Calculating the resolution to print a JPEG poster in InDesign?

[Katherine25238449muqx]

Hi!  I recently took a test and I don't think I answered this question correctly.

I had to design a 22”x28” poster with an 1/8 bleed.  The directions said it will go to a web designer so save a low resolution JPEG with 1200px width.

So...I had no idea how to calculate what resolution I should export as, but online I found that if you want to print a 22”x28” and you want the resolution to be 300 PPI, then you multiply the inches by PPI amount (PPI is just the resolution, right?) and that gives you the pixel amount.  So, if the width of this poster is 22inches, then I'd take 1200 (the width of the final JPEG image) and divide by 22, right?  That gives me 55.  Should 55 be the resolution?  It seems awfully low!

I feel something is amiss.  Any help on this, or directing me to a tutorial or something on it, would be very much appreciated!!

Thank you so much!

Kathy

[Willi Adelberger]

Web Images have no Resolution, only pixel dimensions. The resulting resolution is depending on the size of the screen. 

[Katherine25238449muqx]

Interesting. PPI is not resolution?  The size of the screen varies for each user.

So if it's a 22”x28” poster (I guess we only care about the ratio here?), do you know what I'd do in InDesign to save a low resolution JPEG with 1200px width?

thanks!

[Willi Adelberger]

I never said that ppi is not resolution. But as long as you do not know the size in metric, no resolution is known. I did say that resolution is not relevant on screen, pixels are relevant.

If an image should have 1200px width, make it 1200px width. Where is the problem?

[Katherine25238449muqx]

The "problem" is that InDesign does not ask for the px width.  I have to give a resolution when I export (as far as I know -- I uploaded what I see after pressing Export), so this would call for me to do some sort of calcluation.  Right?  Am I not explaining my problem correctly?  Thanks.

[Scott Falkner]

True, but you cannot tell InDesign to export to a specific size in pixels. InDesign files always have a physical size, even if your units are the wrongly named “pixels” the program uses. So if you have a 22 inch wide document and want it exported to a 1200 pixel wide image you must specify 55 ppi (actually 54.55, but InDesign will round that up to 55 and you’ll get a 1210 pixel wide image).

[Peter Spier]

Your thinking is correct for calculating the size for the web poster, and you are equally correct that this resolution is too low for quality printing at full size (not to mention that a "low-resolution" jpeg is probably unsuitable for printed output at any size).

Requirements for screen and print are quite different.

Willi is also correct. Images have no inherent resolution, only the pixels. Resolution is a function of how large or small that image is physically output.

[Katherine25238449muqx]

Thank you, this is helpful.  Images have no inherent resolution - didn't know that.  Thanks!  Yep, I know print requirements a bit better.  Bleed was only for print, obviously.  Sorry for confusion.  🙂

[Test Screen Name]

There seems to be a problem right at the start. You say it's going to a web designer. Then you talk about printing. But web designers don't print things: they want images measured in pixels. (It's a very strange spec because bleed is only applicable to printing, and not relevant to web graphics either.)

Your calculation of 55ppi is correct, but your view of it as "awfully low" is not: that's thinking as if it will be printed. 
A key thing to know about "resolution" is that the word is used for two entirely different things. 1. The pixels per inch as printed (ppi) 2. The width and height of an image in pixels. You need to know which meaning applies. 

[Katherine25238449muqx]

Oh, great!  I'm so glad I did my calculation correctly.  So I should always just divide the desired pixel width by the width of the JPEG.  Wow, 55 seemed low but I'm glad it's not low for web.

Thanks for the tip about "resolution"!  I'm coming back to this after some years and have forgetten the terminology. 

[James Gifford—NitroPress]

One point of confusion is that image files have no "resolution," even though a value for PPI can be assigned and noted in apps like Photoshop. It's a completely arbitrary value. If you open a file with an assigned PPI of 300 and change that value to 50, it's still the same image/file. (It will be interpreted differently by apps like ID if it's imported and placed without scaling, but the pixel-size-image still doesn't change.)

The only time PPI/DPI matters is when an image is printed or displayed, and then there will be a relationship between the actual pixel dimensions (say 900x1200) and the size at which a screen will display it (at a nominal 92/96/120/144 ppi) or it will print (at 300 dpi, it would be 3x4 inches).

