InDesign is always exporting in proportions and respecting the resolution and size.
What dimensions are you talking about? Pixel? CM? ???
So I’ve been creating some ads for the web in InDesign and then exporting them as .jpgs. My issue is that I need to export them at the dimensions 320 x 50 at 300 ppi.
InDesign measures a pixel as 1/72" so if you export at 72ppi the page's 320x50 pixel dimensions will export unchanged. If you really want the 320x50 pixel image at 300ppi you could uncheck Resample in Photoshop's Image Size dialog and set the Resolution to 300, but it won't make any difference when the image is placed in a browser—HTML only considers pixel dimensions.
So when I export as a .jpg, just keep the res as 72 in the dialog box? Is there here no need for a 300 res jpg with today's sophisticated monitors?
Sent from my iPad
Pixels per inch is a print output measurement and doesn't relate to browser display. When you insert an image in HTML at 100% the relationship between image pixels and monitor pixels is usually 1:1 (except most browsers will detect HDPI or retina displays and upscale images). So the image will appear physically larger on a lower res display because the monitor pixels are larger.
Is there here no need for a 300 res jpg with today's sophisticated monitors?
There is a need for high resolution images for Retina/HiDPI displays, but they're not created by specifying ppi directly. The answer is still as the other posts have said: You use pixel dimensions. The pixel dimensions you use depend on the "scale factor" or "device pixel ratio" of the devices you're targeting. For example, most Apple Retina displays use a 2x scaling factor, so for the ad you mentioned earlier which is 320 x 50 pixels, the Retina version of the ad would be (2x 320) by (2x 50), or 640 x 100 px at no specific ppi.
The InDesign issue is unrelated. You need to leave the JPEG export ppi at 72 to work around the bug that incorrectly uses the ppi to adjust the export pixel dimensions.
For example, most Apple Retina displays use a 2x scaling factor, so for the ad you mentioned earlier which is 320 x 50 pixels, the Retina version of the ad would be (2x 320) by (2x 50), or 640 x 100 px at no specific ppi.
to work around the bug that incorrectly uses the ppi to adjust the export pixel dimensions.
It's not a bug, Indesign allows you to set pixels as a unit of measurement and for ID it is equal to 1/72" (it has to be a static unit because the page object has no resolution). So the export math works correctly—if you make a page that is 10"x10" and change the rulers to pixels its dimensions will be 720x720. When you export at 72 pixels per inch you get a Photoshop image that is also 720x720 (10"x72=720). If you export at 300ppi you'll get 3000x3000 (10"x300=3000)