Crafting a Custom Focus Target for Your Coolscan
The Coolscan family sits in a category of its own. Whether you run a V ED, an IV ED, an LS-50, or a later LS-9000, the lens and sensor resolution is high enough to expose subtle focusing errors that would otherwise hide in lower-resolution workflows. A properly built focus target turns that sensitivity into a calibration tool, giving you repeatable sharpness across hundreds of archival slides.
Most beginners chase focus by feel, eyeballing the preview pane and trusting the auto-focus built into Nikon Scan. Auto-focus on a Coolscan is genuinely capable, yet it drifts over time as the AF sensor ages, the film holder wears, and ambient temperature shifts the lens mount by fractions of a millimetre. A printed target gives you a neutral, repeatable reference that does not depend on any particular slide being sharp to begin with.
In Australian conditions the case for an external target becomes stronger. Workshops from Brisbane to Perth run hot in summer, and the coastal humidity that settles across Sydney and Melbourne's bayside suburbs can leave mounted slides slightly curved after years in storage. A focus target sits flat on the glass, isolating optical performance from those storage quirks and giving a fair point of comparison if you ever take a slide to a local service desk.
Once you have built the target and dialled in focus for the first time, you will probably realise how much soft work you have been keeping. The project costs almost nothing, takes an afternoon, and pulls you back into the physical side of photography that draws many of us to film.
Why a Focus Target Matters for High-Resolution Film Scanning
The Nikkor lens inside a Coolscan resolves around 4000 to 6300 samples across a 35mm frame, depending on the model. That is roughly a 4000 dpi scan, well above what most inkjet prints record. With that resolving power, even a few microns of focus error shows up on screen, especially on fine grain patterns, hair, fabric weave, or distant foliage scanned from landscape Kodachromes.
Critical focus differs across models. The LS-50 and earlier IV ED use a focus motor with limited travel, so the acceptable window is narrower. The LS-5000 and LS-9000 use a high-resolution stepping motor with finer increments, but they are more sensitive to film flatness and holder alignment. Whichever unit you run, you are chasing the inflection point where the modulation transfer function peaks, and the only honest way to find it is with a known target.
A custom target also helps you decide whether a service is worthwhile. If your focus test reveals asymmetric blur, the lens block may need a rebuild by a specialist workshop. If both sides of a Siemens star blur equally, the lamp housing is likely drifting and may need a gentle clean. For day-to-day use, the target becomes your sanity check between archival jobs.
Choosing the Right Test Pattern for 35mm Work
There is no single correct pattern, but a few have earned their reputation over decades of optical bench work. The USAF 1951 chart, with three-bar elements arranged in groups, is the classic choice because each element corresponds to a known line pair per millimetre value. A Siemens star is faster to read visually, because the point where the wedges blur into grey tells you roughly where the resolution limit sits, even if it is less precise than a labelled USAF chart.
Many owners combine elements. A small USAF 1951 block gives you absolute numbers, an NBS 1963A spatial frequency chart lets you confirm contrast response, and a Siemens star offers quick visual confirmation during routine sessions. The lower corner can host a Cobb slanted edge, handy for computing a modulation transfer function in software.
Whatever pattern you choose, make sure it is symmetrical and includes black-to-white transitions in both orientations. Stage tilt tends to show up as horizontal-versus-vertical asymmetry, and a target with only one orientation will hide that flaw. Always work from a printed copy rather than a screen image, because screens have their own pixel structure that interferes with the test.
Materials and Tools You Will Need
The list is short and inexpensive, which is one of the pleasures of this exercise. You will want a laser printer or photo printer capable of at least 600 dpi, a sheet of matte presentation paper around 160 to 200 gsm, a precision steel ruler, a scalpel or fresh hobby blade, a clean glass plate from a picture frame, and a small spirit level or a phone-based level app.
