GIF exporting from Photoshop at low resolution and at a larger size

[ryanc45517540]

I have a Photoshop file set-up as per the attached image. 180mm x 50mm, resolution of 200ppi.

I want to export it as a GIF with a transparent background and at the size and resolution as the file set-up. 

When I do export the file, either 'save as', 'export' or 'save for web (legacy) it saves the file at 499mm x 140mm and a resolution of 72ppi, see image attached.

Firstly, why is it doing this?

Secondly, when I open the exported GIF in Photoshop and resize it and change the resolution and save it, it says it's saved, but when I close and open it again, it's reverted back to 499mm x 140mm and a resolution of 72ppi.
I have no idea what is going on, in my 15 years as a designer, I've never seen this happen. I've attached my Photoshop file if yoy want to see how it's set-up.

Thank you

[Nancy OShea]

200PPI, CM and MM are all print values. They have no meaning on the web. 

For digital art and web images, I prefer to work with pixels.  I start with an oversized canvas with a transparent background to which I add my other elements. 

Save first as PSD. 

Then export to JPG, PNG or GIF and resize as needed.

[Warren Heaton]

When saved as a GIF, it has to be flagged as 72 dpi.

I find it helpful to make a duplicate of my print resolution documents (Image > Duplicate...), adjust the Image Settings based on Pixels at 72 dpi and then use Save for Web (Legacy).  You can close the duplicate without saving or save it with somethign like "for Web.psd" at the tail of the filename.

[davescm]

When you export as a Gif, the file does not have any ppi value set. The GIF file format has no ppi value information, as it is meaningless for its purpose of display on a screen , where only pixels matter.  So whether the source file is set to 720 ppi or 72 ppi is irrelevant. The GIF itself will not have a ppi value.

When you re-open the file, any file with no ppi value set is allocated, by Photoshop, a value of 72 ppi. That allows Photoshop to have a nominal value ready for printing and to use with on screen rulers.

Dave

[Conrad C]

The animation below was created from the file that was attached: The Image Size dialog box from the original vs the GIF export. The image size, in pixel dimensions, did not change.  Other than the expected file size change, the only other change was the assumed physical dimensions (cm), which had to happen because the ppi changed when the actual number of pixels did not.

Why did the ppi change? It isn’t just that ppi etc. have no meaning on the web, I don’t think the GIF format has a way to store inches, mm, or ppi. The only way to express dimensions in the GIF format is pixels, as far as I know. Trying to shoehorn any other unit of measure into GIF will require conversion to pixels. When Photoshop opens a GIF, it assumes 72 ppi because there is no ppi info in the file.

If you want to save as GIF, you must accept that the ppi value will be discarded.

If you want ppi to be respected, Save As to a format that can store a ppi value, such as JPEG.

Why Save As instead of Export As or Save for Web, for a JPEG? Save As preserves maximum metadata, while the latter two do optimization for the web, so they discard any metadata not required for web browsers, including ppi, to keep file size as low as possible.

Also, nothing has changed recently in this area, everything above has worked that way in Photoshop for many years.