If you are designing for print, the effective DPI is important; if the printer wants a "300DPI file" it must contain 300 pixels per intended print inch. If you are designing for web, it's a lot looser as images can easily be rescaled on the fly; working to a nominal 100PPI for approximate screen sizes (300 pixels wide if you want the image to be about three inches wide on a "standard" screen) is adequate.

But outside of very specific situations, such as magazines or catalogs that create and maintain images for their specific requirements, a "300 dpi image" is a meaningless designation, topped only by things like "a 2MB image." In all cases, it's the actual pixel dimensions that matter, interpreted as necessary by an output DPI/PPI/desired size.

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[Katherine25238449muqx]

Thank you!!  This is so helpful.

[Katherine25238449muqx]

Oh wait!  ... when I went to export 55ppi, it wouldn't let me.  Please see attachment - the lowest option to select seems to be 72, and I don't know a way to customize it....  

[James Gifford—NitroPress]

ID is not the tool for wonky image processes. Export to, say, 150ppi and then use a tool like Photoshop to scale it — not to an arbitrary ppi, but to an exact pixel size.

There are other ways to get there from within ID, but it involves setting up a working layout that is exactly the right multiple of a standard export resolution. That is, if you have to deliver a 1200 pixel width, set it up at 8 inches in layout width and export at 150ppi, or at 4 inches in width and export at 300ppi, etc. Your work size should be proportional to your finished size at a standard export resolution.

All of the parameters you're trying to work to are somewhat in conflict, as noted above. If what you need is a 1200-pixel image, all the "22z28 inches" is irrelevant. Always work from what you need as an end point back through standard export options to guide how you lay it out in ID. It's just many extra steps and frustration to lay things out in an arbitrary size or way and then try to get to another specific, but arbitrary end size or resolution.

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[Katherine25238449muqx]

Oh, I was totally confused on this test, it seems.  How can I learn all this?  Is this what a person learns in Graphic Design school?  I'd love to do this professionally but it seems hard!!

So, whenever you're exporting for web, inches have nothing to do with it?  JPEG sizes don't have to do with inches?  I had to export for print with the bleed, and I also had to export the same poster for web.  So I guess the test-writer was just giving me the inches for the print version of the poster.  I didn't need to take that into consideration for the web JPEG.  I think I get it.

[rob day · Accepted solution]

Oh wait! ... when I went to export 55ppi, it wouldn't let me.

Hi @Katherine25238449muqx , the Resolution dropdown is a collection of presets, but you can also type in any integer (54.54 will get rounded up):

55ppi will export to a 1210 pixel width (55 x 22 = 1210). If you need an exact 1200 pixel width set the Image Size Resolution to 55.54 with Resample checked in Photoshop:

[Katherine25238449muqx]

WOW, thank you SO much for this!!!  I have stored it away in my files for reference!

[Mike Witherell · Accepted solution]

Notwithstanding all your excellent comments, I have a feeling the question wanted a simple answer of "72 ppi".

[Katherine25238449muqx]

Haha!  🙂

[rob day]

If exporting to an exact pixel width is important, you can try this JPEG export script, which includes a fixed pixel width field:

https://shared-assets.adobe.com/link/e62626e4-9f44-45a3-6da0-0a1b51e8ecbf

Here’s the dialog:

The Exported JPEG file:

[Katherine25238449muqx]

Wow, did you make this?  I downloaded it -- thanks!!

[rob day]

Yes, it exports the JPEG via PDF, which solves some known quality issues with the built-in JPEG export.

[James Gifford—NitroPress]

Well, unless the OP is misstating the points, the whole question doesn't quite make sense. It conflates a "22x28 print poster" with a specific web image size, which is next to nonsense.

I was trying to get to a sensible end point with it all. 🙂

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[Scott Falkner]

It isn’t hard to imagine a scenario where this would be required. A company that produces lots of posters for online sale might require web assets for their catalog, including a large view of the poster.

[James Gifford—NitroPress]

Not at all. But producing the poster layout for print and a web asset are two completely different tasks with different workflows. It is difficult to do such a layout that can easily be exported to, say, 300ppi PDF and any exact web-image size, hence the usefulness/need for a pass through Photoshop.

The OP may have been incomplete in the test example steps; we all responded to the question as it was posed/discussed. 🙂

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