Forget about the dpi (dots per inch for printer spec) or even ppi (pixels per inch). If you downsize you can lose pixels, and therefore quality. Best to make HQ origianl and keep it, then make smaller copies as needed. So think you are confused as to what you think you are getting. What is the end result you want?
What you need to focus on is image size of height and width in pixels. Access this in Image/image size.
Thanks Curt & Janelle
Curt says "forget about the dpi". Why? I'm making a poster that has a scanned watercolour as part of the background. Surely the dpi directly affects the print quality? Why is it wrong to supply the image at A3 300dpi, A4 150dpi, and A4 72dpi say?
The blog post Janelle recommends mentions the 'Save for Web & Devices…' feature but that seems optimised for screens not print as the measurements are all in pixels (otherwise that would have been exactly what I wanted, I think).
The final video tutorial in Janelle's answer has good beginner tips on resizing (e.g. use Bicubic Sharper) and suggest that for making A3 300dpi into A4 72dpi I should just use the standard image resize feature and then just click 'Undo' to revert to the original (rather than having to maintain multiple copies). I'm a little nervous about that though - if I forget to undo I'll have lost the original.
It's a shame there is no feature akin to 'Save for Web & Devices…' but for differing sized prints.
I would recommend you watch the following links, they are advice on how to prepare images for printing.
I think you and Curt are saying the same thing. As long as you keep the original render intacted.
Why the 72ppi? If it is for the web or any video screen the ppi is irrelevent and therefore not required, you only need to deal with pixels.
As for your question on if there is a quality loss, that depends on the resample check box when you resize the image in photoshop. That check box when not checked disables the ability to alter the actual size in pixels. When checked you can type in size size in pixels.
This also affects the size of the print when adjusting the ppi. Because the pixels are locked the size of the print must change, or if you alter the print size the ppi must change.
When the resample check box is checked, then the print size can stay the same when altering the ppi but the size in pixels changes.
Use this forumla:
width in pixels / ppi = width in inches
width in inches X ppi = width in pixels
width in inches / width in pixels = ppi
You can insert height and metric system where appropriate.
For help speeding up the process of changing the sizes that are required. Take a look at the batch command, image processor script and actions. Using these can help automate your process. The image processor does not alter the original image, it make a copy in a seperate folder that is usually located inside the folder of the original image.
Silkrooster asked "Why the 72dpi?". I want an A4 72dpi version for email attachments. A4 as it'll keep the ratios of the origional and thus still look like the poster, 72dpi as that's the resolution of many screens. Given that the project is conceived of in paper sizes (it is a poster) I'm not sure that sudenly swapping to think in pixels is useful.
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[...] for making A3 300dpi into A4 72dpi I should just use the standard image resize feature and then just click 'Undo' to revert to the original (rather than having to maintain multiple copies). I'm a little nervous about that though - if I forget to undo I'll have lost the original. [...]
You won't lose the original if you do it to a duplicate document. Menu Image > Duplicate.