File Re-sizing (Mega Pixels)

[PhilBJohnson]

From my basic understanding, reducing a picture image size in the same proportion in height an length (X,Y), with the right software, puts pixels closer together, so that when a file is enlarged again, it does not distort original picture clarity (it just spaces the pixels further apart).
A problem might have occurred in reducing picture file sizes in very small percentages, so that pixels started to stack in file reduction, producing an image once enlarged again, that was less clear than an original file.
Someone in software development might have thought about mega micro pixels, so that micro pixel dots, might be compressed closer together for file storage space.
Please can one advise on what they thought was the best software for file re-sizing, maintaining original picture clarity for Apple Mac and Microsoft Windows, if that software was not available in Camera picture export?

[Padfield]

Hi Phil. You can change the print size of a file by changing its notional dpi - which is what I think you mean by putting the pixels closer together. Thus, a picture saved at 150 dpi will print off at half-size if you change it to 300 dpi. This is 100% reversible and has no effect on either the file size in MB or the appearance on screen (to the best of my knowledge). Equally, if you are coding a website, for example, you can reduce the number of pixels a picture will occupy on screen by hard-setting the pixel size of the displayed image (without changing the image itself). This does not put pixels closer together but rather modifies the image at the level of the browser, so some pixels are conflated. This is also reversible, as it does not interfere in any way with the image file anyway. Some forms of image compression will reduce file-size in MB without losing any information but these have no effect on the print size, resolution or display of the image itself. Any form of compression that actually loses information is obviously not reversible. Nor is any reduction in the pixel dimensions of the image itself.

I am not aware of any other kind of digital manipulation of file size. If there is such a thing, I hope someone will add to my reply and enlighten me! I'm not an expert, but I do manipulate images for screen and print all the time.

Guy