Book Design and Imaging

Image size - inches vs pixels

When I resize images, reducing pixel size from say 6000 to 2000, the image is now 2 inches square instead of  7×7. Does this matter for print repro? Any suggestions for resizing for blurb formats?

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Posted by
kconnell
Sep 8, 2007 1:03pm PDT
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kconnell
 

I am guessing you see it as 2" square in a photo editing program? The problem is to do with how photo editing programs use DPI (dots per inch) when converting pixels to inches (cm, feet etc). When a program of this sort tells you the size in real world units like inches, what it’s actually  converting the number of pixels to inches by dividing by a DPI figure. The important thing for Blurb is that the DPI doesn’t matter (if you’re using the standard templates). A 2000 pixel image has the same resolution regardless of DPI. The DPI is only interesting if you want to specify a particular print size in inches/millimetres and know the exact DPI of the printer.

Using the Blurb templates and the rescaling function in Booksmart you don’t need to know anything about DPI. Just roughly follow the size recommendations in pixels that the template provides and you’ll be fine. It’s OK to go a bit under the recommended figure, but going over is just wasted storage space.

Posted by
Claustral
Sep 8, 2007 11:49pm PDT
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Claustral
 

Thanks for the info. Just to clarify, when I go to Image size in Photoshop, all I have to change are the pixel dimensions, ignore the size in inches and dpi? These change automatically when I adjust the pixels

Posted by
kconnell
Sep 10, 2007 12:00pm PDT
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kconnell
 

Got it! While you can ignore DPI altogether, it wouldn’t hurt to set the image to 300dpi.That’s the resolution used by the printers. But don’t worry about the inch value Photoshop produces. It doesn’t matter.

Posted by
Claustral
Sep 10, 2007 12:30pm PDT
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Claustral
 

Begging your pardon Claustral, but it certain does matter.  If kconnell takes her reduced 2000 pixel 2" image and tries to make it fill a 7"x7" page it will look horrible.

Kconnell – always work at 300 dpi to get an accurate idea of what your image is doing.  If you find that Photoshop locks all the numbers together, check the box "Resample Image" in your Image Size dialog box and you will find that it will let you change them independently now.

Posted by
niftycurly
Oct 1, 2007 1:59pm PDT
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niftycurly
 

do one or the other, but don’t change  the pixel dimensions and the  ppi/dpi in photoshop at the same time.

while you’re in the digital world you can forget about ppi/dpi and pay attention to the strict pixel dimensions of your image.

if you do want to set your mind at ease and change your image resolution in terms of "pixels/inch" make sure you uncheck "resample image" unless you want to interpolate your image – blowing it up or shrinking it down.

ppi only matters in photoshop if you’re doing any output to say your color printer, and you’d like your image to be printed out so that 300 pixels is equal to 1 inch. In your case, if you print your 2×2" image from photoshop, it should take up 2×2" on your paper print out. If you want to spread those 2000 pixels across 10 inches, then change your ppi to 100 pixels/inch in PS and print.

for blurb, just work in absolute pixel dimensions.

Posted by
brokendreams
Oct 1, 2007 4:40pm PDT
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brokendreams
 

oops, that should be:
2000 pixels printed at 10 inches, change your ppi to 200ppi
2000 pixels printed at 2 inches, change your ppi to 1000ppi

Posted by
brokendreams
Oct 1, 2007 4:42pm PDT
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brokendreams
 

In absolute terms: the number of pixels in the image will determine the maximum size that you can print that image at any given resolution. Use ppi (pixels per inch) when displaying images on screen and dpi (dots per inch) when describing printer resolution.

Assume 300 dpi printer resolution (output resoltion in your image editing program) as the printer resolution… because blurb use that resolution and at that dpi setting, the eye will not be able to determine differences between that and a higher printewr resolution.

If you have a 2inch square image (actual pixels), your image MUST contain a minimum of 600×600 px (300ppi) if you want to print it at its actual size on paper (at 300dpi). To print the same image at 7×7 inches, you must have a minimum of 2100×2100 pixels (7inches x300ppi) before you have sufficient pixels to print at the linear dimension you want to.

Claustral is mistaken. Altering the actual size that an image prints onto paper MUST require a change in the number of pixels divided by 300ppi else you will have a low resolution image.

Posted by
jayemcee
Oct 1, 2007 4:51pm PDT
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jayemcee
 

This is a classic thread here on the Blurb forums.

Here is the thread from when we visted it last, there were several very good posts.

Hope that helps.

-Ben

Senior Technical Suport Specialist

Posted by
bmartinek
Oct 2, 2007 10:58am PDT
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bmartinek