Image Resolution Question 150dpi vs 300dpi in Final Layout

[WebPiper]

In the final offset color printing of a file, does it make a difference if the images in the document are 150 dpi or 300 dpi? I know they shouldn't be less than 150, but does making them higher dpi make them really better? Or does it just make the file larger? If you think it does make the image better, please let me know why. I'd be particulary interested to hear from someone who works with a printing house. Or if there is something on Adobe's website addressing this issue, please add the link.

Thank you,

Marilyn

[Peter Spier]

First, are we talking about the resoution of the image at its saved dimensions or the "effective" resolution at the size it is being printed? The one that matters is effective resolution.

Second, there are a lot of factors that affect what the optimal resolution is going to be, not least of which is halftone screening method and the screen density. Choice of a screen is often dictated by the quality of the paper stock -- coated papers are less absorbant and will allow a finer screen and finer details. Theres a vast difference between what you need for the Sunday comics and a coffee-table fine art catalog, and there is no single simple answer to a question as open-ended as yours. Is there going to be a difference? Probably. How significant a difference, and whehter it makes things better or worse depends on all the factors involved in the job.

The rule of thumb is you need an effective resolution equal to twice the linescreen used to print the image. Like all rules of thumb this is more a guideline that will insure you always get good results rather than an absolute. I know people who routinely send images with an effective resolution of 1.4 times the linescreen. I believe, also that stochastic screening methods will work well with lower resolutions, but stochastic screens are uncommon in offset work still, I think.

If the resolution is low, there is usually no point in simply upsampling and sticking the image back in the document, if that's what you are asking. It isn't likely to improve how it looks.

[Peter Spier]

I forgot to include the cardinal rule: Talk to the printer. He should be able to tell you if your images will print acceptably on his equipment using your selected stock.

[Scott Falkner]

The term is PPI for Pixels Per Inch. Dots Per Inch is used to measure the resolution of an output device. 300 ppi is the usual default for sending files for high quality printing. If you use a smaller value you will be sending less data and getting less detail in your output. There is a reason that the PDF export settings for Press Quality and PDF/X-1a default to 300 ppi. These are by far the most used methods for sending files to printers. Most printers will preflight your file and often will call you if images are below 300 ppi.

If I were to preflight your file and noticed that all imges were below 300 ppi, all had the same resolution, and that resolution was a nice round number like 200, I'd call because I'd assume you exported at 200 ppi instead of 300. I'd encourage you to send a new file. You might be billed for the extra prepress to process your file twice (but I doubt it).

[Jongware]

Yes, it does make a difference.

These numbers do not come out of the blue. A typical laser photosetter has an output resolution of 2400 dpi

. Each of the halftone dots that make up halftone images can be one of 256 values of gray (a hardcoded PostScript limitation). That would make the absolute minimum size of each dot 16 x 16 output pixels. 2400 divided by 16 is 150, hence, you can fit 150 complete halftone dots across an inch.

However, the calculated position of the halftone dots does not take the actual photosetter halftone position in account (your image may not start at exactly the internal start of a halftone dot). Each of your single dots may be broken into two because they may not be "aligned" with the dots the output device produces. So, on every relatively sharp color boundary of two input pixels (which are output as exactly 2 output halftone dots), the color of this dot will be rounded to both the left and right input pixel (and upper and lower one).

Doubling the input resolution means than more than a single input pixel is taken into account per halftone dot (of which at least one will be entirely inside the dot), smoothing things out.

There is also an upper limit on which it will not matter anymore whether you throw in more input pixels per halftone dot -- the typical upper useful value used by InDesign and Distiller is 450 dpi -- images above this are downsampled to 300 dpi, so (apparently) the difference between 300 and 450 dpi is neglectable.

Modern machines may advertise a higher resolution, even upwards of 3000 dpi, but it's at the operators discretion to switch back to a lower rez to save time and memory -- and no skilled operator will (or should) go below 2400 dpi for halftone images.

[WebPiper]

Ah, I see that I have asked the question incorrectly. For my .tif files, is 150 ppi high enough resolution when I place them in my InDesign document? Does it make any difference to the final file if they are a higher resolution, such as 300 ppi?

