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.
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