APO vs Plan Achromatic vs Achromatic Objectives: Which Does Your Inspection Need?

This article is also available in 简体中文.

"APO", "plan achromatic" and "achromatic" describe how many colour errors an objective corrects and how flat its field of view is — which together set the smallest feature it can resolve and the widest field it can keep sharp. This guide defines the three classes, places the MPO APO series and the separate MPT microscope-unit line in context, and maps feature size to objective.

1. What the three correction classes actually correct

• Achromatic — corrected for two spectral lines, typically the red and blue ends. A residual "secondary spectrum" remains, so a small colour focus shift and some spherical aberration persist. Achromats are the simplest and least expensive class, fine for routine brightfield overview.
• Plan achromatic — an achromatic correction plus a field-flattening ("plan") correction. Colour correction is still two-line, but field curvature is corrected, so the image stays sharp from centre to edge rather than only near the axis. Plan long-working-distance objectives are the class POMEAS MPT microscope units are documented to accept.
• Apochromatic (APO) — corrected for three spectral lines, with residual spherical aberration also corrected. That yields the highest colour fidelity and the highest numerical aperture (NA) per magnification, which is why APO designs dominate sub-micron inspection. APO objectives are often plane-corrected too, but "APO" and "plan" are separate properties — confirm both.

2. Where the POMEAS MPO APO series sits

The MPO APO range covers five fixed magnifications, 2× to 50×, with NA rising from 0.055 to 0.42:

Working distance is not monotonic: the 5× objective is longest at 45 mm, while the 50× works at only 20.5 mm. Resolution tracks NA via ≈0.61λ/NA (λ ≈ 0.55 μm) — 5 μm at 2× up to 0.7 μm at 20×. Depth of field narrows the other way, 91 μm at 2× to 1.6 μm at 50×, so focus stability is the practical constraint at the high end. Listed field sizes span Φ12 mm at 2× down to Φ0.48 mm at 50× (FN-24 ÷ magnification).

3. MPT microscope units: the caliber is set by the unit, not the objective

MPT is a separate POMEAS product line — an infinity-corrected microscope unit with fixed magnification, not a zoom body and not the MPO series. Its compatible objective caliber must be read from the unit's own datasheet. The two documented units differ mainly in image field and mount:

Both are documented as infinity-corrected, with coaxial illumination and a working temperature range of -10 °C to +50 °C. Both are stated to accept mainstream brands' plan achromatic long-working-distance objectives, and both offer modular options including brightfield/darkfield illumination, DIC modules, an objective changer and extra camera ports. That is the extent of the MPT datasheets: do not assume that an MPO APO objective, or any specific objective, fits an MPT unit — confirm the caliber against the datasheet for the exact unit before ordering.

4. Reverse lookup: from required feature size to objective

Start from the smallest feature you must resolve, not from magnification:

Two qualifications. First, the camera can become the limiting factor: with a 3.45 μm-pixel camera at 9×, pixel-limited resolution is ≈0.38 μm — finer than the 2× objective's 5 μm rating, so the optics set the limit at the low end and the sensor can matter at the high end. Second, an objective rating describes the objective alone: in a full stack, total magnification = zoom body × TV tube (camera adapter) × objective, and no official zoom-plus-APO combination table exists — a 6.5X zoom body with a 2× APO spans roughly 1.4×-9× (computed by multiplication, not yet verified for this combination). Treat combination FOV and vignetting as estimates until verified for your setup.

5. How to choose — and what to confirm

1. Fix the smallest feature size and the colour sensitivity of your inspection.
2. Pick the correction class: achromatic for tolerance-friendly overview, plan achromatic when edge sharpness over a wide field matters, APO for sub-micron or colour-critical work.
3. Set NA from the MPO table, then use the zoom range for framing — not the reverse.
4. Budget the working distance: APO objectives place the front element 20.5-45 mm from the sample — add fixture clearance.
5. Match sensor caliber to image circle: larger sensors need higher tube magnification — in the 4K series chart, 1″ needs at least a 1× tube and φ32 mm needs 2×.
6. Confirm practical behaviours: parfocality across objective steps, vignetting acceptance at the low-magnification end (attachment lenses in the 0.25×-2× range on large sensors are the documented risk), and lighting through the objective.
7. Ask for a verified combination sheet for your exact zoom, tube, objective and camera.

Need help matching a correction class, NA and working distance to your sample? Contact POMEAS with your feature size, sensor format and WD budget.

Related reading: the zoom lens with APO objectives guide covers combined magnification and low-magnification vignetting, and the complete motorized zoom lens guide compares the zoom-body families, such as the LZ-650104 motorized zoom lens (0.7×-4.5×, WD 82±2 mm, 2/3″).