I got dpi and ppi confused for a second.

Please let me know.

Thank you.

[Peter Spier]

Again, you haven't told us enough. It doesn't matter if the images are TIFF, JPEG or PSD. If matters how big they are in physical size compared to the saved size and resolution. Suppose you saved an image 2 inches square at 150 ppi in Photoshop. Now you place it in your ID document.

If you place it at 2 x 2 it has an effective resolution of 150 ppi, the same as you saved, but if you scale it to 4 x 4 the effective resolution drops to 75 ppi. At 1 x 1 the effective resolution increases to 300 ppi. All the same image and you haven't made any changes to the pixel information, but the quality of the image in print will be very different among the three.

You also haven't told us a thing about the output. Printing on newsprint you'd be unlikley to use a screen finer than 100 lpi, so an effective resolution of 200 ppi  would be acceptable, 300 ppi would be overkill, and 150 ppi would be in the low range of workable. With screen frequency of 85 lpi, which isn't abnormal, 150 ppi is quite good. A brochure on gloss coated stock would probably be printed at 150 lpi, and anything less than 210 ppi effective resolution is going to be problematic, with 300 ppi being the target value. A fine art book might be printed at 200 lpi, and would require images with even higher effective resolutions.

Peter

[WebPiper]

Yes, you are right. I need to be more clear. I prepare the layout in InDesign. Once it is finalized, I go back to Photoshop and size the images to the final dimensions they should be in the layout. For instance if a photo will be 3" x 4" in the final brochure, and the image I got from my client is 1200 pixels x 1700 pixels at 1 ppi (yes, I actually do get those), then I resize it, crop it, fix the color & exposure, and make the final resolution to be 150 ppi and save it as a .tif. The documents are printed on high quality paper—not newsprint. I export the PDF for the printer using the Press Quality settings.

Does this help?

And are you saying that if I am laying out something like a book with photographs, then consult with the printer first to find out what the resolution of the photos should be and what settings to use in Acrobat?

Many thanks. I appreciate your patience.

[Peter Spier · Accepted solution]

I would not, as a matter of course, save an image that has been sized to it's final dimensions at less than 300 ppi unless there were not enough pixels to do so without upsampling, in which case I would save at the highest resolution the pixels will support at that size. It's easier to downsample later if you find you don't need that much resolution than to not have the data you need.

The advice to talk to the printer is not resticted to books. He will tell you what sort of image resolution you will need for any project, and he may well have a custom set of PDF options to give you.

[WebPiper]

Early in my career—15 years ago, I was told by other designers and printers that I did not need images at a resolution higher than 150 ppi at their final dimensions. I wondered if this philosophy had changed. I've never had a printer question the files I've sent to them, and the projects have all turned out looking fine (clear, not muddy). However, I will start saving the images at a higher resolution for their final dimensions. Thank you for all of your advice. It is never too late to ask questions and improve practices.

[Scott Falkner]

I was told by other designers and printers that I did not need images at a resolution higher than 150 ppi at their final dimensions. I wondered if this philosophy had changed.

Assuming you are dealing with high resolution printing at 133 lpi or higher, then no, this philosophy has not changed: it has always been wrong.

[WebPiper]

Thank you, Scott. I will check with the printer before any news projects to find out the lpi.

[lars_p]

Another point to consider: the motive of the image.

If there is any text, line art or fine structures, even 300 ppi is quite low resolution and 600 or more ppi would be better.

Company logos and other things that should ideally be vector art should have a greater resolution than 300 ppi as well.

[Jongware]

Lars -- you are right in pointing this out, but this is only valid for monochrome bitmaps. If the image is not a bitmap, it will go through the halftoning engine, and the apparent resolution will change (for better or for worse) to the halftone lines per inch.

Pure bitmap images will never display as halftone dots, and in that case, the more dpi the better (although 1200 dpi is a reliable upper limit, since each single pixel will be 0.06 points wide and high, well below the usual limit of the minimally recommended 0.25 points line thickness).