How far can the focuser move before defocus shows in your image? This uses Don Goldman's New Critical Focus Zone (NCFZ) — which, unlike the old 4.88·λ·f² rule, accounts for your aperture, seeing and how much focus error you're willing to accept.
Optics & conditions
Focus tolerance
New Critical Focus Zone
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microns (total)
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Effective focal length—
Effective f-ratio—
Focuser steps across the zoneat your step size—
Suggested autofocus step≈ ⅓ of the zone per sample—
At a strict 10% tolerance—
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Why focus tolerance matters so much
Focus is the one variable you cannot fix in processing. If your stars are bloated at capture, no amount of stacking or sharpening will recover the detail. The critical focus zone (CFZ) tells you how much room for error you have: the band of focuser travel within which stars stay acceptably sharp. The faster your telescope, the smaller that band becomes — often just a few microns.
Critical Focus Zone vs the New CFZ (NCFZ)
The classic CFZ formula was based purely on the diffraction limit. The New Critical Focus Zone refines this by factoring in real-world detector and atmospheric tolerance, giving a slightly larger, more practical figure that matches what imagers actually see at the camera. For modern CMOS astrophotography the NCFZ is the better target, and it is what this calculator reports.
How focus tolerance shrinks with focal ratio
Because depth of focus scales with the square of the focal ratio, a fast astrograph is dramatically less forgiving than a slow one:
Focal ratio
Relative focus tolerance
f/4
Very tight — electronic focuser essential
f/7
Moderate — careful manual focus possible
f/10
Forgiving — roughly 6× the tolerance of f/4
How to nail focus and stay in the zone
Use a Bahtinov mask or, better, an automated focus routine that measures star size across a range of focuser positions and parks at the minimum. Most importantly, account for temperature: as the night cools, your tube contracts and focus drifts out of the CFZ. An electronic focuser with temperature compensation, or a scheduled refocus every degree or two, keeps you sharp from dusk to dawn.
The New Critical Focus Zone
When you rack a focuser in and out, stars stay tight over a small range of travel, then bloat. That range is the critical focus zone. For years it was estimated with CFZ = 4.88·λ·f², but Don Goldman showed that formula is flawed — it treats some focus error as "non-measurable," when in reality all focus error is measurable. His New Critical Focus Zone (NCFZ) instead asks: how much focus error has a negligible effect on the image, given your seeing?
The formula
NCFZ = 0.00225 × θ × √τ × A × f² (microns), where θ is total seeing FWHM in arcseconds, τ is the focus tolerance as a percentage of the seeing, A is aperture in millimetres, and f is the effective f-ratio. You can be off by half of it in either direction (±NCFZ⁄2). Example: a 106 mm f/5 scope in 3″ seeing at a 15% tolerance gives 0.00225 × 3 × √15 × 106 × 25 ≈ 69 µm (±35 µm).
Why seeing and tolerance matter
In poor seeing, stars are already fat, so a little defocus matters less — the NCFZ grows. In excellent seeing it shrinks, demanding tighter focus. The tolerance τ is your call: a stricter 5–10% keeps focus error well below the seeing for critical work; 15–20% is a practical everyday target. Because the zone scales with the square of f-ratio, fast systems are far more demanding.
Frequently asked questions
How is this different from the old CFZ?
The traditional 4.88·λ·f² depends only on wavelength and f-ratio and assumes a fixed wavefront criterion. Goldman's NCFZ replaces wavelength with your actual seeing and a chosen tolerance, so the answer reflects the conditions you're really imaging in.
What tolerance should I use?
15% is a sensible default for deep-sky imaging. Drop to 5–10% for premium refractors, planetary or lucky imaging where you want focus error well inside the seeing; relax toward 20% in poor conditions.
How does this relate to autofocus step size?
Pick an autofocus step that moves the focuser by roughly a third of the NCFZ, so several samples land inside the sharp zone and your V-curve is well defined. You'll need your focuser's microns-per-step to convert.
Does temperature change focus?
Yes — tubes shrink as they cool, shifting focus by tens of microns over a few degrees. If your NCFZ is small, plan to refocus through the night or use a temperature-compensated focuser.
Formula: Don Goldman's New Critical Focus Zone, NCFZ = 0.00225·θ·√τ·A·f² (GoldAstro / GoldFocus). A guide; your focuser's resolution and real-time seeing also matter.
Frequently asked questions
What is the critical focus zone?
The critical focus zone (CFZ) is the small range of focuser travel within which stars stay acceptably sharp. Move outside it and stars visibly bloat. The CFZ shrinks as focal ratio gets faster.
What is the New Critical Focus Zone (NCFZ)?
The New Critical Focus Zone is an updated formula that accounts for both diffraction and sensor tolerance, giving a more realistic focus tolerance than the older critical focus zone equation.
Why does focal ratio affect focus tolerance?
Faster focal ratios concentrate light into a steeper cone, so the in-focus region is much thinner. An f/4 system has a far smaller CFZ than an f/10 one and demands far more precise focusing.
How do I use the critical focus zone?
Keep your focuser within the CFZ for the whole session. If the CFZ is only a few microns you will need an electronic autofocuser and should refocus as temperature shifts the focus point.