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For an archaeological project, our client is requesting that all pictures taken should be in "300 ppi" (not 300 dpi).

I'm having trouble understanding what it means.

For now we have been using different cameras :

  • Sony HX400v

  • Panasonic Lumix FZ300

  • Samsung Galaxy Tab A7 2021

Looking at the pictures file properties I can see that pictures from the Sony are 350 ppi (vertical and horizontal) and pictures from the Lumix are 180 ppi (vertical and horizontal).

Does it mean that only the Sony is acceptable for the deliverable?

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    \$\begingroup\$ You might need to provide some more context around their request - ppi (or dpi, for that matter) is a factor in how you display or print an image... it's not quite so relevant when you are actually taking the photo to begin with... perhaps they want you to print/display at 1:1 size with 300ppi? Or do they want the images to be enlarged/reduced? \$\endgroup\$
    – twalberg
    Commented Mar 15, 2023 at 14:52
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    \$\begingroup\$ My advice is to ask the client for more information because the answers here are just guesses. \$\endgroup\$
    – Bob Macaroni McStevens
    Commented Mar 15, 2023 at 20:45
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    \$\begingroup\$ If you actually want to be helpful to the customer, you might ask how they're using the images (display/print) and at what size. They're probably used to specifying 300dpi for prints on large paper, and really just want a photo that they can print to that resolution. \$\endgroup\$
    – Cullub
    Commented Mar 16, 2023 at 17:28
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    \$\begingroup\$ @Cullub Indeed. Of course this depends on whether your contact at the customer is someone able to consider overall objectives and process beyond their own job, or someone in a bureaucratic or pathological organisation who is concentrating on just following instructions. They may have been instructed to receive 300ppi images. \$\endgroup\$
    – bdsl
    Commented Mar 18, 2023 at 12:37
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    \$\begingroup\$ I never depend on a client's technical spec. Always ask what the end use and expectation is. \$\endgroup\$
    – user41829
    Commented Mar 18, 2023 at 20:34

6 Answers 6

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I'm having trouble understanding what it means.

What it means is that whoever wrote the request probably doesn't have a clue how JPEGs are rendered using most current image viewing applications.

PPI (pixels per inch) is the way we refer to how many pixels of a digital image should be rendered per inch of physical page space when the image is embedded in a document intended for printing, such as would be produced by a page setting program. Other than that, the PPI/DPI number in an image's EIXF information is pretty much meaningless.

What they should be telling you is the minimum number of total image pixels in width and total image pixels in height they require.

Having said that, it is pretty easy to set the PPI at 300. For best results, so that the images are ready to go right out of camera, set it in camera if your camera has an option to set ppi (some cameras may call it dpi, but it really means ppi when it's the camera EXIF tag you're setting). If your camera doesn't allow that, then use an image editing program to change the value in the image metadata. Then save a full resolution version with 300 dpi selected in the output settings (which is actually 300 ppi, even though the standard EXIF tag is often labeled as dpi).

If you're using JPEGs straight out of camera with no additional editing, then you can use an EXIF editor to change the value without having to decompress/recompress the image.

Please see this answer to How do I generate high quality prints with an ink jet printer? for a very thorough explanation of the actual relationship of ppi to dpi. PPI is pretty much irrelevant for viewing and printing full page photos with most current applications.

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    \$\begingroup\$ "These parameters are often set by people who haven't the faintest idea what they actually need. " Hahaha, indeed... that's exactly what I thought. There is no other requested resolution details about the pictures (no min size, or megapixels) we have to deliver so I think we are fine... \$\endgroup\$
    – tonyduky
    Commented Mar 15, 2023 at 15:46
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    \$\begingroup\$ Just be sure you're saving at the largest resolution and "fine" compression and you should be OK. You can always downsize the images later if you need. \$\endgroup\$
    – Michael C
    Commented Mar 15, 2023 at 15:47
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    \$\begingroup\$ "What it means is that whoever wrote the request probably doesn't have a clue" wanting to upvote this so many times for this \$\endgroup\$
    – Philip Kendall ♦
    Commented Mar 15, 2023 at 17:40
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    \$\begingroup\$ If the specification also included image dimensions (in either pixels or a physical unit of length), requesting the the image metadata be set to make them display or print at a desired default size would not seem unreasonable. \$\endgroup\$
    – supercat
    Commented Mar 16, 2023 at 15:59
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    \$\begingroup\$ Changing the metadata shouldn't require redoing the compression at all. If you're working from the RAW originals or something, then yeah you'd have to select a compression quality like "fine", but if you already did that an just want to change the PPI metadata then decompress + recompress is just going to lose quality for no reason. Use a metadata editor that doesn't have to recompress at all. (You can check if two JPEGs decompress to bit-identical RGB or YUV pixels; if so there was no loss introduced by the program that changed metadata.) \$\endgroup\$
    – Peter Cordes
    Commented Mar 16, 2023 at 17:56
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While other answers are correct regarding the general usage of ppi/dpi in digital images, there is a specific and important usage that may apply here. As you said it's for an archaeological project, does it involve taking photos of recovered artefacts?

In document scanning and general archival, a specification of 300 ppi means that for every inch of the object the photo should have 300 pixels across.

It doesn't make sense to specify such things in total megapixels, as that depends entirely on the size of the object and the distance and focal length of the lens.

Just taking photos with the highest settings and changing the EXIF data is not going to satisfy such a requirement. The need is to have sufficient detail of the object(s) for study and reproduction.

