How Many Megapixels to Scan a Document With a Phone?

To scan a document of ordinary type for OCR, a phone needs to put about 8 megapixels on the sheet itself, so a 12-megapixel camera is enough when a letter-size page nearly fills the frame. How many megapixels the camera advertises matters less than how many land on the paper, which distance, zoom and lens choice decide.

How many megapixels does a letter-size page need?

A letter-size page needs about 8.4 megapixels on the paper to match a 300 dot-per-inch scan. The sum is 8.5 inches times 300, which is 2,550 pixels across, by 11 inches times 300, which is 3,300 pixels down. Multiply the two and you get 8,415,000 pixels.

That figure counts only pixels that fall on the sheet. A camera frame is a different shape from a page, so some pixels always land on the desk. A frame in the common 4:3 shape is a little longer, relative to its width, than letter paper turned to match it. As you move the phone closer, the sides of the page reach the sides of the frame while a thin strip of desk still shows past the top and bottom.

Here is what a 4:3 frame gives a letter page held so that it runs side to side across the frame, and what it gives when the page covers only 80% of that width:

Each figure is the short side of the frame divided by 8.5 inches. Legal-size paper is 14 inches long, which is longer in proportion than the frame, so its length sets the limit: 4,000 pixels over 14 inches is about 286 per inch from the same 12-megapixel camera. Why 300 is the target, and what the number means for a file with no resolution recorded, is explained in the guide to DPI in a phone photo.

Megapixels, distance and how much of the frame the page fills

Megapixels on the paper fall with the square of the distance. If the page spans 70% of the frame in each direction, it receives about half of the camera's pixels. Step back until it spans half the frame each way and it receives a quarter. A 12-megapixel camera held too far away is a 3-megapixel scanner.

This is why two photos from the same phone convert so differently. Work it through with a fictional file. A client, Marisol Dufresne, photographs a letter-size lease page while standing over a table. Her 12-megapixel picture is 4,000 by 3,000 pixels, and the page occupies 2,000 of the 4,000. That is 2,000 pixels over 11 inches, or about 182 per inch. She takes a second picture from about half the distance, with the page reaching the sides of the frame, and gets about 353 per inch from the same phone.

That closer photograph of the lease is the one to keep if the page may be documentary evidence. Small type in a clause needs those pixels before you try to pull text. Check the rent figure at full zoom, save the sharper file for authentication, and record which phone took it so the chain of custody starts at the capture.

You can measure any photo you receive in three steps:

1. Crop a copy of the picture to the edges of the paper.
2. Read the cropped image's pixel dimensions in the file's properties.
3. Divide the long dimension by the paper's length in inches.

A result near 300 is what an office scanner delivers at its 300 setting. A result under 200 on a page with small type is a reason to ask for a closer picture before you spend time correcting the text. Pixels can also disappear after the shutter closes: the post on sending photos without losing quality covers channels that shrink a picture in transit.

Do 48 and 200 megapixel phone modes help with documents?

A 48 or 200 megapixel mode helps with documents only when the page is large, the type is small, or you cannot get close. For a letter page of ordinary type that fills the frame, the default mode already clears the 8.4-megapixel mark, and the extra pixels mostly buy a bigger file.

On iPhone the choice is a documented setting. Apple's page on advanced camera settings says that on supported models "you can change the default resolution of the Main or Fusion cameras from 24 MP to 12 MP," and that capturing at 48 MP means turning on Resolution Control or ProRAW & Resolution Control under Settings, Camera, Formats.

A sensor with a very high pixel count has very small pixels. A camera built on one can merge neighbouring pixels into a single larger one for its default picture, a technique called pixel binning, and record every pixel separately only in a full-resolution mode. If your phone offers a 200-megapixel mode, look in its maker's documentation for which behavior you get and in which light.

Three things limit what the big number delivers:

• Light. Small pixels collect less light each. In a dim file room the full-resolution picture can be grainier than the default one.
• Sharpness. A slight hand shake or a soft lens spreads each letter edge over several pixels. More pixels then record the blur in finer detail.
• File size. Full-resolution files are several times larger. Schema Law takes a PDF, PNG or JPEG of up to 20 MB, as the upload step on the homepage says, and Google's photo backup help page lists a limit of "Photos up to 200 MB or 200 MP." Check the limit of wherever the picture is going.

The honest test takes two minutes. Photograph one page in the default mode and again in the high-resolution mode, in the room light you will work in. Open both at full size on a computer and compare a footnote. Keep the mode whose small letters have cleaner edges.

Why digital zoom and the ultrawide lens cost you pixels

Digital zoom and the ultrawide lens both reduce the detail on the paper. Digital zoom crops the middle of the frame and enlarges it, which adds no information. The ultrawide lens takes in more of the room, so the page gets a smaller share of the frame, and straight lines near the edges can bend.

The fix for both is to use the main lens at its 1x setting and move the phone. A dedicated telephoto lens is a real lens and not a crop, but check its pixel count in your phone's specifications before you rely on it.

Leaning in close can change lenses without your asking. Apple's page on macro photos says that on supported models, when you get close to the subject, "Camera will automatically switch to the Ultra Wide camera to enable macro photography." The same page explains how to turn automatic macro switching off from the control that appears on screen. If your close-up of a signature block looks softer than you expected, check which lens took it.

How many megapixels do fine print and footnotes need?

Fine print and footnotes need megapixels in proportion to how small the letters are. Type half the height of body text needs twice the pixels per inch to give each letter the same number of pixels, and that means four times the megapixels on the page. For a letter sheet, that is roughly 34 megapixels on the paper.

You do not need a camera that puts 34 megapixels on a sheet in one frame. Photograph less paper per picture instead:

The arithmetic for a 4,000 by 3,000 pixel frame runs as follows. Half a letter sheet is 8.5 by 5.5 inches, and its 8.5-inch side laid along the 4,000-pixel side gets about 470 pixels per inch. A quarter sheet is 4.25 by 5.5 inches and gets about 706 per inch, twice the single-frame figure. That is what a camera with four times the megapixels would put on the whole sheet. Overlap matters: leave at least a few lines in common so you can confirm no line was skipped when the text is put back together. The same tiling method scales up to plats and drawings, covered in the post on scanning a large document with a phone.

Whatever the pixel count, read the small print in the converted text against the picture. Pixel arithmetic tells you whether the letters were recorded. It does not tell you whether every figure in a footnote was read correctly.