Reported Batch DPI change.. not working?

[BobK]

After doing a Batch Resize, I decided that since I was reducing the size of large images to 30% of original, I would increase the original DPI from 72 to 200 for better quality.
After the second run (to a different directory) the resulting file sizes of the 200 DPI run were identically sized as the first, 72 DPI run.

Using the IMAGE Information, I found the first run showed 72 DPI (same as the original large images) and the second run showed 200 DPI.

Shouldn't the 200 DPI images be much larger file size than the same sized 72 DPI images? Am I misunderstanding what 'file size', 'image size' and 'DPI' mean in Irfanview?

Thanks..

[j7n]

The file size is mostly determined by number of pixels. The amount of detail and noise is also a factor in case of compressed file formats. If you were to resize an image so that each horizontal and vertical dimension was 30% of the initial, the area of the image would decrease to 9%. The file size will change to a similar fraction; it may be slightly bigger than that because the density of detail has increased in the new image.

DPI describes the area than the image would cover in the event that it is printed. It is a kind of "tag", a numeric property, which can be set without changing the pixel data or quality. DPI usually has no effect for on-screen viewing, since you can zoom in and out of an image as needed, or display 1 image pixel onto every screen pixel for maximum fidelity. Some software may use the DPI value embedded in the image to set the initial zoom at which the image is displayed, for example if it is inserted into a word processor document, which works with "inches".

DPI is meaningful for scanned images, designs for printing, such as adhesive labels, but not photographs. It describes the physical size of the original document that was placed on the scanner. If you start with a 600 dpi original, and decide to resize it to 30% in each dimension, the new accurate DPI value would be 180. You would typically use either 33.3% or 25% "round" scaling factor. When downsampling, the dpi becomes smaller. Regardless of file size, photographs typically are assigned some "standard" dpi value, either 72 or 96 dpi, which approximates the pixel density of a traditional monitor, or none at all.

If you want to have better quality, then you must have more pixels. A more efficient compression format could be used to keep the file size the same. If the current format is PNG, you could choose JPEG instead and approximately double the number of pixels without noticeable artifacts.