question about picture quality

[jyeh74]

I am viewing two different jpg pictures over the years and noticed some changes in picture file size and the dpi sizes.

The newer picture is 2300 x 3300 resolution. Horizontal and vertical is 300 dpi. Size is 500 kb.

The older picture is also the same 2300 x 3300 resolution. Horizontal and vertical is 72 dpi. Size is 3 MB

I looked up the difference between 300 dpi vs 72 dpi. From research, 300 dpi allows you to print high quality photos, whereas 72 dpi is for online images. How come the size of the picture decreased from 3 MB to 500 kb however, the dpi increased? I thought if dpi increases to allow for better higher quality printing, then the total size of the picture will increase?

[davexnet]

DPI is not particularly important, it's really like a default print size and can usually be overridden when ever the image is printed

[jyeh74]

I always thought that the larger the file size, the higher the quality. That’s why I thought the new file at 500 kb was worse quality than the old picture at 3MB. But I checked the resolution is the same on both. So how did they shrink it to 16% of the original size and still maintain the same resolution?

[jyeh74]

DPI is not particularly important, it's really like a default print size and can usually be overridden when ever the image is printed

Are you saying if I wanted to print the older picture at 72 dpi, I can somehow change it to 300 dpi before we print it? Where is that configured?

[davexnet]

DPI is not particularly important, it's really like a default print size and can usually be overridden when ever the image is printed

Are you saying if I wanted to print the older picture at 72 dpi, I can somehow change it to 300 dpi before we print it? Where is that configured?

It's been many years since I printed anything on a home printer, but I remember there was always a way to set a print size
(in inches) and that is implicitly related to DPI.

For example, assume you have an image at 640x480 resolution and you print it at 8 inches by 6 inches.
That's an efective DPI of 80, (640/8) - doesn't matter what the file says when you display the properties

[davexnet]

I always thought that the larger the file size, the higher the quality. That’s why I thought the new file at 500 kb was worse quality than the old picture at 3MB. But I checked the resolution is the same on both. So how did they shrink it to 16% of the original size and still maintain the same resolution?

If it's a jpg file, it's a lossy format, there are various quality levels you can set when the file is saved (in some programs).

It's like comparing an mp3 music file to the original WAV/PCM uncompressed source

In both cases (jpg and mp3) lower "quality" setting = smaller file

[thecoalman]

The newer picture is 2300 x 3300 resolution. Horizontal and vertical is 300 dpi. Size is 500 kb.

The older picture is also the same 2300 x 3300 resolution. Horizontal and vertical is 72 dpi. Size is 3 MB

Assuming the old picture is source file for new picture whenever the DPI was changed the photo was resampled. DPI is just meta data and changing the DPI by itself doesn't change the image data.

A pixel has no native dimension, DPI gives it dimension. It's sets the default size for when you have a physical canvas, e.g. if you have a 100px*100px image and the DPI is 100 the default physical size is 1 inch by 1 inch. If you open your word processor the canvas is inches, cm or whatever. The old photo would actually be too big to occupy a standard sized page. The new photo is going to occupy a space about 4.6 inches * 6.6 inches.

As another example a browser can care less about DPI, both images would display exactly the same size.