by
genevh » Thu Jul 24, 2008 12:33 pm
I checked a picture I just resized for the web in CS3 for the latest LR/ACR challenge, and when I opened it again it also shows as 240 ppi in the CS3 workspace.
Now, where I'm getting confused in your post, you say when you are doing a save for web operation, you are setting your resolution to 72 ppi. I'm not seeing where you can do that in the save for web window. When you do Image -> Resize in save for web, and set the width and height, you are not setting resolution but how many pixels wide and high your image will be.
My understanding of the save for web is that it provides a convenient method for setting a picture size (based on pixel count, not pixels per inch) and it also makes the conversion to the sRGB color space, which is used for web/monitor display, along with resampling the image to meet the size you specify. For instance, when I upload a picture for insertion into a post, I usually set the longest side to 450 - 475 pixels to minimize scrolling and keep the size of the file manageable. An image that starts out at 4278 pixels wde by 2848 pixels high and is a 5.2 Mb file, when resized to 475 pixels wide by 317 pixels high in save for web ends up turning into an 87 kb file. A significant difference, and a lot of resampling is being done on the image as a lot of pixels are being thrown away.
I think the 240 you are seeing in the bottom left corner of the CS3 workspace is the native resolution (ppi) CS3 displays at. You should also see your resaved picture is a lot smaller than what it started out at even though CS3 says its at 240 ppi. You can of course adjust that resolution via Image -> Image size, and if you do and resample and go to 72 ppi, that image is going to shrink quite a bit on your display. This is probably what you are seeing and are concerned about. Take that same image without resizing in the workspace and start the save for web process again, and you will see on the image size tab that it is still at the pixel dimensions you set the first time you saved it.
To illustrate my point, do this exercise on the CS3 workspace and reset the PPI of a photo and uncheck the resample box. The document size will change as reported in the resizing window, but you will not see the image change size on your display, and CS3 will report that image as being whatever PPI you chose to set it at. Print at that size, and your printer should print it at whatever document size the resizing dialogue said the picture would be. A lower PPI will give you a larger physical print, a higher PPI will give you a smaller (but higher quality) physical print. Resizing this way does not add or subtract pixels from your picture. It will either spread them out more (lower PPI) when you print, or squeeze them down (higher PPI) into a smaller physical dimension when you print. You will also see that the pixel dimension section in the resize window now sets itself so you can no longer change that dimension. The pixel width and height in this scenario doesn't change, but the size of the pixels (when printing) do.
Now, resize and check the resample box. You will now see the image size on your display change accordingly, but the resize window will not report a change in document size in inches, however, you will see the pixel dimensions change. Up the resolution and the pixel dimension will increase. Lower it, and the pixel dimension reduces. Document size in inches will stay the same either way. The reason your image resizes on your monitor at this point is because you are pretty much stuck at whatever your monitor resolution is (72 ppi for CRT and around 96 ppi for LCDs). The image resizes accordingly, and you are also adding or subtracting pixels in your picture when resizing with the resample box checked. This is also why picture quality is affected during resampling. CS3 is taking an "educated guess" on how to handle the pixels in the picture.
I hope this makes some sort of sense.