A borescope's depth of field is the working distance window over which the tip renders a usable focused image, and it is a tip-level rather than a system-level spec on modern VideoProbe platforms such as the Mentor Visual iQ+ [S1]. Direction of view (DoV), field of view (FoV) and DoF are coupled optical parameters, so a 90° FoV selection is never independent of the focus range the same tip can deliver [S1][S2].
Industrial videoscope FoV is quoted in horizontal degrees (H°) or H° × V°, and FoV inversely tracks on-screen magnification: a wider angle shows more area but smaller features, a narrower angle shows less area but larger features [S2]. Standard borescope FoV offerings span roughly 40° (normal) up to 120° or wider on articulating video probes, with one product line listing 90°, 105°, 120°, 135° and 150° options for interchange [S3][S6].
How DoF and FoV interact on the same optical tip
FoV tells the inspector how wide an area the tip can see at once, and it is a primary driver of how much context fits in a single frame [S7]. A 120° FoV is a "wide" optic that orients the inspector quickly inside a turbine cavity, combustion liner, or large pipe, but features such as cracks, pits, weld undercut and corrosion pitting appear smaller in the frame at any given stand-off [S2][S7].
A 90° FoV sits closer to the "normal" lens range that most inspection guides recommend as a default: GLC describes about 40° as a typical "normal" objective, and Teslong notes that 90° framing lets the user see the entire frame at once without sweeping [S3][S4]. The trade is concrete: a 90° tip renders a feature at, for example, twice the on-screen size of a 120° tip at the same distance, which directly supports measurement and acceptance decisions [S2]. DoF is layered on top: a tip with a stated 6 mm to infinity focus range will lose usable detail if the tip is pushed inside that minimum, and the far end will go grainy under low light, regardless of whether the FoV is 90° or 120° [S1].
Decision matrix: 90° vs 120° FoV across the criteria that matter
The two main options should be compared on the criteria that drive an inspection outcome. Below is a criterion-by-criterion read for the two most common videoscope FoV bands (90° and 120°), grounded in the research: [S3]
Area covered per frame. 120° FoV shows a noticeably larger patch of the target surface, which speeds general surveys and reduces the number of articulation moves needed to map a chamber [S2][S7]. 90° FoV frames roughly 30–40% less area at the same stand-off, so more probe repositioning is required to document a region [S2].
On-screen feature size. A 90° FoV makes a given defect appear larger than the same defect under a 120° FoV at identical stand-off, which improves the ability to identify small cracks, pits, and weld features [S2]. 120° FoV shrinks the same feature in the frame, which can push marginal defects below the documentation threshold [S2].
DoF compatibility. Wide-FoV optics (e.g. 120°) are commonly paired with shorter focus ranges suited to close-up work; standard/narrow FoV tips (around 40–90°) are usually offered with both close-focus and long-focus variants to cover stand-offs from a few millimetres out to a defined maximum [S1][S6]. Choosing FoV without checking the matching DoF band is the most common cause of soft, unusable images in the field [S1].
Orientation vs measurement. 120° FoV is the better orientation tool when first entering an unknown cavity; 90° FoV (or narrower) is the better measurement and documentation tool once the target is located [S2][S7]. Many spec-driven workflows therefore run a wide-FoV tip for navigation and a normal/narrow-FoV tip for the acceptance stills and video [S2].
Probe diameter constraint. Small-diameter probes (≤2.8 mm) used in aerospace turbine blade work are typically limited to a narrower set of FoV/DoF combinations than 6 mm and larger petrochemical or power-generation probes, so the 90°/120° choice may already be fixed by the probe OD [S2].
When to choose 90° FoV (and when not to)
Choose 90° FoV when the inspection is a documentation pass: weld visual examination, blade leading-edge damage measurement, corrosion pit depth assessment, and any acceptance decision that needs a clear, large-on-screen feature. It is also the right default when the access path is straight and lighting is marginal, because a narrower FoV tolerates lower light better per unit area [S2][S3].
Do not choose 90° FoV when the inspector first needs to find the target in a large volume, such as a 12-inch-diameter pipe or a combustion chamber, where sweeping with a narrow FoV costs time and frame coverage [S5]. Do not force 90° FoV onto a 2 mm or smaller micro-borescope where the tip's optical prescription may not support that FoV at the required DoF band [S2].
