I was pointed to a couple of blog postings yesterday, one called The Myth of DPI and the other titled Why DPI Does Not Matter.

Their arguments are logical and faultless. Their reasoning is sound. The message they are trying to get across is correct. But they are both wrong.

So why are they wrong?

Because they are writing about images on the web, and mentioning DPI. When you set the image size and resolution in Adobe Photoshop you never have the option of choosing the DPI of an image. This is what you get if you choose inches as your preferred units:

and this is what you get if you choose metric:

The option you have is of choosing “pixels/inch” or “pixels/cm”. Not “dots/inch” or “dots/cm”. Adobe Photoshop is concerned about your digital file. It doesn’t give a monkeys what you intend to do with it later. So it offers you ppi, NOT dpi, which is used in printing terminology. 300 pixels per inch equals 118.11 pixels per centimetre.

Our earnest bloggers are absolutely right in that ppi is irrelevant when you’re showing images on a screen. But dpi is relevant when you print those images, because then you can figure out how large you can print the image before those annoying little pixellations get in the way and become visible. 300 dots per inch is generally enough for the human eye to be tricked into seeing continous tone.

fotoLibra has always demanded that images uploaded to its site should be 300 ppi, for two perfectly valid reasons — and one utterly compelling one. You can read about them here.

I agree with my friends Ben Gremillion and Svein Wisnaes that resolution counts for nothing on the web. So it generally doesn’t matter, until you come to selling your photographs. When you are selling photographs to book publishers who need to print them at 300 dots per inch, it’s common courtesy to supply them at 300 pixels per inch.

Otherwise they might buy them from someone who took the trouble to go the extra step.

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11 Responses to “Why DPI Does Matter”

  1. Svein says:

    Thank you for taking the time to read my blog post. You are absolutely correct. It should have stated PPI instead of DPI.

    But guess what? It does not change one single thing. It is still correct. PPI does not matter in screen based media. BUT – DPI does not matter either! As it only applies to the printing process, how can it apply to screen based media (not only web – it also goes for TV)? The whole printing process does not matter for screen based media.

    So I think it would be wise of anyone reading this and for one single second thinking this has anything to do with print is to say STOP!

    It is ONLY for screen based media. DO NOT interpret anything that has to do with print into it. Do NOT assume that a customer delivering something to screen based media will give the same to print in a book. Contrary to popular belief, customers are not stupid. But if we use a lot of technical language when asking them for material for either print or screen based media, we are shooting ourselves in the foot. NEVER mention either ppi or dpi to a customer. It does not make sense unless you educate them. And that is not why they hire you. They hire you to avoid that kind of talk. The only thing they need to know is this:

    What we come back to, again and again, is that the important numbers are the pixels in the original picture. Then let the print people deal with it their way and the screen based people deal with it their way. Simple as that. Ask your customer for a picture that is minimum 2000 pixels wide. Or whatever suits you. Ask for EPS for logos. Then you fix the rest yourself. This is what you are paid to do.

  2. Gwyn Headley says:

    And of course you are correct as well, Svein. But easily the largest market for our member photographers is print, specifically book publishing. Here resolutions of 300 dpi really do matter. The more photographers who are persuaded that ppi / dpi is genuinely irrelevant, the more problems we have with frustrated uploaders who haven’t thought through the different demands of screen and print, of RGB and CMYK.
    We have two sorts of customers: the publishers who buy from us and the photographers who upload to us. We have to supply what they want, but it is not a one way street — they in turn have to supply what we want. From the publishers we want money, and from the photographers we want images that meet our Submission Guidelines.

  3. Svein says:

    The only problem is that you have to check the context of the two blogposts.

    Both specifically states that they are for screen based media. And as such, both are correct. Yes, there are inaccuarices, but it does not change the overall picture (no pun intended :-) )

    If I go on a blog about cars and in a discussion about what gasoline based car uses less fuel states that an electric car uses less fuel, I am not even sure they would let it stay there. It is irrelevant.

