72 dpi or 96 dpi - What's best for web images?

[Saz]

Originally posted by Egg Bramhill:
I've been to sites where the creator has put a 1200 dpi image on a web page and just resized it
inside their web editor to fit a page. This doesn't reduce the file size, just the size (and quality) of the image. But what your trying to say has nothing to do with what we're discussing
Egg

You edited your post while I was replying. http://www.talkgraphics.com/images/smilies/frown.gif

I don't get you. What are we discussing then?

[Egg Bramhill]

And that's exactly the point I'm making. On the web dpi or ppi makes absolutely no difference to a web graphic. The only thing that matters is the length in pixels and the height in pixels. You can set ppi to whatever you wish~ for web work it's meaningless, as is the physical size in inches.

I agree the images are different in their properties, but there just 640 x 450 pixel images to the web browser. The file size is the same between the two.

See here


Egg

[Saz]

As I've said, why the file sizes have come up the same I don't know. Perhaps it's to do with the image quality setting I used. I'm really stumped on that.

All I know is that one of my images loads quicker than the other. I even swapped them over a few times to be sure that it wasn't just the browser loading the first image first, if you get my meaning. I admit the time difference wasn't much, but I'm on a 1mb broadband connection so I didn't expect it to be.

[Egg Bramhill]

No Saz, it's not to do with your image quality settings. Image (a) has 600 x 450 pixels jammed into 2" x 1.5" (300ppi). Image (b) has 600 x 450 pixels spread over 8.3" x 6.25". In other words the same number of pixels but over diifferent REAL world areas.
Monitors don't understand inches, only pixels. If you have an 800 x 600 pixel display and look at either image in a browser they display at 600 x 450 pixels. Thats why the file sizes are the same.

The only way you could check on image download times would be to have 2 different web pages each with your different images on them, then test them over and over again, timing each one. It wouldn't make a jot of difference though as both images are the same file size.

Egg

[RTK]

Saz,

I'm not sure if I can explain it in an easier way than, Egg. But I'll try...

First of all your images are identical in size (in pixel dimensions and file size...)

Both identical images are 600 by 450 pixels --- consisting of 600 pixels X 450 pixels = 270000 pixels. That's why they are the same size...

DPI ONLY matters when printing. 96 dpi will be printed large (the 270000 pixels will be spread out.) And 300 dpi will be printed smaller with the 270000 pixels "packed" closer.

DPI is just "rubber" it tells how much the image should be stretched out when printing...)

Export quality drops colour information. Less data to keep track on = smaller file size. It doesn't drop pixels, it simply makes very similar colours (pixels) identical. A huge compression will take wider spectrum of similar colours, and make them identical.

Exporting an image at 10000dpi and at 4dpi will result in an equally large image (in pixels and file size). But they will be displayed the same size on-screen as your monitor is showing you pixels (not embedded in the file, dpi DATA).

[Saz]

Well that makes sense. I've never seen it described like that before, I will admit. Sorry Egg. I really think we've got off on the wrong foot here and been at loggerheads for no reason! It's just that everything I've ever read said the opposite. Plus there's this high res images taking longer to download thing, which I know you're not going to accept but I have had it happen to me while on dial-up.

If this is the case though, why do people say web images should be a certain ppi? In the google search I did, nearly every one said 72.

I'm of the understanding that monitors used to run at this resolution, so that would make sense, but if it makes no difference to the image display or load time why bother?

Then there's digital cameras. Mine and both of our children's automatically produce 72ppi images and there's no way to change that. Now mine is about 4 years old, but the kids only got theirs for their birthdays last July so are relatively new. If monitors run at 96 and the web doesn't matter, why don't cameras produce images at 96ppi (with the exception of those which allow for higher resolutions for printing purposes of course)?

Risto, refreshed the thread before submitting this and noticed you've posted while I was typing this. Thanks for your explanation too. http://www.talkgraphics.com/images/smilies/smile.gif

[Egg Bramhill]

Saz
Sorry if I gave you the idea we were at loggerheads. I just thought it was a great discussion and exercise. Like you I've searched the web re information on this issue and like you have found it conflicting and bias towards the 72 dpi. I just want to get to the truth of the matter.

Plus there's this high res images taking longer to download thing, which I know you're not going to accept but I have had it happen to me while on dial-up.

No, I agree. A hi res image of 24000 x 24000 pixels sent in an image format with no compression is horrendous. But I think that just confuses the current issue.

Thanks for your input Risto. I keep intending to create a tutorial re this issue, but it does tend to hurt your head!

Egg

[Saz]

Well I think we can safely say that the truth of the matter is that image resolution means diddly squat unless you're printing!! http://www.talkgraphics.com/images/smilies/biggrin.gif

[Egg Bramhill]

Amen

[RTK]

Not sure what "loggerheads" means, but it sounds kind of cute! http://www.talkgraphics.com/images/smilies/biggrin.gif "Hey, look at that couple out on their first date - they are all "loggerheads" with each other - awww!" http://www.talkgraphics.com/images/smilies/smile.gif

The reason why you see tutorials on the web mentioning 72 dpi and 96 dpi - is because they are normally written for people who re-size images once in a blue moon. And using simple applications that only mention dpi (or ppi). 72/96 or 300 dpi in those applications are normally just a simple approximation (guideline). For most people, exact file sizes don't really matter. Exporting at "web resolutions" is close enough for people sending out an occasional email with some family snapshots attached.

I did a search on Google for 72 dpi / 96 dpi tutorial and found a really LOOOONG one:

http://www.scantips.com/basics1a.html It also explains this obesession with 72 dpi (in the end of it). I think Gary or Klaus has also mentioned it here at TG before?! Yes, at one time 72 dpi was exactly one inch - on a 1984 Macintosh computer... Monitor sizes and resolutions has come a long way since then... So has the ability to display images on-screen.

As for the "difference" between DPI and PPI - Instead of making this post even more long-winded - also look at this article at my favourite camera review site (Steve's) - http://www.steves-digicams.com/techc...uary_2005.html