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Say NO to 72dpi!
dpi

72dpi, 20K in JPEG-file in Photoshop 6.0 using the former BoxTop Software's ProJPEG plug-in 5.2, quality setting 85 and smoothing 5
72dpi
300dpi, 20K in JPEG-file and with exactly the same settings as for the 72 dpi image to the left
72 dpi for the web
72dpi resolution
filstorlek för bilder
Is it true that all images for the Web should have a resolution of 72 dpi?
Not necessarily. Dpi is for printing. It does not matter to screen or web page if you store 72 dpi there, or 7 dpi there or even 7200 dpi there. It simply does not matter what it says there, because on the screen, that number is ignored. The original correct number is no better on the screen, it only affects printing, on paper, where inches exist.
Even Adobe has problem to understand that. Or they give us what we want, as when saved for the web, Photoshop automatically sets the resolution to 72dpi even if it is meaningless and has no effect in any way on the image or what you see. The numbers affects only the future printing resolution and size? My advise is that you take control yourself! The mythical "72" dpi was maybe fairly accurate once on a PC 14 inch 640x480 VGA display, but way off an 17 1280x1024 inch monitor which might have a resolution of 96dpi.

My original test image was scanned at a resolution of 300dpi (1144x1668 dpi,3,8 x 5,56 inch and 5,46MB in Photoshop format).

  • On a copy of the same image the resolution was reduced from 300dpi to 72 dpi (the resampling mode was Bicubic) and the size was, because of that, reduced to 274x400 dpi.
  • Saved as a JPEG-file in Photoshop 6.0 using BoxTop Software's ProJPEG plug-in 5.2. Unsharp mask
  • I then resampled a copy of the original image to 274x400 pixels, dpi remains at 300 dpi (the resampling mode was again Bicubic.
  • Saved as a JPEG-file in Photoshop 6.0 using BoxTop Software's ProJPEG plug-in 5.2
  • Opened the both JPEGS again
  • Arranged the 300dpi version as a layer on top of the 72 dpi version
  • Set layer mode to difference. Histogram now shows exact match and also Levels Adjustment-layer does not reveal any difference.
Both images now appear in exact the same size in a monitor as I gave both images the size of 274 x 400pixels. This example is maybe a little academic. Just wanted to show that you do not need a resolution of 72 dpi even to get a small filsize in combination with JPEG compression. (Remember that how large an image appears on-screen depends on a combination of factors––the pixel dimensions of the image, the monitor size, and the monitor resolution setting.)
However Wayne Fulton writes in his excellent book: A few scanning tips:
"My advise is to forget about 72 dpi. It won't help you get better results, and it is counterproductive to understand how it really works. I can't tell you what 72 dpi means and how to use it, because it's not real and has no use. It is never mentioned in the monitor's specifications. Numbers like 640x480 and 600x800 are always mentioned however, and this is what is important. .....Nevertheless, some do scale their images to 72 dpi for the screen. You can do if you like, it won't hurt anything, but it is pointless, as it will be ignored and will have no effect at all until you print the image. Even if you want to print it, printing it at 72 dpi is pretty coarse on the printer. "
So you see it is not necessary to save in 72 dpi resolution for the web for the purpose to get a small file size! It seems that the myth of 72 dpi is so deep that many people will argue strongly that it can be no other way, but obviously they are not thinking about it , and obviously are without any understanding of it.
filstorlek för bilder
So when does the dpi value have any meaning?
When you print and when you scan of course. (See also my documents about the relations between file size, resolution and print size)
If I import two of the above images into PageMaker or Word they will appear different in size.
A) Printing:
1) The image " 72dpi.jpg" will on paper have the size 3,82 x 5,57 inch= (275/72 x (400/72) inch
2) The image "300dpi.jpg" will on paper have the size 0,92x1,33 inch= (275/300 x (401/300) inch.

B) Scanning:
Wayne Fulton further writes:" It is very common to hear the advice : "Monitors can only show 72 dpi so scan all your Web images at 72 dpi." Don't believe that. You only have to test it once to know it's wrong. On any one monitor, we can easily see that there is obviously a tremendous difference in viewing the same photo scanned at 72 dpi and at say 200 dpi (the 200 dpi image is much larger). The 200 dpi image contains greatly more detail, and our so-called 72 dpi monitors will certainly show all of that detail, because the screen simply presents every image pixel, one by one. We are obviously NOT limited to 72 dpi, and 72 dpi is not a valid concept. The 200 dpi image is simply larger on the screen, perhaps too large, so that it may or may not fit our screen size without scrolling, but all the pixels are there. No pixels are discarded to limit out at some magic 72 dpi resolution limit. 72 dpi video has no meaning in that context. It is not a limit, it is not real, the 72 dpi property simply does not exist."

3) Learn more about Image size and Resolution
Barry Haynes, the co-author of the excellent book Photoshop 5 and 5,5 Artistry replies in an e-mail to me June-2000:

I generally agree with your comments on 72 dpi. If you look at an image on the screen at 100% in Photoshop, or in a browser, you will be seeing every pixel. It is the pixel dimensions that will determine how large the image will be on the screen. I believe people use 72dpi as an average number of pixels that get displayed on a computer screen per inch. If you take a 150 dpi image and display it at 100%, it will be approx. 2x as large in each dimension. The 72 dpi setting just gives you something close to Photoshop's inch dimensions when displayed on a monitor at 100%. Some monitors display at 60dpi and some at 100dpi so nothing is perfect. Does this make sense and help?

Word of advice: Don't scan with a resolution of 72dpi for the web. Scan at about 200-300 dpi and reduce the image size in Photoshop/PaintShopPro to the size in pixels you want the image to have on your webpage. If it is a photo save as a JPEG and compress as much as possible to maintain acceptable image quality.( See also my info about "Image Size and Resolution"

Note: Why do I not say 72 ppi instead, which should be more true? It would sound funny as 72 ppi. Search www.google.com for "72 dpi" and you get 627,000 links while "72 ppi" gives 13,200 links. I will go with the 50x greater usage. That's the name it has always had.

This page was last updated: September 25 Year 2004 Return to main page