THE MYTH OF 72 DPI

This information was prepared by Roy Killen and is copyright. It is part of the supplementary training materials for members of the Belmont 16 Ft Photographic Club and is not intended as a complete, stand-alone instruction package.

Comments and suggestions on this tutorial are welcome - they should be emailed to Roy Killen.

ADJUSTING IMAGE SIZE

Photographers often need to produce images of a particular size, either because they want a particular sized print or they want a particular sized digital image.

In Photoshop there are two different approaches to changing the size of an image.

1. You can change the "pixel dimensions" (the number of pixels there are in the image), or
2. You can change the "document size" - the size that the image will print at a particular resolution.

There is often confusion between these two very different things, perhaps because the two different types of size change are made from the same dialogue box.

The iinformation in the "Image Size" dialogue box tells us these important things about the size of the image:

1. PIXEL DIMENSIONS. It is 4248 pixels wide and 2608 pixels high. These dimensions determine how large it will appear on our computer screen if it is displayed at "actual size" or "100% size". [The actual image shown above is not 4248 x 2608 pixels because what you see above has been reduced in size for the purposes of this tutorial. You will just have top believe me that the actual image is 4248 x 2608!]

2. DOCUMENT SIZE. If this image was printed at a resolution of 300 pixels per inch the print would be 35.97 x 22.08 cm.

If you are going to change the size of an image you must first decide if you want to change the pixel dimensions (the number of pixels in the image) or the document size (how big it will be when you print it) or both.

If you change the document size without changing the pixel dimensions this is called RESIZING.

If you change the pixel dimensions this is called RESAMPLING.

CHANGING THE DOCUMENT SIZE (RESIZING)

You can change the size of the PRIINT that you produce from an image without changing the number of pixels it contains by unchecking the Resample Image box and then either:

1. Changing the width or height (just type in a new value for one dimension and the other one changes automatically); or

2. Changing the resolution. If you decrease the resolution the width and height will automatically increase (and vice versa) because the number of pixels in the image is not changing - they are simply being spread out or squeezed closer together so that the image covers the area you have set with the document size dimensions.

It is very important to remember that the "Resolution" referred to in the Image Size dialogue box is the PRIINT RESOLUTION. It has NOTHIING to do with how the image appears on your computer screen.

Likewise, the width/height numbers in the "Document Size" area of the Image Size dialogue have NOTHIING to do with how your image appears on the computer screen.

The size of the image on your computer screen is determined by its pixel dimensions. Images that have the same pixel dimensions will appear the same size on your computer screen regardless of the width/height/resolution values in the Image Size dialogue box - provided they are displayed at 100%.

The best way to undersand the above points is to open an image in Photoshop, open the Image Size dialogue and experiement with making changes and watching what happens on the screen.

CHANGING THE PIXEL DIMENSIONS (RESAMPLING)

If you change the pixel dimensions of an image this is called resampling.

If you reduce the number of pixels in the image it is called downsampling. When you do this, information is deleted from the image because pixels are removed - they are lost, gone forever!

If you increase the number of pixels in the image it is called upsampling. When you do this, new pixels are created. The original pixels are spread out and Photoshop makes its best guess at how to fill in the gaps.

To resample an image you have to put a check in the "Resample Image" box and you have to select an interpolation method to tell Photoshop what method to use to either add or delete pixels. Several options are available for interpolation, but the best ones to use are:

Bicubic - for the smoothest tonal gradations.

Bicubic Smoother if you are enlarging the image (increasing the number of pixels).

Bicubic Sharper if you are reducing the size of the image (removing pixels). If Bicubic Sharper gives too much sharpening of the image then use Bicubic.

To see the difference in the results produced by the different interpolation methods you will probably have to magnify the image to the point where you can see individual pixels on your screen.

When you are resampling you need to have a check in the "Constrain Proportions" box so that when you change one dimension (e.g., width) the other dimension will automatically change by the correct amount. This keeps the aspect ratio of the image the same so that it does not distort.

