Zoom Lens + APO Metallurgical Objectives: The Configuration Matrix

Five APO objectives and four TV tubes on one 6.5X zoom body cover 0.39X to 247.5X — this is the zoom lens with APO objectives grid that decides which cell you order.

Every figure here is a published specification or a calculation from published inputs, marked calculated where it is one. Combination depth of field is deliberately absent: the DOF values belong to the objective, not the assembled stack.

Why the APO Route Exists

A zoom body is a capable microscope substitute up to its resolution ceiling, and the table below is that ceiling for every body in the range.

That published ceiling is comfortable for 5 µm defects and fine for 10 µm features. It stops being fine at 1 µm, where no zoom position creates resolution the front optic does not have.

An APO metallurgical objective changes the optical system rather than adding an accessory to it: the objective becomes the imaging front optic and the zoom body relays and scales what it delivers. That is why its published 5 µm down to 0.5 µm becomes your new ceiling. It is also why NA (numerical aperture) rises with it, from 0.055 at 2X to 0.55 at 50X. The "APO" part is about colour and shows up as reduced fringing on bright metal edges — APO versus plan achromatic versus achromatic covers the correction grades.

Objectives use an M26 × 0.705 thread; adapters cover the usual interfaces, with 75401 the 20.3 mm APO adapter and 75402 the 26 mm APO adapter leading the range. If you have not yet decided that you need an objective at all, the four-path configuration guide starts one level earlier.

The Effective Magnification Formula: TV × Zoom × APO × 0.55

Three multipliers and one constant. The TV tube sits behind the zoom body, the body sits behind the objective, and each contributes a factor:

effective magnification = TV factor × zoom position × APO magnification × 0.55

The 0.55 is a relay factor for this configuration — a confirmed input, not a measured magnification. Leaving it out removes 45% of the naive product (calculated) and is the most common arithmetic error on this route. Two worked lines on the 6.5X body with a 2/3" sensor (8.8 × 6.6 mm):

• 5X APO + 1X TV: 1 × 0.7 × 5 × 0.55 = 1.925X at the wide end and 1 × 4.5 × 5 × 0.55 = 12.375X at the narrow end, so 1.93X–12.38X (calculated), with HFOV from 4.6 mm down to 0.7 mm (calculated).
• 50X APO + 2X TV: 2 × 0.7 × 50 × 0.55 = 38.5X to 2 × 4.5 × 50 × 0.55 = 247.5X (calculated), with working distance pinned at the objective's 13 mm throughout.

Working backwards from a field of view uses the same arithmetic: the magnification-to-FOV walkthrough and the FOV calculator tool.

The Complete Matrix: 5 APO Objectives × 4 TV Tubes

Twenty cells, one per objective-and-tube pair, all calculated on the 6.5X body across its full 0.7X–4.5X zoom span. Read a row to see what one objective does as you swap tubes. Read a column to see what one tube does to every objective.

All 20 cells calculated as TV factor × zoom position (0.7X–4.5X) × objective magnification × 0.55. The 0.67X tube appears in some POMEAS documentation but has no SKU in the selection chart, so treat that column as TV SKU to confirm with POMEAS before ordering.

Reading the columns. The column steps are not equal. The 0.5X to 0.67X step multiplies magnification by 1.34, 0.67X to 1X by 1.49, and 1X to 2X by 2 (calculated). Row steps are larger: 2X to 5X is 2.5, 5X to 10X is 2, 10X to 20X is 2, and 20X to 50X is 2.5 (calculated). The consequence matters mechanically as well as optically: a TV step buys magnification without moving the objective, so it buys it without changing working distance, while an objective step resets both. If a 10X objective already sits at a designed 34 mm clearance, a 2X tube takes that row from 3.85X–24.75X to 7.7X–49.5X and the 34 mm stays where it was.

The 2X row is the survey row — widest fields, lowest magnification: 0.39X with the 0.5X tube is the widest field in the whole matrix, and 9.9X with the 2X tube is the top of the row (calculated). Nothing else in the table reaches below 0.77X.

At 5X you buy compromise: 0.96X–6.19X, 1.29X–8.29X, 1.93X–12.38X and 3.85X–24.75X across the four tubes (calculated), all of them at 35 mm, the longest working distance of the five objectives.

Widest coverage of the mid tiers belongs to the 10X row: 1.93X with the 0.5X tube through 49.5X with the 2X tube (calculated), and its 34 mm standoff is only 1 mm shorter than the 5X objective's.

Clearance starts to bite at 20X: 3.85X–99X across the four tubes (calculated) at a 20 mm working distance. Above 99X only the 50X row is left.

