Part 4, and we’ll begin with Mark’s question. Mark profoundly confuses physical print size with image size. (Which is likely why he asked me to start this missive in the first place…)

As pointed out in Part 3 of this Encyclopedic treatise, a graphic image has a fixed size, say 3000 x 4000. The size on printing that image has to do with how many of those 3000 “dots” are used in a given inch.

If you print that image at 3000 dpi, the printed image will be one inch wide. If you print it at 300 dpi, the print will be 10 inches wide. If you print it at 30 dpi, the print will be 100 inches wide.

The DPI is an OUTPUT specification, and has nothing to do with the image itself.

Let’s go into Photoshop, and make a new document. When you do that, you’ll get a dialog box asking you to specify the size. Make sure the pop up menu says you’re specifying pixels (the list is pixels, inches, mm, cm etc).

Enter 320 for width and 240 for height, and as you do so, look in the lower right of the box. You’ll see the “Image size” change. That’s the size of the file. If you increase the size to (say) 600 wide, the file size will grow larger. Reduce either dimension, and the file size will grow smaller.

Now stop making changes, and look at the file size. Remember it, and go to the “resolution” field. Enter 9000. The file size does not change. Enter 72. The file size does not change.

That’s because the DPI is a specification for how the file will be printed- how many of those pixels of width will be used in a single inch when you print it out.

Now let’s do it the opposite way.

First, put 72 back into the resolution field.

Change the popup menu to inches instead of pixels. (Now you’re specifying the output size – the inches of printed width.)

Put 5 in as width (inches) and 7 as height in inches. If you have color: rgb and 8 bit in the other menus, then your image size will be 531.6K.

Briefly, switch the “inches” back to pixels. You’ll get 360 as the width because 5 x 72 = 360. Switch back to inches again.

Change the resolution to 300 ppi and watch the image size: it will balloon up to 9 Megabytes. Switch the width popup to pixels, and you’ll see that now the file is 1500 pixels wide, because 300 ppi x 5 inches = 1500.

So, at this point, I hope I’ve made clear how the dpi/ppi thing works. A given image file is some absolute, fixed size (whatever that may be, such as 3000 x 4000) and that can be expressed either simply (3000 x 4000) or as output-size/dpi (10″ x 13.3″ at 300 dpi or 5″ x 6.6″ at 600 dpi). Each of those describes the exact same file, just at different output resolutions.

If this isn’t complete clear, the rest of what follows will only be confusing, so I’d suggest you go back and re-read until it is, before continuing.

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Your monitor, which is an output device, has a fixed display resolution. You can figure out what it is. First, go to the Displays preference panel and find the width in pixels of your monitor at its native setting (or look it up in the book that came with it. My 23″ Cinema Display is 1920 x 1200, for example.) Next, grab a ruler and simply measure the width of the display area (no plastic, no black, non-image parts… just the illuminated width). Finally divide the resolution from step one by the measured width. That’s your screen’s ppi. Mine is 98.14.

(Actually, if you do this, you’l likely find that the horizontal and vertical resolution are not the same! (Mine is 98.14 H and 96.0 Vertical. That’s apparently a manufacturing necessity. But I’d not worry about it: we’re talking 2/100’s of an inch here, so we’re close enough for govt. work.) I used the 98.14 width.

In photoshop, you can actually enter that amount in its preferences so that when you set an image to 100%, it will really be displaying one pixel per pixel… or darned close, eh?

To test this, set your ppi as determined above into the Photoshop/preferences/units&rulers/screen resolution.

Then make a new document, and use inches as the setting. Try 8 inches wide, with a resolution of your ppi – the same thing you just set in preferences. 98.14 in my case.

When the document appears in the window, make sure you’re viewing at 100% (see lower left corner, or just double-click on the zoom tool) and grab your ruler again. Measure the width of the document, and you see that it is exactly 8″ wide.

OK… now, let’s go back to that 12Mp photo you have; about 3000 x 4000. If you load that into PS (Photoshop) and view it at 100%, you will not be able to see everything in the image because your screen is not 3000 pixels wide and 4000 pixels tall (or larger) which you’d need to see an image that big at a 1:1 pixel ratio (100%).

To see that whole picture, you’ll probably have to view it at about 25% (750 x 1000) which will fit your (say) 1920 x 1200 screen.

What follows is a bit involved, but not really difficult, so bear with me. It’s only a long-winded explanation of the ramifications of what has gone before… with the practical result of ending up with the correct DPI to use when printing for a given target size.