You don't need to worry about the "Scale Styles" check box unless you are resampling an image that contains layers that have styles applied to them.

WHY CHANGE THE PIXEL DIMENSIONS?

Sometimes you need to change the pixel dimensions of an image. For example, many competitions that accept digital images will ask for them to be no more than 1024 x 768 pixels.

If you change the pixel dimensions while the "Constrain Proportions" box is checked (which you should do for photographs - otherwise they will become distorted) you need to change only one dimension - the other one will change automatically. The Document Size (width/height) will also change but this does not matter - it is of no importance because you are not going to print the image.

If you are trying to produce an image of a certain pixel size (e.g., 1024 x 768) pay NO attention to the Document Size information. In particular, pay NO attention to the Resolution because this is PRINT resolution - it does NOT change the way the image appears on your screen (unless you simultaneously change the pixels dimensions of the image).

DO NOT try to produce an image with a resolution of 72 dpi (or 72 pixels per inch) as this has no meaning for an image displayed on a computer screen or through a digital projector. (See explanation below.)

THE MYTH OF 72 PIXELS/INCH (OR 72 "DPI")

You will often hear people suggesting that digital images that are to be displayed on a computer screen (or through a data projector) should have a resolution of 72 pixels per inch (very often mis-stated as 72 dpi). Sometimes you will see 96 dpi recommended. Whenever you see this suggestion it is an indication that the person who is making it does NOT understand why they are making this meaningless suggestion. If you want to know why - read on.

The dpi information in an image file is an instruction to the PRINTER about how to print the file (how to space the pixels on the paper). For example, if you specify a print resolution of 300 dpi your printer will place more dots per inch of paper than if you specify a print resolution of 200 dpi. If an image with the same pixel dimensions is printed at 300 dpi and at 200 dpi the 300 dpi version will be smaller becasue the pixels have been pushed closer together.

Video drivers (the part of your computer that tells the screen what to display) does NOT use the print resolution information. It uses the pixel dimension information in the image file. So, if you have an image that is 1024 x 768 pixels the video driver will tell the screen to display exactly that (an image that is 1024 x 768 pixels). How much of the screen is taken up by this image will depend on the resolution of the SCREEN. So if your screen is set at a resolution of 1024 x 768 the image will take up the full screen. If your screen is set to a resolution of 800 x 600 the image will be larger than the screen (so you will not see it all without scrolling if you have it displayed at 100% size). If your screen is set to a resolution of 1280 x 1024 the image will take up only about 80% of the screen width and about 75% of the screen height.

Your video driver and your screen do not care what print size or what print resolution information is attached to the image because they cannot use this information. You might just as well try to tell your screen that you have an Epson printer rather than a Canon printer - the screen does not care!

If you are not convinced about what I am saying, try this on your own computer:

1. Open an image in Photoshop (or some other image editor). Open the Image Size dialogue box and take note of the information (Pixel Dimensions, Document Size and Resolution).

2. Make sure that the Resample Image check box does NOT have a check in it.

3. Decrease the Resolution value - for example, if it was 300 pixels/inch change it to 72 pixels/inch. Note what happens. The Document Size will increase (because you are telling the printer to spread out the pixels as it prints them, but the Pixel Dimensions DO NOT change (because you have not resampled the image).

4. Click OK and see what happens to the size of the image on your screen - exactly NOTHING. You have changed the Print Resolution but the size of the displayed image has not changed because its pixel dimensions have not changed and it is the pixel dimensions that determine what is displayed.

5. Open the Image Size dialogue box again and change the Resolution to 600 pixels/inch (still without a check in the Resample Image box). Note that the Document Size will decrease because you are telling the printer that you want the pixels to be printed closer together. Note also that the Pixel Dimensions do NOT change.

6. Click OK and observe what happens to the size of the displayed image - NOTHING.

If you save each version of this image (the 72, 300 and 600 pixel/inch versions) you can then open them in some other image previewing program and you will find that they all display at the same size (because they all have the same pixel dimensions). If you try printing these three images you will find that they PRINT at different sizes but this has nothing to do with their size on your computer screen.