Only one row passes 100X: the 50X, at 9.63X at the bottom with the 0.5X tube and 247.5X at the top with the 2X tube (calculated), at 13 mm — which constrains how the part may be presented more than it constrains the optics.

The diagonals. Because the matrix multiplies, cells on a diagonal are numerically identical. 1.93X–12.38X is reachable as 5X + 1X TV or 10X + 0.5X TV. 3.85X–24.75X is reachable three ways: 5X + 2X TV, 10X + 1X TV, 20X + 0.5X TV. 7.7X–49.5X is reachable two ways: 10X + 2X TV and 20X + 1X TV (all calculated).

This is the most useful property of the table: identical magnification does not mean identical hardware. Same zoom position, same field of view, and the working distance, NA, resolution and depth of field can all differ. A later section works the 3.85X–24.75X triple through in detail.

The 0.5X column halves every row's band against that row's 1X cell (calculated) and is where the wide end of the matrix lives: 0.39X on the 2X objective, 9.63X on the 50X. It is also the most demanding column optically, because the smallest relay factor leaves the beam closest to the edge of the clear aperture — which is why the wide end of a 0.5X configuration is the first thing to check on a real target.

Between 0.5X and 1X sits the 0.67X column, at ×1.34 over the 0.5X cell (calculated). It appears in some POMEAS documentation but has no SKU in the objective and tube selection chart, so quote it as TV SKU to confirm rather than as a delivered part, and be ready to redesign around the 0.5X or 1X column if it is not available.

Reference for the whole grid is the 1X column: no magnification is added behind the objective, so the band is simply the objective's magnification times the zoom span times 0.55. That is why 1.93X–12.38X on the 5X objective and 3.85X–24.75X on the 10X objective are the two bands a 1X tube gives you (calculated) — and the two that appear in existing drawings most often.

Doubling the 1X band is what the 2X column does (calculated), and what makes 247.5X reachable on the 50X objective. It also ends the useful life of the low objectives: 2X with a 2X tube tops out at 9.9X (calculated), so if the target sits above 10X the 2X objective is out of the running whatever tube you already own.

Where the matrix stops helping. Three cells need care before they go on a drawing:

• The 50X row needs 13 mm of clearance and near-sub-micron focus repeatability, so it usually arrives with a dedicated fixture rather than a retrofit.
• The 2X row at 0.5X TV is the widest field and the most forgiving cell, but it is also the one where a 1" sensor is most tempting and most wrong, because the body's image circle is 2/3".
• The 0.67X column carries an unresolved SKU, which makes it the only column where the optics are known and the part number is not.

In millimetres. Magnification spans are hard to feel in a design review, so here are five cells as horizontal field of view on a 2/3" sensor (8.8 mm wide), each calculated as 8.8 ÷ effective magnification.

Note the direction of the trade: one row down multiplies magnification by 2 to 2.5 and divides field of view by the same factor (calculated), while working distance falls from 35 mm at 5X to 13 mm at 50X. A 4 mm field of view comes from the 5X + 1X cell at 35 mm; the 20X + 1X cell gives 1.1 mm at 20 mm. Same table, different job.

One row, four tubes. The same numbers as a vertical slice make the tube decision concrete. Below is the whole 5X row in millimetres of horizontal field on 2/3", with the working distance that never moves; HFOV is calculated as 8.8 ÷ effective magnification.

Read the second and fourth rows together and the trade is obvious: the field shrinks by a factor of four across the row (calculated) and the only thing that changed is a tube behind the objective. If a moving part needs 8 mm of field at 35 mm of standoff, the 5X objective covers it — at the wide end with the 0.5X tube, and further up the zoom with the 1X tube.

If your body is a 12.5X, every cell shifts. The zoom span becomes 0.58X–7.5X, so 5X APO + 1X TV becomes 1.595X–20.625X instead of 1.93X–12.38X (calculated); every other cell moves by the same two ratios — 0.829 at the low end (0.58X ÷ 0.7X) and 1.667 at the high end (7.5X ÷ 4.5X) (calculated).

Do not copy a number out of this table into a 12.5X quotation — recompute it. The 12.5X body earns its place on distortion and on 3.05 µm resolution from 4.5X, not by inheriting this grid; APO magnification and vignetting covers the optical side of that body choice, and the configuration guide on which body to start from covers the body-level comparison.

Working Distance on the APO Route Is Set by the Objective

On an A-class route the front objective changes working distance and magnification together. The APO route does not: the objective is the front optic and owns the working distance, while the body and tube behind it only scale the image.

The 5X objective demonstrates it cleanly: 0.96X–6.19X with a 0.5X tube, 3.85X–24.75X with a 2X tube (calculated). Magnification and field of view both move by a factor of 4; working distance stays 35 mm.

