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I am designing a banner which will be printed at 7ft x 4ft.

I am using Photoshop CS4. The images are not vectors. The original poster artwork has been done at 300 ppi for A4 printing. I was hoping to re-size the canvas of the poster (after saving as a new file) to keep the 300 ppi resolution but working on a smaller scale to reduce the file size. This is mainly to avoid having to copy all the layers in to a new file to recreate the existing image.

I have looked around at various questions already on the site, but can't seem to find an answer. The common recommendation seems to be that banners or other large-scale pieces that will be viewed from a distance only need to be around 75-100 ppi, but I was hoping to work with a higher resolution and smaller size, instead of lower resolution and larger canvas.

Any and all advice or suggestions are greatly appreciated.

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A5 @ 300 ppi = 25MB, A6 @ 600ppi = 25MB. 1/2 the size, double the resolution, same file size same CPU processing etc etc – horatio Mar 13 at 21:00
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I meant to add: A4 is about 12" on one side, so the unaltered unresampled artwork is going to be approx 40ppi @ 7 ft. To get 100ppi, you'd need ((7x12)x100)) by ((4x12)x100)) pixels. Also the A5 in first comment is a typo, should be A4 – horatio Mar 13 at 21:10
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Speak to whoever will be printing/reproducing the banner. They'll have specifications to follow. Anything else is guesswork - sometimes good guesswork, but still guessing. – Scott Mar 13 at 22:33
Thanks for your quick replies! I will get the contact details for the printers. At least if I don't manage to get in contact with them, I'll be able to work it out. Thanks for your advice. – AoifeUnudottir Mar 13 at 23:02
This should have been added as an answer, @horatio! I suggest you elaborate a bit more and add it as one. – kevin9794 Mar 14 at 6:09
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1 Answer

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Decision on dpi varies according to banner positioning in space, content and printing technology. Often 100dpi is enough - but don`t take it for constant.

7x4ft in 100dpi resolution means 8400x4800px image.

For estimating image size, there are handy dpi calculators like this http://www.photokaboom.com/photography/learn/printing/1_calculators.htm#Print

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a quick rule of thumb for file size is divide the each pixel dimension by 1000 (actually 1024, but keep it simple) (shift the decimal 3 places to the left), multiply them, then multiply by the number of colors (RGB=3, CMYK=4). The result is the approximate file size in MB. ( 8400x4800 > 8.4x4.8x3 > 120 ). Add a color for each Alpha channel. This does not take into account layers. – horatio Mar 14 at 14:24

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