[Definition] Resize vs. Resample

[Rhonda]

When changing the size of an image, what's the difference between resizing and resampling? Is one better than the other? In one of the forums on scanning it was suggested to scan high and resize (but not resample) and that confused me. Thanks.

[Ed_L]

Hi Rhonda,

Welcome aboard. I hope I can explain this properly. If you change the document size, but you do not resample, the pixel dimensions remain the same (and the information remains constant), but the resolution changes. Let's say you have an 8 inch by 10 inch document, and the pixel size is 2400 X 3000. This equals 300 ppi resolution. If you change the document size to 4 inch by 5 inch, and you do not resample, the pixel size is still 2400 X 3000, but the printed copy will be reduced to 4 inch X 5 inch, and the resolution will be 600 ppi (2400 (pixels) divided by 600 (pixels per inch) equals 4 (inches). When you resample, you either add pixels (if you make the document larger), or you discard pixels (if you make the document smaller). When you do that, Photoshop guess which pixels should be eliminated (when you make the document smaller), or it guesses what the added pixels should look like (when you make the document larger). If you do not resample when you make the document smaller in size, you keep all the information available. To see this for yourself, open an image, then go to "image size". Then change the document size, with "resample" checked, and again with "resample" unchecked. Make note of the pixel dimensions and the resolution as you do this. If you resample to a larger document size (upsampling), you wind up with more pixels, but they do not represent true information. I hope I got this right, and it helps.

Ed

[Rhonda]

resize vs. resample

Thank you for such a quick reply. I think I've been doing it backwards. If I have a small image (say 2X3) I usually scan it at 1:1 and make the resolution pretty large, say 600. It them opens in PS as a large document...both size wise (which is what I want) and resolution (which I don't want). I then go into "image size" and change the size to 5X7 and reduce the resolution to 125 or 300, depending on what I'm doing. It usually looks ok. I've never noticed the resample box. I guess I'm throwing away a lot of information doing it this way. I'm going to go into PS now and see the difference. Maybe there's a better way to make a small photo larger. Thanks.

[Ed_L]

You're very welcome. I guess the next question would be "what is your final output going to be". If you want it strictly for viewing on a monitor, you don't need the higher resolution. We normally think of 72 ppi as good for monitor viewing. If you are going to make prints, 240 ppi to 300 ppi is usually thought of as being good.

When you say that the small photo scanned at 1:1, and 600 ppi resolution, opens in Photoshop as a large document, I think what you're probably seeing is the image viewed at 100% (actual pixels), or at least something larger than the print size. But if you go to "View"/Print Size, you should see a small image on screen. Again, go to "Image Size" to see the pixel dimensions and the print size, which might surprise you if you don't understand any of this.

One other word. Don't scan your image at a larger resolution than the *optical* resolution of your scanner, even though it might be available for use. If the resolution listed on your scanner is 600 / 1200, 600 is the optical resolution. Scanning at a higher resolution than the optical resolution, again forces the equipment (scanner) to guess what the added pixels should look like, and Photoshop likely does a better job, when using bi-cupid resampling, than the scanner.

Sending anything more than 300 ppi to your printer is usually considered overkill, because the printer can't use all that information, the file is harder to print, and the file size is much larger. Not being sure where you are with Photoshop, I hope I didn't give you information you already knew. If you have other questions, please ask. There are many people with a lot more knowledge than I have about Photoshop, and they're willing to help.

Ed

[Rhonda]

Hi Ed...

I think you're right, the image was still the same size 1:1 when it opened in PS and the resolution was high (600). I did it this way because when I scanned a small photo in at a reasonable resolution (125) and resized it up to 5X7 it looked really bad. If I scanned at a high resolution and then resized up to 5X7 it usually looked good viewed at 100%. But because the file was so large I then dropped the resolution to 125 and felt I had a good image to work on. Again , there must be a better way to make a small image larger.

Most of the stuff I do goes straight to my ink jet for printing. I pretty much like 5X7 prints on 8X10 paper...back to my days in the darkroom with sloppy black borders.

Thanks...Rhonda