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Image formats

Why JPG makes text blurry: JPEG compression explained

By DeckleUpdated

JPG makes text look blurry because JPEG compression throws away fine detail to make files small. That works well for photos, where colours change gradually, but sharp black-on-white edges get soft halos and blotchy artefacts. For screenshots, scans of text and diagrams, use PNG and convert with PNG to PDF. For photos, JPG is fine, and JPG to PDF adds no further loss.

JPG to PDF

Choose a source for the job

Photograph

Inspect colour and detail

Diagram

Inspect lines and labels

Scan

Inspect frames and text

A static PDF page does not preserve motion

Conceptual illustration · image format and output page behaviour are separate decisions.

How JPEG compression works

JPEG splits the image into small blocks of 8 × 8 pixels and describes each block as a mix of smooth patterns. To save space, it keeps the broad patterns and drops or rounds the fine ones. The quality setting decides how much is dropped.

Photographs survive this well, because a sky, a face or a wall changes colour gradually. Text does not: a letter is a hard edge between two colours, which needs exactly the fine detail JPEG discards. The result is a faint ripple or haze around each letter, often called ringing or mosquito noise, and sometimes visible block edges in flat areas.

Image content How JPEG handles it
Landscapes, portraits, product photos Very well; differences are hard to see
Scanned colour pages with photos Well
Black text on white Soft edges, halos around letters
Screenshots and interface text Visible blur and coloured fringes
Line diagrams and charts Fuzzy lines, blotchy flat colours

Every save loses more

JPEG is lossy every time it is saved. Opening a JPG, editing it and saving again compresses it again, and the losses add up. A screenshot shared through two messaging apps and saved twice can look noticeably worse than the original.

This is why the source matters more than the conversion. Use the earliest copy you have, ideally straight from the camera, scanner or app that made it.

What Deckle does with JPGs

JPG to PDF copies the JPEG data into the PDF byte for byte. It is not decoded and saved again, so converting adds no compression at all. The PDF looks exactly like the JPG, artefacts included. Choosing Fit inside A4 draws the image smaller on the page but keeps every pixel.

The exception is a photo over the device's pixel limit: 16 megapixels on a computer or Android device, 4 megapixels on iPhone and iPad. That photo is scaled down and saved again as a JPEG at quality 90, and the review lists it before you convert.

Two other Deckle tools do create JPEG data:

  • HEIC to PDF and TIFF to PDF save each image as a JPEG at quality 90. At that quality photos look unchanged at normal zoom; crisp black-and-white text can soften slightly when you zoom right in.
  • PDF to JPG turns PDF pages into JPG images. For pages full of small text, PDF to PNG keeps the edges sharper.

How to keep text sharp in a PDF

  1. Save screenshots, diagrams and text scans as PNG, not JPG. Most screenshot tools already use PNG.
  2. Open PNG to PDF and add the files, or drop them onto the page.
  3. Choose Fit inside A4 for a printable page, or Match each image.
  4. Choose Create PDF and zoom in to 200% to check small text.

If you only have a JPG of text, converting it to PNG first does not help: the blur is already in the pixels. Use JPG to PDF as it is, or recreate the image as PNG from its source.

When JPG is the right choice

JPG is not a bad format, just a photographic one. Use it for:

  • Camera and phone photos, where it gives small files with no visible loss.
  • Colour scans of photos or illustrated pages, where PNG would be several times larger for no visible gain.
  • Large photo sets, such as an inventory or an album, where file size matters for email.

A 12-megapixel phone photo is typically a few megabytes as JPG and many times that as PNG, with no difference you can see.

Tips

  • Zoom in to judge. JPEG artefacts are easiest to see at 200% on the edges of letters.
  • Avoid re-saving JPGs. Turn sideways photos with the rotate button in JPG to PDF instead of re-saving them, since rotating there does not re-compress.
  • Choose PNG at the source. If your scanner app or design tool offers PNG, choose it for text.
  • Keep photos as JPG. Saving a photo as PNG makes it bigger without making it sharper.

Frequently asked questions

Does JPG to PDF reduce quality?

No. The JPEG data is placed in the PDF unchanged, so the PDF matches the JPG exactly.

Why does my scanned document look fuzzy as a JPG?

The scanner or app saved it with JPEG compression, which softens text edges. Scan to PNG, TIFF or directly to PDF instead.

Will converting a JPG to PNG fix the blur?

No. The detail was lost when the JPG was saved. PNG keeps what is left, but cannot restore it.

What JPEG quality does Deckle use?

Deckle does not re-compress JPGs. For HEIC and TIFF images, which it must decode, it saves JPEG at quality 90.

Related tools

  • JPG to PDF to place photos in a PDF without re-compression
  • PNG to PDF for text, screenshots and diagrams
  • PDF to PNG to export text-heavy pages without JPEG artefacts

Sources