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Resolution is one of those subjects that seems to cause a lot of confusion for photographers. But it doesn't have to be that way. There's actually only a few things you need to keep in mind when it comes to resolution. There are two basic concepts that I think are important related to resolution. One is the quantity of information. How much information do you have in an image and therefore, how large a print can you produce, for example. The other issue is the distribution of that information and that relates to the final output size. One way you can examine resolution is to take a look at the Image Size dialog. To do so, you can go to the Image menu and choose Image Size. And that will bring up a dialog that allows you to change the overall size of your image.
But also to review the details about the resolution of your photo. And here we have represented both types of information, the quantity of information, in other words the overall pixel dimensions, and the density of that information, how many pixels per inch. But the pixels per inch only matter when we're printing. Let's take a look at how these two settings interact with each other. The image itself contains a fixed number of pixels. In large part, this will depend upon the particular camera that was used to capture the image.
In other words, how many mega pixels was the image sensor? And in fact, mega pixels is one way that we can describe the overall quantity of information within a photo. Another method is simply referencing the pixel dimensions. How many pixels across by how many pixels down. Of course pixels is a little bit of an abstract term. After all pixels don't have a fixed size. So when I say the image is 1500 pixels wide, that doesn't necessarily resonate with all photographers. You might be able to think of it within the context of your monitor display for example but still that's not an absolute when it comes to size.
This is where the output resolution comes into play. And mostly the pixel per inch resolution allows us to get a better sense of just how much information we're dealing with. It provides some context for the quantity question. If you turn off the Resample Image check box in Image Size, then the pixel dimensions can't be changed. You can't change how many pixels are in the image. What you can change is the distribution of those pixels, using the pixel per inch value for resolution. Most monitor displays are somewhere around 100 pixels per inch in terms of their resolution. So you can see that on a typical monitor, I might expect this image to be displayed somewhere around 15 inches wide.
That will vary significantly depending on the size and resolution of the monitor itself. But this gives you some sense of how you can interpret the information contained in your photo. When printing we'll typically use a resolution of around 300 pixels per inch up to perhaps around 360 pixels per inch for a photo inkjet printer. So, for example if I were sending this image to an offset press printer to have it included in a calendar or a book perhaps then I could set the resolution to 300 pixels per inch. And now once again I'm not changing the number of pixels in the image, I'm simply adjusting a setting. I'm changing the pixels per inch setting for the image. In this case 300 pixels per inch with an image that is 1500 pixels by 1000 pixels results in a print that would be five inches by three and 1/3 inches. In other words, I have enough information in this image, a quantity of information to produce a five by three inch print.
Now, I could always add to that information. For example, let's assume that I want to make a ten inch wide print. Well, then I would need to turn on the Resample Image check box and then I can increase the size of the image. Let's make this ten inches for width for example and now you'll see that the pixel dimensions are each doubled. But that's actually quadrupling the amount of information in the image because we've doubled both the width and the height. At the top of the image size dialog, you'll see an indication that this file was 4.29 megabytes in terms of the total quantity of information.
But now, based on the re-sizing I'm intending to apply, it is 17.2 megabytes. That's a rather significant increase and obviously that should cause some concern because if I'm inventing that much information than quality will probably suffer at least a little bit. Now I'm not actually going to resize this image. I'm not preparing it for printing for example, I just wanted to illustrate some of the concepts around resolution. But again, the key thing to keep in mind is that for given output, you need a certain amount of information. And it's important to recognize whether or not a photo contains enough information for what you're trying to accomplish.
If you're sharing an image online then you frankly don't need all that much information. You don't need much resolution in the image. Whereas, if you're making a large print, you need a lot of information. And so there's a bigger issue around resolution. Just keep in mind that when we're talking about resolution, we might be talking both about the total quantity of information. In other words how many pixels are in the image but also the distribution of that information. In other words, how are we going to spread out those pixels in order to produce a print? At the end of the day, though, it's all about information. And so it really comes down to those pixel dimensions. The resolution is just a way to get a better sense of how many pixels we need for a given print size.
But ultimately, I think the most important thing is to take the time to try to understand resolution. But don't let it be a cause of concern. It's not nearly as scary an issue as some photographers make it out to be. And if you follow some very basic guidelines, it's not something you have to worry about too much in your overall work flow.
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