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Richard S.

Technical Question for Printers

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Hi everyone.

I have a technical question for any commercial printers, or someone who has genuine first-hand knowledge of the printing process.

Firstly, let me say that this question is exclusively aimed at dpi and printing, and NOT aimed at ppi and screen display.

Ok, so my question is - when you create a new image file, and initially set the dpi to 300, what does this dpi setting actually do to the file? Does it set some kind of hidden meta-tag, and if so what could that meta-tag possibly do that would benefit the quality???

The reason I am asking is that in theory, the dpi setting should be TOTALLY irrelevant, providing that the image is created at the correct size. Let me give an example.

Suppose I wanted an image printed out at 5 inches by 5 inches. If the commercial printer has told me that they need a 300 dpi image file, then in theory, as long as I create my image at 1500  by 1500, the image should print out at that size. (1500 divided by 300 = 5). If I created the image with smaller dimensions, say 750 by 750, then it would obviously end up 2.5 inches by 2.5 inches (750 divided by 300 =2.5).

Therefore I would like to know the technical process of what the dpi setting does to the actual file itself, and what possible benefit it could offer.

The only reason I can see to use the dpi setting, is if you do not enter pixel dimensions when initially creating the file, and instead set the inch settings. That way you would obviously need to set the dpi setting, in order for the image editing application to multiply the inches size you set, by the dpi you set, in order for it to work out the required number of pixels to create the canvas at.

Thank you all in advance.


High-End Photographic Prints

 

 

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14 minutes ago, Richard S. said:

...

Ok, so my question is - when you create a new image file, and initially set the dpi to 300, what does this dpi setting actually do to the file? Does it set some kind of hidden meta-tag, and if so what could that meta-tag possibly do that would benefit the quality???

The reason I am asking is that in theory, the dpi setting should be TOTALLY irrelevant, providing that the image is created at the correct size. Let me give an example.

Suppose I wanted an image printed out at 5 inches by 5 inches. If the commercial printer has told me that they need a 300 dpi image file, then in theory, as long as I create my image at 1500  by 1500, the image should print out at that size. (1500 divided by 300 = 5). If I created the image with smaller dimensions, say 750 by 750, then it would obviously end up 2.5 inches by 2.5 inches (750 divided by 300 =2.5).

...

DPI in an image is informational only. It can help some/most applications when an image containing a valid DPI field initially place the image so 100% size results in the computation that you show above.

Some/most automated image processing routines at (especially) on-line printers will use the same computations to process your images. Which is why they want the images to have the DPI information and or just the express number of pixels.

There is X amount of metadata in an image, how much information depends upon the image type. DPI is always (mostly?) used in all of them. But it is just that, metadata and has no bearing on the image quality.


My computer is a nothing-special Toshiba laptop with unremarkable specs running Windows 10 64-bit.

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Hi Mike,

So my understanding is basically correct?

In that case, if the commercial printing press is set to print at 300 dpi and the image sent to them is set to 1500, then the image should still print out at the correct size and quality, hence making the dpi setting irrelevant in this particular case, even if it was set to 72.


High-End Photographic Prints

 

 

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2 minutes ago, Richard S. said:

Hi Mike,

So my understanding is basically correct?

In that case, what would happen if I created the image at 1500 by 1500, but set the dpi to 72? If the printing press is set to print at 300 dpi and the image is set to 1500, then the image should still print out at the correct size and quality, hence making the dpi setting irrelevant in this particular case.

Yes, what you expressed is how DPI computation works.

It's a good question as regards an on-line photo print establishment. As regards my own experience with such places, DPI is used and in lieu of DPI, the image is expanded/shrunk so one dimension or the other fits the size of their template (the size of print you are ordering). Only one has ever warned my wife an image has too low of "resolution" when she ordered some prints. It confused her for a few minutes until she realized what was happening.

Other print establishments that deal with traditional print generally also have preflight checks (say images contained in a PDF) but in general what you send is what they are going to print.

As to what would actually happen in your explicit example? That likely would depend upon the on-line print establishment you choose. Most all have some sort of FAQ section that would indicate the requirements for their services. All the ones we have used, image processing is done against their sizes they will print. I doubt if you wanted the 1500x100 px image printed on 5x5 in paper—assuming they offered it—would raise a preflight flag.

 


My computer is a nothing-special Toshiba laptop with unremarkable specs running Windows 10 64-bit.

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Thanks Mike.

It is always interesting and helpful to get other experienced people to provide their understanding on certain issues.

Im the kind of person who likes to know exact details of how things work, in order to better understand them ;)

Thank you.


High-End Photographic Prints

 

 

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