And this explains why, when you are preparing images to be a particular pixel size (such as 1024 x 768) you can completely ignore the width/height/resolution information in the Image Size dialogue box. If you resample your image in order to change its pixel dimensions this information will change (because you will have checked the "Resample Image" box) but this does not matter at all because you are not printing the image.

BUT WHY . . .?

I can imagine that you might be thinking - if the above information is true, why do I keep hearing people talk about "screen resolution being 72 dpi" and telling me that images for digital projection have to be at 72 dpi? Is it possible that so many people are wrong?

Well, if you did the little experiment I described above you saw for yourself, on your own computer, that, YES, all those people are wrong. And there is an historical reason for it.

The first computers to be able to display text on a screen at approximately the same size as that text would appear when printed were Apple computers. Since text size is measured in "points" and one point is 1/72 of an inch these computers were designed with a screen resolution of 72 pixels per inch. On such a screen, a 12-point font could be made to be 12 pixels tall and that would make it close to 12/72 inches tall - the same size as that font prints on paper. This gave Apple a WYSIWYG (What You See Is What You Get) display for text - a great marketing point in the early days.

These days, the "apparent" resolution of a screen varies according to its construction and the resolution that you have instructed it to display. Consider, for example, that you have a laptop with a 17" screen, its actual screen width will be around 14.4". If the native resolution of the screen is 1680 x 1050 pixels then the horizontal resolution of that screen is 1680 pixels divided by 14.4 inches which is 117 pixels per inch - a far cry from the mythical 72 ppi. On this monitor, ANY image that was 1024 pixels wide would display at approximately 8.75 inches wide - regardless of the print resolution that had been selected for that image (so long as the program you are using has been instructed to display the image at 100%).

WHY 1024x768?

Most competitions for EDIs (Electronic Digital Images) require the images to be 1024x768 pixels. The basic reason is that when this standard was developed a lot of computer screens and data projectors had a "native" resolution of 1024x768 pixels. On screens (or data projetors) with these pixel dimensions the following happens:

(a) An image that is 1024x768 pixels displays full screen (each pixel on the computer screen corresponds to a pixel in the image) and it displays full size when projected.

(b) An image that is smaller than 1024x768 (e.g., 800x600 pixels) will not fill the screen (it will be reproduced using just 800x600 of the screen pixels if the program is set to display the image at 100% size) and it will not fill the full projection area.

(c) An image that is larger than 1024x768 (e.g., 1500x1000 pixels) will 'overflow' the computer screen if the program is set to display it at 100% size. Most data projectors are smart enough to prevent this 'overflow' and will dspaly the full image at the maximum possible size. However, in doing so, the projector will have to interpolate (make a best guess at how to compress say 1500x1000 pixels into 1024x683 pixels - keeping the proportions correct). In this process, the data projector will degrade the quality of the image.

DISPLAYING IMAGES AT "PRINT SIZE" IN PHOTOSHOP

In Photoshop you can display an image at approximately the size it will print by doing the following:

1. Calculate the actual resolution of your screen by dividing the screen's pixel width by the width of the display area. For example, you might have a monitor that is 1650 pixels wide and has a display width of 18.5 inches. This gives a screen resolution of 1650 divided by 18.5 which is approximately 89 pixels per inch.

2. Open the Photoshop Preferences dialogue, select the Units and Rulers section and set the Screen Resolution to the value that you calculated for your screen (89 in the example above).

3. Now when you have an image displayed you can select View>Print Size from the menu and the image will display on your screen at close to the size it will print.

CONCLUSION

If your aim is to PRINT an image at a particular size and quality then you need to be concerned about the width/height and resolution of that image. The pixel dimensions will influence the quality of the print but not its size.

If your aim is to produce an image with particular pixel dimensions (for dispay or projection) then you can ignore its width/height and resolution.

All images on this site and all documents and tutorials linked to this site are copyright.

This page last updated 13th June 2009 (RK)