Borescope depth of field (DOF) is the working distance range where the probe can produce a usable focused image. If the target is closer than the minimum distance or farther than the usable range, the image can become blurry, grainy, poorly lit, or unreliable for inspection decisions.
For inspectors, depth of field is not just a camera term. It affects whether cracks, corrosion, pits, weld features, blade damage, deposits, and surface texture can be seen clearly enough to document. It also works alongside borescope field of view and borescope direction of view, because focus range, viewing width, and tip angle all affect what the inspector can actually see.
Depth of field tells you how close or far the borescope tip can be from the target while still showing a usable image. A stated range may not guarantee field performance because lighting, surface reflectivity, probe angle, focus, and inspection geometry can change the usable image.
On modern VideoProbe systems, depth of field is usually tied to the optical tip being used. The updated Mentor Visual iQ+ optical tip tables list different DOF ranges depending on probe diameter, tip direction, field of view, and measurement type. That is why depth of field should be checked with the actual tip and inspection setup, not treated as one fixed number for the entire borescope.
For example, if a tip has a usable range from a few millimeters out to a defined maximum distance, moving the tip too close can soften the image, while moving too far away can reduce detail, light, and inspection confidence.
Borescope optical terms are connected. Depth of field describes usable focus distance, while field of view and direction of view affect how the target is seen.
Why Depth of Field Matters in Real Inspections
A borescope may have a published focal distance, but the inspection environment decides how useful that range feels in the field. A bright reflective surface at short range is different from a dark tank, pipe, turbine cavity, or engine area.
The original AIT depth of field note warned that manufacturer specifications are not always determined in real-world conditions. Light output can limit whether the stated range produces a usable image.
What Can Reduce Usable Depth of Field?
A published depth of field range should be treated as a starting point, not the only selection factor. Image usefulness can change when the inspection area is dark, reflective, curved, oily, wet, irregular, or difficult to approach squarely.
When possible, move the probe slightly in and out before capturing the final image. The sharpest image is often found by controlling distance, lighting, angle, and articulation together.
How to Select a Borescope With the Right Focus Range
Do not choose a borescope only by diameter or screen size. Focus range should be checked against the actual inspection target and access path.
Pre-Inspection Depth of Field Checklist
Key Takeaways
- Borescope depth of field is the usable focus distance from the objective tip to the target.
- A published range may not guarantee real-world clarity in dark, reflective, or difficult inspection areas.
- Lighting, working distance, viewing angle, surface condition, and tip choice all affect image usefulness.
- Depth of field should be evaluated with field of view and direction of view, not in isolation.
- Before buying or renting, match the focus range to the actual asset, target, access path, and reporting need.
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