An iPhone can send a fax without an app

Open Notes, start a note, tap the camera icon and choose Scan Documents. It finds the edges of the page, corrects the angle you are holding the phone at, and gives you a PDF. Tap the scan, use the share button and choose Save to Files; then on this site tap the upload area, choose Browse, and pick it.

This is worth the extra twenty seconds because of what a fax page is. The receiving machine prints a grid of 1728 × 2156 pixels and each one is black or white — there is no gray in the format. A scan has already been flattened and straightened before it reaches that grid. A hand-held photograph has not, so everything the room did to the paper is still in the image when it gets thresholded.

To get from a photograph to black-and-white pixels, something has to pick the brightness at which gray becomes black. We pick it with Otsu’s method, which reads the split off the picture’s own histogram rather than assuming a value — a photograph of white paper is almost never actually white, so a fixed threshold throws away either the text or the page.

It is one threshold for the whole page, deliberately. If your hand or a lamp puts a shadow across the bottom third, the histogram has two humps in it, the split lands between them, and the shadowed third comes out solid black with the text inside it gone. Adaptive thresholding would paper over that. We would rather show it to you, because the recipient’s machine is going to print whatever this produces and you are the only person who can go back and retake the photograph.

Flat, even light. No shadow across the sheet. Fill the frame with the page and keep the phone parallel to it. Then look at the preview before you pay — that is what it is there for.

Since iOS 11 the camera saves photographs as HEIC by default, under Settings → Camera → Formats → High Efficiency. Plenty of upload forms reject it, which is why the usual advice is to switch the setting to Most Compatible or convert the files first.

You do not need to do either. HEIC is decoded here directly — it sits outside the PDF and JPEG decoder we use for everything else, so it gets its own, precisely because it is what a phone-first customer actually arrives with. Leave the setting where it is.

The size limit is on the file as it is sent. A photograph is resized to the fax’s own resolution before it leaves your phone; a PDF is sent as it is, so a larger PDF has to be saved as two files, which can go in the same fax.

Accepted from an iPhone

PDF, HEIC, JPG, PNG

Per file, as sent

4 MB

Pages in one fax

Up to 30, cover page included

A four-page form photographed four times is four files and one transmission, at one price. They are transmitted in the order you add them, so add them in page order — or reorder them before you pay. A cover letter as a PDF and three photographs after it is the same one fax.

There is no fax rail in iOS. The share sheet lists apps, and an app that says it sends faxes is a front end for a carrier, usually sold as a subscription with a page allowance. Nothing installed on the phone dials anything; the call is always placed by somebody’s telephony network. That is the case here too — the difference is what it costs and whether there is an account left behind afterwards. See sending a fax without a subscription.

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