How do I export or save smaller sized images?

Question

I have a design that is A3 size (i.e. 297 × 420mm) and 300dpi. I'd like to save out various versions of the image: one at full size, one at 150 dpi, one at A4 (i.e. 210 × 297mm) 150 dpi, and one at A4 72dpi. At the moment I'm doing this by saving different versions of the Photoshop PSD file and then doing a 'Save As' jpg from each of those.
What's the best way to save multiple sized versions of a design?
If I resize an image in Photoshop to be smaller and/or fewer DPI is that destructive or can I go back without loss of clarity? (I'm guessing not as the PSD file sizes are so different.)

Answer 1

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.