How to setup an image for large format printing?

[defaulta0zgawvfvnlx]

Hi there. 

I have a client who wants a cell phone picture printed on a 12 x 5 meters big banner. 

The printing company says they want the output file to be at approx 70-80 DPI at 100% size. However they only accept PDF files. 

When i put in 1200 x 500 cm at 80 DPI, the image will be around 22 GB and can only be saved as PSB-files which the printing company will not accept. 

I have to go as low as 25 DPI in order to be able to export as PDF, and now the printing company says that he will blame me if the printed banner comes out too gritty. So I'm kind of in a pickle here - what would you suggest? 

[Peru Bob]

I've moved this from the Using the Community forum (which is the forum for issues using the forums) to the Photoshop forum so that proper help can be offered.

[D Fosse]

A cell phone image at 12 x 5 meters doesn't sound like a particularly good idea. The most important consideration for things like this is that it really needs to be a top quality image to begin with - good exposure, no burnt out highlights or crushed shadows, tack sharp, no shake, no unintended flare, in short, it should be done by a very competent photographer. This will be up for critical view on a permanent basis, it's not instagram where you quickly scroll by.

That said, I've done similar sizes at 20 ppi and it works splendidly. But then it's a high quality image from a high-end camera. The point is that it will be seen from far away. The eye wants to take in the whole image, so you step back. You can't help it.

The basic rule of thumb is that a good image file will work for any print size - magazine spread, wall sized billboard, same file.

I've noticed that printers often start by asking for unrealistic resolution, for what reason I don't know. Once you discuss it through with them, you can usually settle on a more realistic figure. There's no way 70 ppi is really needed here, unless this is a very special situation where you're required to go right up to it and fix your gaze on one tiny part at a time. But I wouldn't do that with a phone image anyway, so...

[Per Berntsen]

I totally agree with @D Fosse .

An image from a phone is the worst possible starting point for a banner this size.

You're asking for suggestions, and my suggestion is to have the image reshot with a proper camera, and then properly processed for the purpose. I'm afraid that the printed banner will come out gritty, coming from an oversharpened jpg from a smart phone. But of course, you're not to blame for this.

You can't make a silk purse out of a sow's ear.