Published POMEAS objective specifications. The DOF column is the objective's published depth of field; depth of field for a zoom-plus-objective combination is not published and should not be inferred by scaling these figures.

Two consequences: clearance is a physical constraint you cannot solve optically, so if the fixture holds the part 25 mm away the 20X and 50X objectives are not candidates; and the 10X's 34 mm is only 1 mm less than the 5X's 35 mm, which usually makes 10X the better mid-magnification buy. The 2X, 5X, 10X, 20X and 50X pages carry the drawings; working distance versus depth of field covers the standoff-versus-quality trade.

Same Magnification, Three Different Setups

This is the comparison the matrix exists to make, and it is the one worth memorising: three configurations on the 6.5X body produce exactly 3.85X–24.75X, and they are not interchangeable.

The arithmetic is trivial once you see it. The objective × TV product is 10 in all three cases: 5 × 2, 10 × 1, 20 × 0.5. Multiply by the 0.7X–4.5X zoom span and by 0.55: 0.7 × 10 × 0.55 = 3.85X at the wide end, 4.5 × 10 × 0.55 = 24.75X at the narrow end (calculated). Because magnification matches at every zoom position, field of view matches too — on 2/3" that is 2.3 mm down to 0.36 mm (calculated) for all three.

Magnification band is 3.85X–24.75X for all three rows (calculated). NA, resolution and DOF are the published figures of the objective in each row; they describe the front optic, not measured performance of the assembled stack.

5X + 2X tube: the clearance option. Thirty-five millimetres is the largest working distance in the matrix, which matters when the part is hot, moving, or fixtured in a way you cannot bring closer. You pay in resolution — 2 µm instead of 0.7 µm, a factor of nearly three (calculated) — and you are repaid with the roomiest depth of field of the three at 14 µm, often the difference between a usable image and a permanent refocus battle on a non-flat surface.

10X + 1X tube: the default. Thirty-four millimetres is within a millimetre of the best case, resolution halves to 1 µm and depth of field drops to 3.5 µm. If the requirement says nothing more specific than "high magnification", this row offends the fewest constraints.

20X + 0.5X tube: the detail option. This is the only one of the three that resolves below 1 µm, bought with a 20 mm standoff and a 1.6 µm depth of field — eight times shallower than the 5X row (calculated). It also puts the smallest TV factor behind the objective, which is exactly where corner fall-off becomes a review item rather than a footnote; run the checks in low-magnification corner vignetting before committing to a 0.5X tube at the wide end of the zoom.

One higher-band pair is worth knowing. 10X + 2X TV and 20X + 1X TV both give 7.7X–49.5X (calculated). The first has NA 0.28, 1 µm resolution, 3.5 µm DOF and 34 mm of standoff; the second has NA 0.42, 0.7 µm resolution, 1.6 µm DOF and 20 mm. If the specification reads "see detail at around 30X", both satisfy the magnification clause and only one satisfies the resolution clause.

How to choose between the three: clearance first — which rows survive your fixture. Resolution second — which survivor reaches your feature size. Depth of field third — which survivor tolerates your surface flatness. Illumination last — whether the survivor still leaves enough light at the wide end. Teams that start from a magnification number usually end up buying a second objective.

What the zoom span itself contributes. Inside any cell, the body's own 0.7X–4.5X travel is a 6.43:1 ratio (calculated), so one objective-and-tube pair already covers a factor of 6.4 without changing hardware. That is why the matrix is read as bands rather than single magnifications: the tube sets the floor of the band and the zoom sets how far above that floor you can travel. A requirement spanning more than 6.43:1 needs two cells — and in practice that means two tubes, not two objectives.

Writing it into a specification. Name the objective and the tube as a pair, because "3.85X–24.75X" identifies neither. State the objective first with its NA and working distance (5X, NA 0.14, 35 mm), then the tube (2X), then the band and the field it produces. The working distance belongs in the same paragraph as the fixture callout, since it is the one figure that survives every tube change and the first number a mechanical designer will look for.

If you expect to change your mind. The tube is behind the objective and the objective owns the working distance, so swapping tubes later is the reversible move: the fixture stays, the band moves, and a 10X objective can serve 1.93X–12.38X today and 7.7X–49.5X next quarter without touching the machine.

Swapping objectives is not reversible in the same way — 5X to 20X takes the standoff from 35 mm to 20 mm and puts the fixture back on the drawing board. When the target magnification is still moving, pick the objective by clearance and use tubes to absorb the uncertainty.

Magnification Is Not Resolution on the APO Route

A matrix full of magnification numbers invites one mistake: assuming a bigger number sees smaller detail. Magnification scales the image; resolution is the smallest detail the front optic can transfer, and the two are published separately.

