Preparing photos for PDF Web Documents

Hi,



I am a newcomer to InDesign but use Photoshop.



If I put together an InDesign (CS3) document for a company website,and this document is to be viewed online or be available for download, should I prepare the images at 300 dpi / ppi or 72? I am concerned about the image quality when people download the brochure and print them off. Should one be at 72 (or higher) for viewing online and another at 300 if for download?



I've tried to research this but only seem to find answers for sending photos via email (Save for Web etc) and can't dind anything specific to preparing InDesign documents for use on the web.



Appreciate your help.



Philby
Preparing photos for PDF Web Documents
Resolution doesn't exist on the web, only pixel dimensions, so the quality of the image doesn't change when you change the resolution.



In print, however, you are working with physical dimensions into which you must fit the pixels. Higher resolutions (i.e. more pixels fit into the space) tend to look better. Keep in mind that the ''effective resolution,'' that is the scaled value at the print dimensions, is the only one that counts. Captured size and resolution are meaningful only in as much as they allow you to calculate the limits to which you can scale without serious degradation. Up-sampling in Photoshop almost never works.



The rule of thumb is that effective resolution should be twice the half-tone line screen of the output. Since 150 LPI is a very standard screen for offset litho on coated stock you get the de facto 300 PPI standard, but the truth is that required resolution is highly dependent on printing method and intended viewing distance, and two times line screen is just a guideline to insure enough data -- not a minimum -- and as little as 1.4 times will often give satisfactory results, particularly on a device which uses FM screening rather than a conventional halftone dot, like a desktop inkjet printer.



My rule of thumb for desktop devices is 200 ppi is nice, 180 ppi is adequate, and as low as 100 ppi will probably work, though it won't look like fine art.



I do a few jobs that have both web and printed versions. I used to do two PDFs for the web -- a screen-only version and a ''printable'' version -- but over time I've found that even the version intended for viewing only (downsampled to 75 ppi during export) prints on desktop devices at a quality the average user wouldn't distinguish from the printable version at 150 ppi downsampling.



Peter
Preparing photos for PDF Web Documents
Hi Peter,



Thank you for taking the time to help me with this.



Your comment that resolution doesn't exist on the web, only pixel dimensions, was a good reminder. For print, I use 300 ppi but 200 or 240 often produces excellent results.



Creating PDF documents that will be downloaded from the web is a new task for me and I should probably experiment a bit with this. Like you, my first thought was to create two documents; I'm still not convinced that 75 ppi will be great but I'm sure your experience with this is greater than mine. I'm more concerned with the print quality if the image fills most of a page. Although I'm keen to keep the document size down, would 150 ppi for the web download document be a reasonable compromise?



Thanks for your help.



Philby

I'm sure some of the answer will lie in the quality of the original images, i.e. the better they are the more likely you are to notice a difference in the downsampling.



I'd make a few single page tests exporting the same sample file to PDF using a variety of compression settings and methods (zip vs. jpeg, for example) and view and print all of them and see if you find one significantly better than another for either purpose.



I'm sure you are going to need to compromise someplace if you go with one file. It may be slightly larger than you'd like for fast downloading and not quite as good in image quality as you'd like for print, but only you can decide if it is good enough, and if you care whether users download a screen-only version and then print that instead of waiting for the larger file, in which case you must add a no-printing restriction.



Peter

Hi Peter,



Thank you for providing an excellent insight into this topic. It seems that compromise is the name of the game here and I'm glad you were able to share your experience. Really appreciated!



All the very best,



Philby