Photoshop Ninja

Image resolution is a very important area to fully understand when using Photoshop, as it dictates the printed output dimensions and quality of your image.

Really printed output dimensions are dictated by DPI (Dots Per Inch), or sometimes referred to as PPI (pixels per inch) - whatever term you use they mean the same thing. This refers to how many pixels get printed in an inch. So, if you have a 600 pixel x 600 pixel image at 300DPI, it will output at 2 inches square. If you change this images DPI to 150, this will mean it will output at 4 inches square. So, as you can see, changing the DPI of an image changes it output size.

However, lowering the DPI too low produces progressively lower output quality. Optimal DPI is dictated by the output medium and size. 300DPI produces high quality output on any medium, but produces large files. I personally use this for anything smaller than A3, unless I need really high quality output. For A3 images I would work to 300-200DPI. For larger items I would use 150dpi, and for really large items like exhibition panels I would use as low as 75-100dpi.

For large format digital prints you can actually get away with amazingly low DPI and still get extremely good quality prints (RIP, printer and operator allowing!), 100DPI is no problem for this kind of print.

Good quality Lytho (or similar) commercial printing really requires you to have your images in the 300-200DPI range.

Screen printing is inherently lower quality than lytho and usually uses a courser screen to print with, so you can get away with going lower than 300DPI with this style of printing.

For home inkjet or laser prints and even digital laser prints you can actually get away with images as low as 150DPI, dont think that because your inkjet printer has an output DPI of 2800 you need your images to be set to this resolution, this just refers to the printed ink density on the page!

Obviously, if you are bringing your Photoshop prepared images into a layout application like Quark or InDesign, you can scale your image to different dimensions - scaling an image larger effectively lowers the actual output DPI, so if you scale our 600×600 pixel 300DPI square image by 200%, you just lowered its actual output DPI to 150DPI.

On the flip side of this, if we where to take the same image, and scale it by 50%, we would end up with an actual output DPI of 600. You may think this is better - it is not! All this actually does is slow down the printing of the document (the RIP has to resize your image), and produce larger jobs to send out to print and archive - this really does you no favours!

When using images in such applications I personally place the image on the page and resize it as required for the design in the DTP application. Once the design is signed off I will then go back and scale the image in Photoshop so that it is sized at 100% in my DTP application. Doing this also allows you to accurately control sharpening of your image.

If you are enlarging an image in your DTP application - again, go to Photoshop and resize the image for all the same reasons above, as well as Photoshops image resize algorithms are fantastic! You also will see how your image quality is degraded and have the ability to combat this in Photoshop, rather than just cross your fingers and hope that no-one notices you resized that image by 250% and its going to look rough - until your client demands a reprint that you have to pay for and get delivered…by tomorrow!

If you want to change the DPI of an image, go to Image Size, tick off the ‘Resample Image’ option, and then type in a new DPI, you will see the image dimensions change to reflect this. Alternatively, if you know the desired output size, you can type this in (with Resample Image unticked) and see what DPI your image will output to.

This information only refers to printed material, image dimensions for web and multimedia usage are purely defined by pixel dimensions and they are traditionally set to 72dpi, although I usually prepare my images at 150DPI if im ever unfortunate enough to have to prepare images for Powerpoint - they will then print fine on an office laser or inkjet printer.

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• Karma Kula

This entry was posted on Tuesday, July 19th, 2005 at 8:47 am and is filed under Photoshop Top Tips, Techniques - Print. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.