Unreadable fax: fixing scanning, resolution and contrast

Short answer: nine out of ten "unreadable" faxes have nothing to do with the line, and everything to do with the document being sent. A fax is a pure black-and-white image, with no real greyscale, transmitted at 200 × 100 dpi (standard mode) or 200 × 200 dpi (fine mode). Anything light grey, screened, coloured, photographed at an angle or overloaded with pixels either vanishes or turns into black mush. The three settings that save a fax: scan in black and white at 200 or 300 dpi, increase contrast rather than brightness, and stick to A4 with margins of at least 10 mm.

About MondialFax. This article is a technical guide, independent of any manufacturer or carrier. MondialFax sends faxes free of charge and without registration to seven destinations (France, Belgium, Germany, Spain, Italy, Portugal, Canada). It does not assign a number and does not handle reception: the reception scenarios mentioned assume a third-party provider. See receiving a fax and our fax-to-email guide.

The scenario is always the same. The transmission report says "OK". The page went out, the call duration makes sense, the page count matches. And yet, an hour later, the recipient calls back: "we can't read it, can you resend?". So you resend. Nothing changes, of course, since you're sending the exact same file.

That's the blind spot of faxing. Hours get spent diagnosing the line, T.38, the SIP gateway — see our guide My fax won't go through — when the problem lies upstream, in the way the document was scanned. Fax isn't a channel that carries PDFs: it's a channel that carries bitonal images. Understanding that constraint changes everything.

What a fax actually carries

The Group 3 fax machine, standardised by ITU-T in recommendations T.4 (image coding) and T.30 (session protocol), was never designed for photographs. It encodes each page as a sequence of black or white pixels, with nothing in between, compressed using MH, MR or MMR (T.6).

There are only a handful of standardised resolutions:

Two immediate practical consequences.

First consequence: standard mode, still enabled by default on many older machines, squashes the image vertically. Text in 9 or 10 point becomes a row of dashes. Switching to fine mode is the single action that fixes the largest number of cases.

Second consequence: there is no grey. A grey fill, a pencil signature, a faded stamp, a passport photo: all of it gets converted by dithering into a cloud of black dots. At 200 dpi, that cloud becomes a smudge. This is why a photocopied ID card comes through badly, and why an Excel table with alternating grey row shading arrives striped.

Threshold: the setting nobody touches

When a scanner converts a colour or greyscale page to bitonal, it applies a threshold. Below a certain brightness value the pixel turns black; above it, white. On most desktop scanners, this threshold is adjustable — often labelled "contrast", "threshold" or similar.

Too high a threshold: pale text disappears and parts of the page arrive blank. Too low: the paper background, its shadows and creases turn black, and the page arrives dirty. On a yellowed document or a photocopy of a photocopy, nudging up the contrast — not the brightness — is the right move. Brightness shifts the whole curve and erases the text along with the background; contrast pulls the two apart.

Scanning properly: a six-point procedure

1. Scan, don't photograph

A document photo taken with a smartphone carries perspective distortion, vignetting, a hand shadow and sensor noise. Mobile scanning apps correct some of these flaws, but they produce compressed JPEGs whose artefacts turn into black dust once binarised.

If you scan often, a scanner with an automatic document feeder remains a worthwhile tool: format detection, duplex, automatic deskewing, direct output as black-and-white PDF. For occasional use in a practice or a law office, a portable USB scanner is more than enough and fits in a drawer.

Failing that, if a smartphone is the only option, place the sheet on a plain dark surface, light it from the side, and above all don't cast a shadow with your upper body. See also our guide sending a fax from your smartphone.

2. Choose "black and white", not "greyscale"

Counter-intuitive, but decisive. Scanning in greyscale and letting the fax service binarise on the fly gives unpredictable results: every provider applies its own algorithm. Scanning directly in black and white (bitonal) lets you see the final result before sending, on screen, at full size.

Simple rule: what you see at 100% zoom on your screen is what the recipient will get, only worse.

3. 300 dpi when scanning, not 600

Scanning at 600 or 1200 dpi serves no purpose: the gateway will resample down to 200 dpi anyway, and brutal downsampling creates moiré on screened areas. 300 dpi in black and white is the right compromise — fine enough for the reduction to stay clean, light enough to avoid a 40 MB file.

4. Strict A4, margins respected

G3 fax transmits lines of standardised width (1728 pixels for A4). A page in Letter, A5 or a custom size will be cropped or rescaled, and the right edge may be truncated. Many receiving fax machines also cut off 5 to 10 mm at the top (header band) and along the sides.

In practice: put nothing important within 10 mm of the edges, and certainly not a file number, a reference or a signature.

5. White background, no fills, no information carried by colour

A chart whose legend relies on colours ("blue curve", "orange zone") is unusable by fax. It has to be reworked with patterns, dashed lines or text labels. Likewise, fluorescent highlighting becomes black rectangles that hide the very text they were meant to emphasise. If a passage needs to stand out, box it with a thin rule or underline it.

6. Check before, not after

Open the final PDF, zoom to 100%, read a random line at the bottom of the page. If you hesitate over a character, the recipient will hesitate too — and they won't have the original in front of them.

