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I would like to address what is probably the most confusing area of digital imaging that ever was and that is resolution. What is the Resolution of this image? Well, you can get all sorts of answers about that depending upon who you talk to. And that's one of the challenges. various people use various types of terminology. Some people would use DPI, Dots Per Inch other use PPI, Pixels Per Inch, still other will use res or 640x480 and some people have been using Megapixels to discuss Resolution and none of them are wrong necessarily, but they are coming at Resolution from slightly different orientation and that some people even use file size to discuss resolution.
So I would like to try to clarify this and at the same time discuss which Resolution values are so important when you're setting up your scanner to do scan of an image. So to help us understand Resolution, we're going to go up underneath image where in Photoshop. I am going to go to a dialog box called Image Size. I don't particularly like the title Image Size, I would rather it be Image Dimension, and typically the way it's set up in Photoshop by default is like this, where kind of everything is turned on, the Resample, the Constrain Proportions and all these numbers are here. Now for a lot of people when you look at this dialog box, you think, oh, my gosh! Quick close that before I hurt myself, right, because there are so many numbers in here.
But what I would like to do is breakdown this dialog box for you and at the same time really gives you an understanding of how to discuss and understand Resolution and how to use that to give you better quality scans. To want to be clear about discussing Resolution, do two things. One is Resolution terminology that matches the building block into your image, and secondly, separate Input Resolution from Output Resolution I am going to click OK, because I want to zoom in our image just to remind us what are the building blocks of our image, and when we zoom way in here, we remind ourselves, this image is made up of pixels.
So when we go back to our Image Size dialog box, we see that Adobe in Photoshop gets it right, they use pixels, not dots, because there are no dots here. Using DPI to describe the Resolution of this image is not just wrong, it's really confusing. So we're going to use pixels. These first two numbers up here, 2200x3000, what this means is if you were to measure this image in pixels and just count them all the way across horizontally, you would count 2200 pixels. If you count out the same number of pixels vertically, boom! You would get 3000 pixels, so this is Pixel Dimensions.
You may be more familiar with terms like 640x480 or 1024x768. It's the same thing, it's Pixel Dimension. So when I am having a discussion about Resolution with somebody or with myself, I am very clear, are we talking about Dimensional Resolution, which gives me the Dimensions of the image in this case, yes. So let's just click OK and go back up, we now know this image is 2200 pixels across and 3000 pixels down. That's the Dimensional Resolution of this image. Back into Image Size. I want to skip the Width and Height and go right to this number down here.
Notice this number is 300, and when we look at the units, it says pixels/inch. What does this mean, 300 pixels/inch? Let's zoom in on our image just once more, and if we were to look at 1 inch of this picture across, see there is 4 inches and coming down here all the way we come to 3 inches. That mean if there's 300 pixels/inch, what's the significance of this number? Well, various kinds of output devices need different densities in terms of pixels/inch.
A high quality printing device such as a Commercial Printing Press, generally requires 300 pixels/inch, whereas, say a Web image, 72 or 100 pixels/inch is all you really need. The point being is you want to pay a very close attention to this Linear Resolution. Now I am adding a new term here where it says, just Resolution, which is not a very good term all by itself, Linear meaning in a straight-line pixels/inch, 300 pixels in every horizontal and 300 pixels in every vertical. So when we're setting up for a scan, we want to set our Linear Resolution for the highest quality output device we're going to be printing to.
If you're printing to one device and you set up for that device. If you're going to be outputting on multiple devices then you'll choose the highest quality output device for your Linear Resolution, and typically 300 pixels/inch is about the highest that you need to go for most high-quality say Commercial Printing Presses. Next, let's come down here and turn off this little Resample checkbox, because its an important little checkbox particularly in Photoshop unless when we turn this off the number of pixels in the image are frozen, which means that no matter what you do down here, the number of pixels in your image will not change.
That is that there's no interpolation, and we're going to discuss a lot more about interpolation a little bit later. But what's important here is notice that at 300 pixels/inch, we can print a high-quality image at 7 inchesx10 inches. If on the other hand, we go up to 100 pixels/inch for the Web or even 72 pixels/inch, notice the Dimensions in inches, which is the Output Dimensions, goes all way up to 30 inches x 41 inches. If we go to 100, it goes to 22 x 30 inches, we're not changing the number of pixels in the image, we're changing the Output Dimension of the document size, by changing the Linear Resolution.
This would be fine for a Web image, but not good for Commercial Printing. For high-quality inkjet device that prints photo quality inkjets, 240 pixels/inch, and by changing that with Resample turned off, we know we could print a 9 inch x 12 inch image. So when we set this up in a scanner, we want to make sure we have the proper Linear Resolution set for the highest quality output device. We also want to make sure that we have the proper Output Dimension. Okay, one other thing to discuss here and hopefully this brings everything back to center and kind of wraps a bow around all this is what is the relationship between the Pixel Dimension and the Output Dimension? So let's go back to our 2200 that we see here.
2200 pixels across, notice if we divide that by 100 pixels/inch, if you remember back to third grade, they said, well, if you divide pixels by pixels/inch, the pixels cancel out and you end up with inches and that's exactly what we get. 2200 divided by 100, lo and behold this 22 inches. 3000 divided by 100 is, you get it, 30 inches. That's the relationship between these numbers in this dialog box. Coming back to the discussion specifically of the scanner, when we set up a scanner then, we want to make sure that we set the proper Linear Resolution for the highest quality output device that we're going to printing to, and make sure we get the document size, the Output Resolution properly, and we want to try to minimize the amount of Resampling that we do to our images, which is why we want to capture them with a proper Linear Resolution and Dimensions to begin with.
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