Resolution for Printing

[mergino]

Anybody know 'rules of thumb' for the minimum size/resolution of a jpeg required to print
* A4 Poster
* A3 Poster
* A2 Poster
* A1 Poster

The reason I ask is I have a jpeg file of a poster (1653 x 2339 pixels - 200 pixels/inch) and need to know the max. size it can be printed on

[StewartR]

The resolution you need depnds on how close your eye is to the picture. In turn that usually depends on how large the picture is: you won't normally be looking at an A1 poster with your nose right up to it.

On the Cambridge In Colour (http://www.cambridgeincolour.com/tutorials/digital-photo-enlargement.htm) web site, there's a calculator to help. For somebody with 20/20 vision, it suggests:
350 ppi when viewing at 25cm
175 ppi when viewing at 50cm
87 ppi when viewing at 1m
18 ppi when viewing at 5m

So for a poster that you're putting on the wall, you'll probably want it to look good at 1m so 87ppi is enough - say 100 ppi to give yourself a bit of latitude and to make the maths easier. Your picture is 1653x2339 pixels so at 100ppi that would print to 16.5" x 23.4". That's almost exactly A2 size. Not bad for a 4 megapixel image, hey? But - don't look at it too closely!