Problem documents, and what to do about them

Photographed ID documents and supporting papers

These are the worst candidates: guilloche backgrounds, halftone photos, holograms. The result: a black mass. Solution: scan in black and white with a high threshold, even if it means losing the photo, and retype the essential details in plain text on a cover page. One caveat, though: sending an identity document by fax carries GDPR obligations, detailed in our article on fax confidentiality.

Prescriptions and handwritten reports

Light blue ballpoint handwriting comes through badly. A black ink pen — a simple 0.7 mm rollerball — improves the result dramatically compared with pale blue or pencil. Practices that fax regularly all end up making black ink an internal rule.

Financial tables

Tight columns, 8-point type, half-point rules: this is the textbook case where superfine mode earns its keep. Another workaround: reformat the document in landscape, increase the font to at least 11 point and accept one extra page. An additional page costs a few cents; a misread figure costs far more.

Documents that have already been through a fax

Every transmission cycle degrades the image. A fax that was received, printed, then resent has undergone two successive binarisations: characters thicken and counters (the hole in an "e" or an "a") fill in. Whenever possible, always start from the original digital file, never from the paper version you received.

File size and transmission time

A well-compressed bitonal page weighs a few tens of kilobytes. A page derived from a colour photo converted in a hurry can weigh several megabytes, because random dithering ruins MMR compression — which works by repeating patterns, and a cloud of dots has none.

The practical consequence on classic G3: transmission time per page can go from 20 seconds to more than two minutes. On a VoIP line, the longer the session lasts, the higher the chance a network incident cuts it off. A clean document is therefore also a document that arrives.

Indicative figures, heavily dependent on document density and hardware.

On the receiving end: what isn't your fault

Some defects come from the other end, and no sending setting will fix them.

• Vertical black bands or regular white lines: the glass or roller of the receiving fax machine is dirty, or the thermal head is worn.
• A page that stretches or compresses: a paper feed problem on the receiving side.
• Page truncated at the bottom: the negotiated maximum length was reached, or the receiver cut the session.
• Print that fades over time: thermal faxes fade within months. This is a genuine archiving issue, covered in our guide archiving your faxes.

On that last point, an underestimated detail: poor-quality thermal fax paper fades in less than a year, even in a closed drawer. If the machine is still in service for reception, immediately scanning any document worth keeping remains the only defence.

A cover page that actually earns its place

The cover page isn't a formality: it's the guarantee that the recipient will know what to do with the document, even if a page arrives degraded. It should be set in large type and include:

• the sender, with a phone number where they can be reached;
• the named recipient, not just the department;
• the total number of pages, cover page included;
• the subject in one line;
• a confidentiality notice if the content warrants it.

Set in 14 point, with no screened logo, it survives almost anything. A gradient logo, on the other hand, will turn into a smudge — converting it once and for all into a monochrome line version settles the matter permanently.

For sends going abroad, adding the country in plain text on the cover page avoids a lot of misunderstandings; dialling codes and local conventions are detailed in sending a fax abroad.

Frequently asked questions

Is it better to send a PDF or an image?

A native text PDF — one generated directly from a word processor, without going through a scanner — always gives the best result. Bitonal rendering is done from crisp vector outlines, with no sensor noise. A JPEG image is the worst case, because its compression artefacts turn into black dots.

Is superfine mode always better?

No. It doubles transmission time and adds nothing on a document that is already crisp in 12 point. It becomes useful for small type, dense tables and drawings. Above all, it must be accepted by the receiver: if the receiver doesn't negotiate it during the T.30 phase, the session drops back to fine mode, or even standard.

Why does my handwritten signature come out dotted?

Because it was made with a light ballpoint or a pencil, and the binarisation threshold cut it in half. Sign in black ink, press normally, and avoid signing over an already printed area.

Should error correction (ECM) be disabled to improve quality?

No — in most cases it's the opposite. ECM (Error Correction Mode) retransmits corrupted blocks and protects image quality. You only disable it occasionally, as a last resort, on a very unstable line where it causes sessions to be abandoned.

Can colour be sent by fax?

Colour fax exists (T.30 annex E), but it is barely deployed and requires both machines to support it. In practice, in 2026, you should assume that every fax is black and white.

In summary

• A fax carries a bitonal image at 200 dpi: no grey, no colour, no shading.
• Fine mode (200 × 200 dpi) should be the default setting; superfine is reserved for dense documents.
• Scan directly in black and white at 300 dpi, never in colour "just to be safe".
• Adjust the contrast, not the brightness, when the source document is pale or yellowed.
• Stick to A4 and leave a 10 mm margin on every edge.
• Ban grey fills, fluorescent highlighting, screened logos and colour-coded legends.
• Always start from the original digital file rather than a paper version that has already been faxed.
• A clean document weighs less, goes out faster and stands up better to the hazards of a VoIP line.

Technical sources: ITU-T recommendations T.4, T.6 and T.30 (International Telecommunication Union) for Group 3 fax coding and protocol; T.38 for real-time transport over IP.