures two to four times more light than an F1.8 lens at the same distance.
The lens is the least discussed but most consequential component of an explosion-proof camera specification. Once conduit runs are sealed and the camera is bolted to its housing, changing the lens requires a hot-work permit and potentially a new housing seal if the camera must be removed from the classified area. Getting lens selection right at the design phase prevents costly field changes and coverage gaps. This guide covers the key variables: focal length, aperture, varifocal vs. fixed, and environmental considerations specific to hazardous area installations.
Focal Length and Field of View
Focal length (measured in millimeters) is inversely related to field of view angle. A shorter focal length produces a wider angle; a longer focal length produces a narrower, more telephoto view. The relationship between focal length, FOV, and sensor size is fixed by optics — for a given sensor size (typically 1/2.8″ or 1/3″ in industrial cameras), doubling the focal length halves the horizontal field of view angle.
The following coverage distances apply to a 2MP (1920×1080) camera with a 1/2.8″ sensor at detection-level monitoring (20 pixels/meter at target). For 4MP cameras, multiply detection distances by approximately 1.4 times.
| Focal Length | Horizontal FOV | Detection Range (2MP) | Best Use Case |
|---|---|---|---|
| 2.8 mm | ~100° | ~100 m | Compressor room interior, small enclosed area |
| 4 mm | ~80° | ~130 m | Process area general coverage, equipment yard |
| 6 mm | ~54° | ~190 m | Corridor, access road, medium-range process coverage |
| 8 mm | ~40° | ~260 m | Gate entry, loading dock, offshore deck perimeter |
| 12 mm | ~27° | ~390 m | Tank farm, long-range perimeter, flare stack base |
| 25 mm | ~13° | ~800 m | Marine vessel monitoring, long-range perimeter fence |
Varifocal vs. Fixed Lenses for Hazardous Areas
Fixed focal length lenses are simpler, less expensive, and have fewer moving parts to potentially fail in a hazardous environment. For installations where the coverage requirement is well-defined and unlikely to change, fixed lenses are the lower-risk choice.
Varifocal lenses allow the focal length (and therefore the field of view) to be adjusted during commissioning and, in the case of motorized varifocal lenses, remotely via the camera’s web interface. This is valuable when camera positions are determined on paper but final coverage requirements may need fine-tuning after installation. In hazardous area installations where physical camera adjustment requires a permit, a motorized varifocal lens allows coverage optimization from the control room without accessing the classified area. The tradeoff is higher cost and slightly greater mechanical complexity. For cameras in Zone 1 or Class I Division 1, the varifocal motor mechanism is enclosed within the explosion-proof housing and does not add ignition risk.
F-Number and Low-Light Performance
The F-number (aperture) of the lens determines how much light reaches the image sensor. The lower the F-number, the wider the aperture, and the more light the lens passes. An F1.2 lens passes approximately 2.25 times more light than an F1.8 lens at the same focal length — a significant difference for night surveillance in areas without dedicated lighting.
For outdoor hazardous area cameras without IR illuminators, an F1.2 or F1.4 aperture is recommended for adequate night-time performance. For cameras with built-in IR LEDs (common in Veilux explosion-proof camera models), the aperture requirement is less critical because IR illumination provides active lighting — an F1.8 or F2.0 lens is adequate when paired with good IR LEDs. Be aware that extremely wide apertures (below F1.2) have shallow depth of field, which can result in out-of-focus images if the focal point is not precisely set.
IR Cut Filter and Day/Night Performance
Cameras with mechanical IR cut filters (ICR) switch between day mode (filter in, accurate color rendering) and night mode (filter removed, higher sensitivity to IR light). This ICR switching allows the camera to use its IR illuminators effectively at night while providing color images during daylight. Most Veilux explosion-proof cameras include an ICR, but verify this in the specification if color imaging at all times is required — cameras without an ICR have a permanent color balance compromise.
For applications in areas with steam, dense fog, or smoke — such as flare stacks, cooling towers, or hot-process areas — consider whether a standard optical camera can maintain useful imagery. Thermal cameras are better suited to these environments. The comparison between thermal and optical imaging for hazardous areas is covered in our separate thermal vs. optical camera guide.
Lens Considerations for Specific Hazardous Environments
Offshore and coastal environments: Salt spray deposits on optical glass degrade image quality over time. Specify hydrophobic coated lens windows that reduce salt adhesion. Some Veilux housings use sapphire glass windows for superior scratch and salt resistance. High-vibration installations: Compressor rooms and mining operations expose lenses to significant vibration. Varifocal lenses with motorized focus should be specified with vibration locking to prevent focus drift. High-temperature process areas: Proximity to steam headers, heat exchangers, or flare radiation can cause thermal focus drift in lenses without athermalized optical designs. Check the camera’s operating temperature specification against the ambient temperature at the installation position.
When reviewing camera specifications alongside housing requirements, the explosion-proof camera housing selection guide covers window material, housing temperature ratings, and the interaction between lens assembly and housing certification.
Frequently Asked Questions
What focal length is best for outdoor explosion-proof cameras?
4mm for 20-50 meter coverage; 8mm for 50-100 meters; 12-25mm for long-range perimeter. Varifocal lenses allow adjustment during commissioning without lens replacement.
Varifocal vs. zoom lens — what’s the difference?
Varifocal requires refocusing after focal length adjustment. True zoom maintains focus throughout. In explosion-proof cameras, motorized varifocal is most common and allows remote angle adjustment.
Does F1.2 always give better night vision than F1.8?
F1.2 passes more light, but cameras with built-in IR illuminators are less dependent on aperture because they provide their own light source. For unlit areas without IR, F1.2 or F1.4 is recommended.
Can I change the lens in the field on a classified zone camera?
Opening the housing in a classified area may require a hot-work permit and recertification review. Plan lens selection carefully during design. Some cameras have external adjustment without opening the housing.
What lens for license plate recognition at a hazardous area gate?
LPR requires 160 pixels/meter at the plate. For a 5-meter lane at 10-15 meters distance, use a 2MP camera with an 8mm or 12mm lens. Camera should face traffic head-on within 30 degrees.
Veilux offers explosion-proof cameras with fixed and motorized varifocal lenses in focal lengths from 2.8mm to 25mm, with optional sapphire glass windows for harsh environments. Contact our team to confirm the optimal lens configuration for your coverage requirements.
Key Industry Standards and References
Camera housing optical requirements for hazardous areas are defined in IEC 60079-0. Security camera placement guidelines reference NFPA 730. Lens FOV calculations use standard trigonometry per IEC optical equipment standards.
About the Author
Daniel Fernandez
Daniel Fernandez is a hazardous area security systems specialist with over a decade of experience specifying ATEX, IECEx, UL Class I Division 1, and cUL certified surveillance equipment for oil and gas, chemical, mining, pharmaceutical, and offshore environments. He holds expertise in NEC and IEC area classification standards and has consulted on explosion-proof camera system designs across North America, Europe, and the Middle East.