First, however, there is this one important difference between printers and monitors: monitors have a fixed pixel resolution. (You can change the size, but to get a 1:1 ratio, there’s only one fixed resolution on a monitor, as determined by the number of pixels it actually has.)

A printer, on the other hand, can print at various resolutions. You can tell it to use 300 dots of ink for each inch, or you can tell it to use 10. 300 – 360 dpi is common for photo printers. (Epson printers have a resolution up to 2880 dpi, and that confuses people at first. I can go into it if you like, but basically it’s because they can print a variable size dot of ink.)

The point however is that printers can use a variable output resolution, while your file size and screen size are fixed.

Back to the monitor –

Here we go: at 100%, you’r seeing 1 pixel in the image for each one pixel on the screen. If you change the view size to 50%, you’re effectively (not -really-, but effectively) seeing 2 pixels from the image for each single pixel on the screen. (You can’t physically do that, of course, since one screen pixel is just ONE screen pixel, so you’re really viewing every other pixel from the image.)

And at 25%, you’d be looking at every 4th pixel from the image.

That is, at 25%, instead of 3000 pixels wide -on the screen- you’re seeing 750 pixels wide. And since your screen shows 98.14 pixels per inch, then on your monitor screen, that image will be (750 divided by 98.14 or) 7.642 inches wide.

And to print that image at the same exact size, 7.642 inches, you’d set the printer resolution to (3000 divided by 7.642) = 392 dpi. (Or you could work it from the other side and because you’ve reduced to 1/4th, you’d multiply by four, and you’d get the same thing : 98.14 x 4 = 392.)

Don’t hurt your brain: The 3000 divided by output-width-you-want is certainly easier to remember, and much less convoluted.

So, if you have a photo that is 3000 pixels wide, and you want it to print at 10 inches wide, you’d set the dpi to 300, since 3000 div 300 equal 10.

Again, you can easily see this in PS. Open up your image and choose Image/image size…

Notice in the resulting dialog box that PS keeps the actual image size completely separate, in its own box, at the top (Pixel dimensions), from the output, or “document” size, in a separate box. As we’ve seen, that’s because the document size is dependent on the Resolution.

Before we do anything else, however, look near the bottom of that box, for the checkbox that is next to “Resample Image:” and make sure the the box is NOT checked.

Now, to set your output size of your 3000 pixel wide document to 10 inches, we know that we have to set the Resolution to 300. Put 300 in the Resolution box and bingo, the output width changes to 10″.

Photoshop even makes this easier, and saves you the (admittedly simple) calculations. You can just put 10 inches into the Document Size: Width: and it will instantly change the dpi to 300 for you.

In fact, that’s the fastest way to approach it: but MAKE SURE that the Resample Image checkbox is NOT checked when you do this.

Then when you enter a desired size, you’ll see what the effective dpi is that the printer will use.

So, in answer to Mark and Bruce then, here’s how you can tell what will happen to an image when it’s printed out.

Try this: create an image in PS, and set the size to 320 x 240. Next, and visit the image size box, it will show you what the actual size is in pixels in the upper box – Pixel dimensions: 320 x 240.

The lower box, Document size, will show you what happens when you print it out.(But again: MAKE SURE that the Resample Image checkbox is NOT checked when you do this!)

Printers want at least 150 dpi; photo printers want about 300 dpi… so that’s what you’re looking for in the Resolution field. If your image then is 320 x 240, and you set the Document box width to 5 inches, you’ll see that the print resolution is only 64. You can pretty much bet that will look terrible.

If you set the resolution to what the printer wants (say 300) then you’ll see that the print will be just about 1 inch wide. At 150 dpi, the image will be just over 2″ wide.

NOW…

That all said, you -can- check the Resample Image check box. The first thing you’ll notice is that the Pixel Dimensions box on the top becomes active. That’s because you are about to try to change the -actual dimensions- of the file! You are going to try to make something out of nothing! Remember, the actual data, as captured was only 320 x 240.

Put in 300 dpi and a size of 5 inches… and you’ll see that the pixel dimension have changed to 1500 x 1125. If you now click the OK button and go back and look at your image, you’ll see it’s all blurry… and that’s because you told PS to make up information to fill in the blanks in the change from 320 pixels wide to 1500 pixels wide.

And you’re about to print out that nice, blurry image.

And I hope that all helps to explain what is going on with images, monitors and printers.

Tracy