File size/resolution

[Ed_L]

I keep seeing posts where people talk about file sizes, and they mention saving the image at 72 ppi. I have always thought that the file size was determined by the number of pixels in the image, and had nothing to do with the resolution. Am I missing something? Since this came up before (but never agreed or disagreed on), I opened Photoshop, and made a new image 288 pixels X 360 pixels. I painted a smiley on it, then saved it at 72 ppi, as jpg at the best quality. I then changed the reslolution to 1200 ppi, but I kept the pixel size the same. This was saved as before, with the only difference being the resolution (and file name). File sizes in Photoshop, and as seen in Windows Explorer confirmed that the two images were identical in size . Is there actually a reason to save the file at a lower resolution? It is my understanding that the image will display the same size on a monitor regardless of the Photoshop image resolution, as long as the pixel size remained the same. If I'm wrong in my thinking, please clear this up for me. It drives me nuts every time I see a discussion on it, and it leaves me wondering where, or if, I'm wrong.

Ed

[Doug Nelson]

Don't confuse monitor size and print size. A 1" wide image saved at 300ppi will be 300 pixels wide on your monitor, but 1" wide on your printer.

[Ed_L]

Yes, I know that. But that's not the question.

Ed

[Doug Nelson]

Than I guess I'm missing your point. Save for web can only save at 72ppi, even if your original is 2400ppi. To save as a 2400ppi jpg, you need to use 'save as...'.

[Ed_L]

Let me put it this way. If I use "save as", and save a file that's 500 X 500 pixels and 72 ppi, wouldn't that be the same file size as a 500 X 500 pixel image at 1200 ppi?

Ed

[Doug Nelson]

A 500x500 pixel image is a 500x500 pixel image. The 'dpi' is a marker at the beginning of the file. A 1200ppi image that is 500 pixels wide will print at 5/12ths of an inch wide. If you change that same file to 500ppi (turn off resampling), nothing in that file will have changed except that marker inside the binary file, and it will now print at an inch wide.

But resampling will change the entire file.

[Jakaleena]

There's also an interesting thread on this subject HERE for anyone interested.

Hopefully, someday, someone will be able to explain this thing in a way that easily makes sense. I still get a little shaky in this discussion and have to think in very simplistic, analogous terms in order to get my brain to wrap around it even a little bit...

[Ed_L]

I can't believe I'm having such a hard time explaining this. I meant ppi, or pixels per inch. This should have an effect on *print* size, but not on the image size on a monitor. I think where the whole misunderstanding is the "save for web" feature. Older versions of Photoshop don't have the "save for web" option, and I guess I'm still thinking of the older versions.

Ed

[Doug Nelson]

As I've explained in other threads here, ppi and dpi have very little to do with each other. You can print a 72ppi image at 2400 or 72dpi and they will print at the exact same size. Same with a 2400ppi image.

Confusion comes from the casual interchanging of the terms, but PPI and DPI are very different.

A printer can use many dots to print one pixel, or have one dot represent many pixels. A monitor doesn't have that luxury, which is why 72ppi has become standard for the web (most, but not all, monitors display 72ppi, but even that is relative depending on your display resolution and monitor size, but some sort of standard had to be struck).

Several of our Challenges are only 2 or 3 inches across in reality, but your monitor displays them with a 1:1 pixel ratio. Therefore this tiny image will overflow your monitor.

I know we went through all this before somewhere.

[Grant Hills]

Hi Doug,

After reading this topic I was just wondering if a Basics or FAQ section might be an idea for Retouch Pro? Could be somewhere to direct people with similar inquires - mind you it took me some time to comprehend (I think!) the relationship of file size/resolution etc

love the site.

regards
Grant

[G. Couch]

Grant - There is a Glossary here with lots of good info on DPI, LPI, PPI, etc...along with a lot of other basic info. Feel free to add stuff to it!

[OhThatGirl2001]

I found a great link that clears up a lot of questions and uses photoshop and an image with resizing as an example.

http://www.luminous-landscape.com/un...resolution.htm

Lisa

[Stephen M]

I have come a bit late to this thread - but here is a link to my list of links on resolution and resizing images:

http://members.ozemail.com.au/~binar...V_links.html#R

Regards,

Stephen Marsh.

[OhThatGirl2001]

Thanks Stephen ....

Incidentally, nothing's ever too late!

Lisa