When to choose 120° FoV (and when not to)
Choose 120° FoV (or wider, e.g. 135°/150° where the OEM offers it) for orientation passes, large-bore surveys, and any inspection where losing the target is the bigger risk than missing a sub-millimetre defect [S2][S6][S7]. A 120° tip frames more of the target per articulation move, so the inspector spends less time hunting and more time documenting.
Do not choose 120° FoV as the only tip when the deliverable is a measurement-grade still or a weld acceptance record: at the same stand-off, a 120° image renders the defect smaller than a 90° image, and the same on-screen pixel count now covers a larger physical area [S2]. Do not use 120° FoV at the very close end of a tip's DoF range, because the wide angle magnifies the loss of focus at the frame edges [S1].
DoF selection rules that keep both FoV choices usable
Treat DoF as a tip-level number, not a system-level number, and verify it against the inspection geometry before committing to a still or measurement [S1]. Modern optical tip tables list different DoF bands for each combination of probe diameter, DoV, FoV, and measurement type, so a 6 mm 0° 120° tip and a 6 mm 90° 120° tip on the same platform will not share the same focus range [S1].
Hold the tip inside the published minimum distance only with caution: even on a target that is easy to reach, pushing inside the near-focus limit softens the image, and wide-FoV tips (120°) are the first to show edge-of-frame blur when the stand-off is wrong [S1]. Drive the tip back from the target to recover a sharp image before any acceptance capture, and re-light if the far end of the DoF window goes grainy [S1][S7].
Confirm direction of view (DoV) before selecting FoV. A 0° DoV is best for locating defects as the probe is fed into a part, a 90° DoV is best for sidewall inspection, and dual-view tips give both options for thorough work; the FoV choice (90° vs 120°) is then layered on top of that DoV decision [S2]. Practical tip-selection flow: pick DoV first, pick FoV second based on whether the pass is orientation (120°) or documentation (90°), then confirm the DoF range of that specific tip before entering the part [S1][S2].
Common failure modes and field limits
Soft or unusable images are usually caused by one of three things: tip stand-off outside the stated DoF range, lighting too low to support the far end of the DoF, or FoV/DoV mismatch with the inspection geometry [S1]. A reflective, oily, curved, wet, or dark target narrows the usable DoF well inside the published number, regardless of whether the tip is 90° or 120° [S1].
Probe OD sets a hard physical limit on the FoV/DoF combinations available, with 2.8 mm and below reserved for small-ID aerospace passages and 6.0 mm and above standard in petrochemical and power-generation work, so the FoV debate is moot when the access path dictates the only probe that fits [S2]. Micro-borescopes with sub-millimetre fibre bundles or 2 mm articulating tips exist, but they trade image resolution and tip robustness for access, which is a separate decision from FoV/DoF [S5].
Sourcing, standards, and what to confirm before buying
Manufacturer DoF numbers are typically not measured under field lighting, so the stated range is a starting point rather than a guarantee of usable image quality in a dark turbine cavity or a reflective tank [S1]. The current OEM guidance, including updated optical tip tables on platforms such as the Mentor Visual iQ+, should be referenced at the probe-diameter, DoV, FoV, and measurement-type level before any tip is locked into a work order [S1]. For broader selection logic, an industrial borescope reference lays out the rigid vs flexible vs video decision that sits upstream of any FoV/DoF choice.
Cross-reference the vendor's FoV and DoF numbers with at least one independent product listing: PCE Instruments documents interchangeable FoV options of 90°, 105°, 120°, 135° and 150° on a single videoscope platform, which is a useful sanity check that the FoV band an OEM quotes is real and not a marketing rounding [S6]. Gradient Lens Corporation's rigid borescope line describes about 40° as a "normal" FoV and uses 0°/30°/90° DoV, which gives a concrete baseline for what "narrow" and "wide" mean in practice [S3]. Yateks independently frames DoF as the "focus range" or "effective focus range" and treats it as the main factor in judging whether an industrial borescope is good or bad, reinforcing the rule that DoF and FoV must be selected together rather than separately [S8]. For adjacent selection context where comparable trade-offs are spelled out, the bimetal vs liquid-in-glass thermometer spec map shows the same "narrow range, higher accuracy" vs "wide range, lower resolution" logic applied to temperature instrumentation.
Trackable signals for the next decision cycle: (1) whether your current videoscope platform's optical tip table lists a dedicated 120° tip with a confirmed long-DoF variant for the probe diameters you actually use, and (2) whether the inspection procedure requires 0°, 90° or dual DoV before the FoV/DoF combination can be finalised [S1][S2].
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