You will also want a sturdy mounting medium. Unmounted slides are scanned on the standard glass holder with their edges resting on the lip, so the target needs the same 35 x 23 mm footprint as a regular mount. Some owners mount the target inside a transparent plastic frame from suppliers such as Lighthouse or Print File, both of which ship to Australia through local distributors. If you prefer a strip of unmounted negatives, cut the target to fit a six-frame strip and feed it through the optional strip holder.
If your Coolscan is overdue for a service, it is worth pairing this build with a light diffuser refresh, since focus checks read more reliably when lighting is even. This light diffuser walkthrough is a useful companion for the calibration step.
Printing and Mounting the Target with Care
Lay out your chosen patterns in a graphics editor at the exact 35 x 24 mm slide footprint, using vector shapes wherever possible so edges stay crisp. Print a test sheet on plain A4 first, measure with digital calipers or a steel ruler, and confirm dimensions are accurate to within a tenth of a millimetre before committing to better paper. Many printers stretch slightly along the long axis, so check both directions.
Once the print is acceptable, cut the slide using the scalpel against the steel ruler on a self-healing mat. Hold the blade perpendicular and use light pressure to avoid tearing the paper edges, which can shed fibres onto the glass. If you use transparency film for an overlay, set the printer to the heaviest matte setting rather than glossy, as glossy surfaces throw specular highlights that confuse the auto-focus sensor on some models.
Mount the finished piece as you would any normal slide. Wipe the glass plate and the printed surface with a rocket blower first, because a single dust mote over your test marks undoes the whole exercise. For unmounted strips, sandwich the target between two thin glass carriers from an old damaged slide mount, taped along the edges so it stays flat during travel through the strip feeder.
Calibrating Your Coolscan Using the New Target
Place the mounted target in the centre of the slide holder with the test patterns oriented vertically. Open Nikon Scan or your preferred VueScan profile, switch autofocus off, and run a quick preview at 1500 dpi so you can see the full frame. The Siemens star should pop into view immediately if focus is somewhere near correct.
Now perform a manual focus sweep. Move the focus slider one step at a time from the lowest to the highest setting, watching a specific USAF group near the resolution limit of your scanner. The V ED and IV ED will show meaningful differences across about six to eight steps; the LS-5000 wants finer increments, ideally two steps at a time. Note the exact slider position where the target is sharpest, then move three steps each way and confirm sharpness falls off symmetrically.
If your scan reveals cleaner results toward one end of the slider range, you have probably discovered the true focus sweet spot. Save the position as a preset in Nikon Scan so future sessions start from the correct baseline. Repeat the test whenever the room temperature swings more than about five degrees, especially during a Sydney summer afternoon or a Perth heatwave shift, as lens mount drift is most noticeable in those conditions.
Comparing Focus Patterns Side by Side
Different targets suit different scanners, scanning routines, and patience levels. The four patterns most commonly used by Australian owners, along with the routines they pair well with, are summarised below for quick reference before you commit to a single design.
| Pattern | Best use | Readability | Setup effort |
|---|---|---|---|
| USAF 1951 | Absolute lp/mm measurements | High with chart in hand | Medium |
| Siemens star | Quick visual sanity check | Very high | Low |
| NBS 1963A | Contrast and tonal response | Medium | Medium |
| Cobb slanted edge | MTF calculations in software | Low without tooling | High |
The USAF 1951 remains the reference standard for serious archival work because the chart reads directly in line pairs per millimetre. The Siemens star is the everyday choice when you want to confirm yesterday's scan is as sharp as today's. The NBS chart and Cobb edge are specialist options best added once you are comfortable with the basics. Most owners eventually settle on a hybrid card that combines a Siemens star in the centre with a small USAF block in one corner.
Once built, the target rewards you with sharper scans for years. The pattern sits in the holder, the focus position holds its setting, and your archive of family slides, weekend bushwalking trips to the Blue Mountains, or quiet mornings chasing bream in Corner Inlet will suddenly look the way you remember. Treat it like a workshop instrument and revisit it whenever the scanner feels off, much as you would re-zero a level or sharpen a chisel.