The easiest way to verify the "true" ppi of your images is to include a ruler in the shot (perhaps together with a color calibration key).

photo of an artefact with scale key

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    \$\begingroup\$ This is the right answer for photographs of flat artifacts. The customer still has to be asked if that's what they mean, as 300ppi is plenty enough for some applications, way too much for some, and yet not enough for others. Manuscripts and printed matter could benefit from 600ppi. Whether 300 or 600ppi, good optics are required, of course, and a calibrated flat field. Lens distortion has to be corrected out, otherwise the ppi value is an uncalibrated guide and can't be used for linear measurements on the photograph. \$\endgroup\$
    – Kuba hasn't forgotten Monica
    Commented Mar 17, 2023 at 14:32
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    \$\begingroup\$ I think of this as Inches Per Pixel, which one sometimes can find in discussions of satellite imagery. Good catch regarding context. \$\endgroup\$
    – Yorik
    Commented Mar 17, 2023 at 15:26
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    \$\begingroup\$ @user8356 yes. If for some reason you're doing archive photography of the entire surface of the pyramid. And 1,500 pixels is perfectly sufficient for printing a life-size image of the dagger. \$\endgroup\$
    – OrangeDog
    Commented Mar 17, 2023 at 22:04
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    \$\begingroup\$ @user8356 The point of 300 ppi (or ipp, as it were) is that, along with an actual PPI/DPI value of 300 (in the EXIF data), such an image placed into a document intended for printing will automatically produce a precisely 1:1 copy of the original object well-suited for printing. The five-inch dagger will be precisely five inches in print, which is a big point in archival photography, and a great part of the reason why a lot of archival photography does include rulers for scale. \$\endgroup\$
    – Janus Bahs Jacquet
    Commented Mar 17, 2023 at 23:51
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    \$\begingroup\$ @fabspro HNQ effect. The other answers were posted at least 3 days ago and had accumulated some votes. On the other hand, it is quite surprising that this late answer got upvoted until 2nd highest :) (unfortunately, today is the last day for this HNQ) \$\endgroup\$
    – Andrew T.
    Commented Mar 19, 2023 at 11:56
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What does a client mean when they request 300 ppi pictures?

It means that they have no idea what they're talking about.

Deliver full-size images as you would to any client. You can change the metadata in the files so that the DPI setting is 300 – that will satisfy their requirement, and will make absolutely no difference to your image files.

(For all intents and purposes, Dots Per Inch (DPI) / Pixels Per Inch (PPI) are used interchangeably by people who think these values are some indicator of size/resolution/quality. I'd like to see your client's reaction if you delivered a 300x300 pixel image with DPI set to 300.)

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    \$\begingroup\$ Yep, and to be clear, changing the metadata dpi attribute actually changes the DPI/PPI. Since that's just a rendering setting, not related to the actual image. So you're not "cheating" at all by changing that attribute. \$\endgroup\$
    – Cullub
    Commented Mar 16, 2023 at 17:26
  • \$\begingroup\$ @Cullub Only if the rendering application bothers to read the EXIF tag and then follows it, instead of stretching/shrinking an image file to fit a predetermined space on the rendering device. \$\endgroup\$
    – Michael C
    Commented Mar 17, 2023 at 10:50
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It means some graphics person told them to get 300 dpi resolution images. Explain that any measurement "per inch" is for printing and does not exist in digital images -- there are a certain number of pixels in the image, not pixels "per" anything.

It's only when you translate pixels into a display format (monitor screen, printed page, pointillist oil painting) that a "resolution" of pixels in a given measurement makes any sense.

I might ask how large the images will be printed/displayed, and what the OUTPUT resolution and halftone screen will be (for conventional offset printing). If you are printing images on paper using a 150 lpi halftone screen, you want about 1.5 times (2 times maximum) the screen resolution, so 225 dpi images. Then, the printed size enters the equations. Say the photos will be printed 8x10 inches, you'd need images with 1800 pixels wide by 2250 pixels high.

Luckily, today's digital images have enough pixels for high-quality printing at fairly large sizes, and hard disks can handle gigabytes of data cheaply. It took a lot more planning and careful image-handling harder when disks and RAM were measured in megabytes.

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ppi is a screen resolution [dpi is, of course, a print resolution].
Photographs have neither of these. They are values only for display/print.

These parameters are often set by people who haven't the faintest idea what they actually need. Ask them what pixel resolution they want. "6,000 x 4,000px" or "24 MP" are valid values, 300ppi is not.
You could view a 1MP image, 1000 x 1000px, at 300ppi [which is approximately the resolution of an Apple Retina screen] & get a visible square of about 3". It would still obey their rules.

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  • \$\begingroup\$ Mere factor of ten out;) Fixed. \$\endgroup\$
    – Tetsujin
    Commented Mar 17, 2023 at 16:20
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The reason the 300ppi requirement is confusing is because the notion of pixels per inch is ambiguous between two meanings:

  • For each spatial inch of the item being photographed, ensure 300 pixels of resolution
  • For a printed image of a particular size, ensure 300 pixels per inch of print resulution

If you are photographing relatively flat artefacts (similar to pieces of paper, ancient documents etc), it is very reasonable for the client to ask that you capture at least 300 pixels per inch of detail of the artefact. This is akin to configuring the dpi setting on a scanner.

If the client needs images printed out at the end, you can achieve 300ppi but you will need to clarify what size the printouts will need to be - this will determine how big the digital image will need to be to achieve the desired resolution.

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