The 50X objective is where this matters most. Its published resolution is 0.5 µm — an optical limit of the objective, not a promise that a 0.5 µm feature will be visible. Seeing something that small also needs enough sampling on the sensor, enough contrast against its surroundings and enough light to expose the frame at a usable time.

The same caution applies to 1 µm at 10X and 0.7 µm at 20X: they say what the objective can transfer, and the rest of the chain has to keep up. Why magnification is not resolution works through the sampling argument, and the microscope-to-machine-vision guide covers the sensor side.

Where Each APO Tier Belongs: 2X to 50X

Read this as a routing table, not a ranking: each objective owns a band across the four tubes at a fixed working distance.

The M Plan Apo L 20X solves a different problem: 20× at a 29.5 mm working distance against 20 mm for the standard 20X, with NA 0.34, 0.8 µm resolution and ±2.38 µm depth of field — roughly 9.5 mm more clearance for some resolution and some depth (calculated). Choose it when the fixture, not the optics, dictates the gap; zoom versus objective plus laser autofocus covers wafer and die cases, and the M Plan Apo L 20X page carries the specification.

DOF Budget: 91 µm Down to 0.9 µm

The matrix spans more than 600 times from its lowest to its highest cell (calculated); depth of field moves the other way, by a factor of about 100.

Notice what the table does not contain: a depth-of-field figure for any assembled configuration. That number is not published and should not be manufactured by scaling these values. Treat the objective DOF as your starting budget and confirm the assembled result on your own sample; the real-case walkthrough shows how much part flatness and stage motion consume.

Motorized Zoom + APO: When the Combination Pays Off

A motorized body lets you traverse a cell of the matrix without touching hardware: the 6.5X body is supplied as PMS-LZ-63100 / LZ-650104 at 82 ± 2 mm working distance in the motorized version, 87 ± 2 mm manual and rail, with LZ-650105 as the coaxial variant.

It pays when one recipe must serve two magnifications — an overview pass, then a zoomed pass — because the APO route holds working distance constant, so a motorized sweep never moves the focal plane. It does not pay when the recipe always runs at one magnification; manual versus motorized selection covers that case, and motorized zoom + APO for semiconductor wafer inspection shows the combination at work. Where focus must close automatically at the top of the range, pair the route with an autofocus unit rather than a camera-side algorithm: APO autofocus, motorized zoom lens guide.

Vignetting and Illumination Checks

Two checks before any cell becomes a purchase order. First the image circle: the 6.5X and 12.5X bodies are specified for 2/3", and a larger sensor vignettes however the matrix is configured — see matching sensor size to image circle.

Second, corner brightness at the wide end: a small TV factor pushes the beam toward the edge of the clear aperture, so 0.5X tube at 0.7X zoom is the least forgiving cell in every row. Review it on a real target; if the corners go dark, a larger tube with a smaller objective usually fixes it — vignetting behavior of APO objectives on a zoom body explains why.

Selecting Step by Step (Summary Card)

FAQ: APO Configuration Questions

How do I cover 2X to 50X with APO objectives on a zoom body?

Work from the matrix: find the row whose band contains your target, then pick the TV column that lands inside it. On the 6.5X body the 2X objective with a 0.5X tube starts at 0.39X and the 50X objective with a 2X tube reaches 247.5X (calculated).

Which TV tube should I pair with each APO objective?

Choose the tube so that objective times TV factor equals the band you need, because that product sets the whole zoom span. A 5X objective with a 1X tube and a 10X objective with a 0.5X tube both give 1.93X–12.38X, at 35 mm and 34 mm of working distance.

Why do three different setups give exactly 3.85X–24.75X?

Because the objective times TV product is 10 in all three: 5X with a 2X tube, 10X with a 1X tube, 20X with a 0.5X tube. Ten times the 0.7X–4.5X zoom span times 0.55 gives 3.85X–24.75X (calculated), so only working distance, NA, resolution and depth of field differ.

Does working distance change when I change the TV tube or the zoom setting?

No. On the APO route working distance belongs to the objective: 35 mm at 5X, 34 mm at 2X and 10X, 20 mm at 20X and 13 mm at 50X. Changing the tube or the zoom position changes magnification and field of view only.

Why is magnification not resolution?

Magnification only scales the image; resolution is the smallest detail the objective can transfer, published separately as 2 µm at 5X down to 0.5 µm at 50X. Seeing a feature that small also needs enough sampling, contrast and light, so the figure is a limit, not a guarantee.

When should I choose the M Plan Apo L 20X instead of the standard 20X?

Choose it when 20 mm of clearance is not enough. The M Plan Apo L 20X gives 20× at 29.5 mm with NA 0.34, 0.8 µm resolution and ±2.38 µm depth of field, so you gain about 9.5 mm of reach in exchange for some resolution and depth.