1. How comparable are these Quality option values to each other?

They are not numerically comparable. They share a 0–100 scale and a monotone direction (higher = better), but each number is interpreted by a completely different internal parameter with a different curve. quality 80 in JPEG and quality 80 in JXL are two different things that merely happen to share an integer.

What is comparable is the perceptual tier («near-lossless», “good for web”, “aggressive”), and you can build a per-format mapping for those tiers.

2. What each 0–100 actually maps to 

Two consequences worth internalising:

1. JXL and AVIF are the only two here with a true lossless mode reachable via the quality setting. JPEG and WebP top out at “visually lossless”.
2. JXL and AVIF are inverted internally — a higher ImageMagick quality means a smaller underlying distance/quantizer. So the relationship between the number and the output is not even monotonic in the same shape across codecs.

3. Approximate perceptual equivalence

Roughly equivalent visual tiers for photographic content:

These are estimates, not guarantees. Treat them as starting points and validate on your own content — the exact ImageMagick↔codec mappings have shifted between releases, and content type matters enormously (a screenshot at JPEG 70 is usually fine; a night sky at JPEG 70 is not).

4. File size at comparable visual quality

For photographic content, the size ranking at equivalent perceived quality is roughly:

JXL ≈ AVIF  <  WebP  <  JPEG		

JXL and AVIF typically land 30–50% smaller than WebP at the same visual quality. So if you were previously using one number across all formats, the practical consequence is that AVIF and JXL at a “moderate” setting are already better than JPEG at a “high” setting — and smaller.

5. Practical recommendation: presets, not shared numbers

Rather than pretending the numbers are interchangeable, expose tiers:

/**
 * Per-format quality values that produce a comparable perceptual result.
 * Numbers are NOT interchangeable between formats — these are tuned
 * per codec so that "medium" looks the same everywhere.
 */
const QUALITY_PRESETS = [
    'high'   => ['jpeg' => 90, 'webp' => 88, 'avif' => 65, 'jxl' => 90],
    'medium' => ['jpeg' => 80, 'webp' => 78, 'avif' => 50, 'jxl' => 78],
    'low'    => ['jpeg' => 65, 'webp' => 62, 'avif' => 35, 'jxl' => 62],
];

Usage:

$preset = \PHPThumb\Imagick::QUALITY_PRESETS['medium'];

$thumb = new \PHPThumb\Imagick('photo.jpg', [
    'jpegQuality' => $preset['jpeg'],
    'webpQuality' => $preset['webp'],
    'avifQuality' => $preset['avif'],
    'jxlQuality'  => $preset['jxl'],
]);

This is exactly the argument for a separate jxlQuality option rather than reusing jpegQuality: the numbers differ, so a shared knob would systematically mis-set three of the four formats.

6. Measuring it yourself

Don't trust the table above — measure on your build and your content. Use a lossless PNG as the reference, not the source JPEG, otherwise you measure the source JPEG's own artifacts as if they were the new codec's.

#!/usr/bin/env bash
SRC=reference.png

for fmt in jpg webp avif jxl; do
    echo "=== $fmt ==="
    for q in 40 50 60 70 80 90 100; do
        magick "$SRC" -quality "$q" "/tmp/t.$fmt" 2>/dev/null || continue
        size=$(stat -c%s "/tmp/t.$fmt")
        ssim=$(magick compare -metric SSIM "$SRC" "/tmp/t.$fmt" null: 2>&1 | awk '{print $1}')
        printf "  q=%-4s %9d bytes   SSIM=%s\n" "$q" "$size" "$ssim"
    done
done

Then pick, for each format, the lowest quality whose SSIM still clears your threshold (e.g. 0.98). Those are your real per-format constants.

6.1. Don't forget the effort/speed defines

For the two modern codecs, encoder effort changes file size substantially at identical quality:

# AVIF — slower = smaller, same visual quality
magick photo.jpg -quality 50 -define heic:speed=0 out.avif

# JXL — higher effort = smaller
magick photo.jpg -quality 90 -define jxl:effort=9 out.jxl

So a tuned AVIF at speed=0 can undercut an untuned one at speed=5 at the same quality number — another reason quality numbers alone aren't a reliable cross-format comparison.

7. Caveats

• Chroma subsampling affects JPEG/WebP heavily at lower qualities and is invisible to the quality number itself.
• WebP alpha quality is a separate knob from the main image quality; the quality option here does not control it.
• Graphics/flat colour behave very differently from photos — AVIF/JXL handle flat areas poorly relative to their reputation, and PNG/WebP lossless may win outright.
• The exact ImageMagick mapping is version-dependent. Treat any table (including this one) as a starting point, and verify with the script above if file size or fidelity matters.

8. TL;DR