    If a photographer reads any of the two blogposts and think that it applies to his pictures, he is partially right! :-) Because PPI is just a flag that can be changed. Going between RGB and CMYK, on the other hand is more of a problem as you loose some information in the process. But changing the PPI flag does not change your picture at all. It is just a piece of metadata. A lot of cameras set the PPI to 72. So you would actually have to change it before uploading it to your service.

    So again – a digital photo is screen based as it’s origin today. So PPI does not matter. Only the number of pixels in that picture. When it comes to the content of those pixels, there are other factors – more pixels are not necessarily better. But again – this is a different discussion (same as print) so it does not belong in a discussion about screen based media.

  4. Svein says:

    And maybe you should change the CSS of your blog to not insert the huge smiley faces like this:

    :-)

    Interpretation is very often a bad thing. As this CSS mistake proves!

  5. Svein says:

    Just read your submisson guide and it makes a lot of sense because there, you have everything in context.

  6. Its clear that its the PRINT designers getting upset about this whole thing, as it’s a constant struggle when obtaining source files from clients, however I just wanted to add some further clarification to your point.

    “PPI does not matter in screen based media.”
    While this is true for the final exported file, designers who are using source images to slice and dice will (usually) always prefer to work with high ‘pixel count’ files (regardless of PPI or DPI) so that the final image is nice and crisp when used in its final size/state. Although compression plays a large part in how it looks on the web – once the file is small that’s it – but the bigger it is, the more opportunity you have to make it look nicer in the final format/size. Even just the fact that you have a nice crisp edge to use when you etch an object makes things much nicer.

    I mention this because I can just see clients start sending me their resized ‘low res’ files to work with because they read an article about how the resolution doesn’t matter for digital media.

  7. There’s one more way to look at it: If someone downloads an image from the web that measures 300×300 pixels, they can print it at 300DPI, one inch square. They can also quadruple its inches if they don’t mind printing at 150DPI.

    As you point out, it takes an extra step to change its print resolution. But one still starts with the same number of pixels.

    I haven’t worked in print for a few years, soI’m using “DPI” in the traditional sense. You have a good point that Photoshop itself refers to pixels/inch or pixels/cm. Thanks for the insight.

  8. Old Boy says:

    “300 dots per inch is generally enough for the human eye to be tricked into seeing continous tone.”

    Nope, that’s not what dpi stands for.

  9. Svein says:

    @Nathan

    If your clients read what Ben Gremillion and I have written about this subject and totally miss that it is all about screen, not print, then they need more help than just with their pictures :-)

    I totally agree with you that it is better to get a high resolution picture from a client than a low resolution picture. I always ask for this, even if I am not going to use it at anything bigger than 150×100. Because that mean I can control the resampling and the resulting quality.

  10. coop says:

    Pet peeve: Customer gives you 72 PPI (DPI? whatever) and asks you to print it. You tell them it will look bad and you need something bigger, like 150 PPI or 300 PPI. You tell them outright that there is no way to increase the quality of an image (I mean you can certainly increase the PPI, but it’s like polishing a turd…it will still be a semi-blurry pixely turd in the end). They go home into their expensive Adobe Creative Suite that they purchased for $1500 (because in their minds, to be a graphic designer or web designer you simply need to purchase the programs) and increase the resolution in Photoshop to 300 PPI, leaving you with relatively the same problem, only the pixels are a little smoother around the edges. I could have done this myself obviously. I still don’t know how to properly educate customers about this sort of thing, so any input would be helpful.

    I’m sure my terminology in this post is slightly wrong, so please don’t totally flip out on me for not being an uber nerd. I use PCs too, because they run all of the same programs I use for at least half the cost of a Mac. Plus the Apple stores have all white walls and it hurts my eyes to be in there. Not hating, just stating.

  11. Mick says:

    There seems to be an awful lot of anorak stuff written here, that is all, probably, technically correct. The only thing that counts though, is not the technical stuff, but the basic fact that requires no explanation, no defense:
    fotolibra want pics that comply at least with a minimum size and are uploaded at 300 ppi,
    that is all that matters, technical arguments and explanations are just a waste of time and space.