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One of the more potentially confusing aspects of images coming from a digital camera is the size of the image. “Size” can mean a few different things, and in this case we’re talking about two ways of describing an image: the file size and the image size. File size refers to the disk space taken up by an image: 4MB, 7.3MB, 18MB, etc. Image size refers to the width, height and resolution of an image. Both play an important role.

The file size of a digital image is determined by the capabilities of your digital camera. A 2 megapixel camera can create a file size as big as 6MB, while a high end camera can create a file of 18 MB or more. Of course this is also dependent on the settings used when you capture an image (normal versus fine for example). One way to estimate the maximum file size a camera is capable of capturing is to multiply the camera’s megapixel size by 3. For example, a 2 megapixel camera would roughly translate to a file size of 6 MB, while you could estimate the maximum file size from a 3 megapixel camera to be approximately 9 MB.

Once you open an image in Photoshop you can use the Image Size command (Image>Image Size) to determine how big your file is and, in effect, what you’ve got to work with. In this case the image was captured at fine (or highest) quality, which resulted in an 18.2 MB file.  As you can see, the image is 43 inches wide and 28 inches high, with a resolution of 72 ppi. Don’t let those numbers throw you – the equally important number is the size of the file (in this case 18.2 MB). Many people make the mistake of focusing on the resolution, thinking, “The resolution’s only 72, and that’s not enough”. That might be true if we were going to keep the dimensions the same size, but since we’ll use the Image Size dialog to play with the dimensions, the resolution will not stay at 72.

First you’ll need to uncheck Resample image. When you do, the three values (width, height and resolution) are linked together. That means if you change any one factor, the other two values will also change – but the overall file size will not change. For example, let’s say that you plan to print on your inkjet printer and have determined that 220 ppi is the ideal resolution for printing. Enter 220 in the resolution field and the width and height will automatically change, in this case telling you that you could print a page as big as 14 x 9. (Note now the file size is still 18.2 MB – all we’re doing is “massaging” the pixels to increase the resolution by decreasing the dimensions).

Here’s another example: we want to print an image that’s 10 inches wide. After entering “10” as the width, the height and resolution change, in this case to a resolution of 310, which is probably more than you need. To keep the new dimensions but lower the resolution to a more appropriate value, click in the Resample Image box (to unlink resolution value from the width and height) and enter the new resolution value. In this case the file size will decrease.

You may be wondering why we would bother lowering the resolution in this case from 310 to 150. The simple answer is file size, and therefore processing time. If you can send a 4.27 MB file to your printer instead of an 18.2 MB file, it will take less time to process and print, with no loss of quality. As long as the resolution you use is appropriate for your output device, there is no advantage to using a resolution that’s higher than required.

Let’s pause for a moment to briefly discuss the “how much resolution/file size do you need” issue. When we first opened our example image it was 18.2 MB, which gives us lots of room to play. We’re going to want to keep one copy of this image at that file size, “just in case”. Then – after we’ve done all my image editing – we’ll use the Image Size method we just discussed to create (and save) a copy that’s appropriate for printing.

As we discussed earlier, the file size can be one of the best methods to determine how big a file you need, and whether your existing file is big enough. For example, you want to end up with an 8 x 10 printed on your poster-sized ink jet that uses a resolution of 240 ppi. To determine how big a file size you need, use the New command (File>New).  In the new document dialog box, enter a width of 10, height of 8, resolution of 240 and make sure the mode is RGB. Don’t click OK, but instead, look at the Image Size value. In this example it’s 13.2 MB. Compare this to a 3 megapixel camera (with a maximum size of 9 MB) and the bottom line is, that 3 megapixel camera can not create a file that’s big enough. Although this is over-simplifying the math a little big, it’s a pretty good way to estimate the type of camera you need (and the size of the file that’s required). It also confirms that if someone sends you a 300K image from their web page, there’s not a whole lot you can do with it!

One extremely important note about resizing images using Image Size: always make the file size smaller (or keep it the same). In other words, do not take a 1MB file and attempt to “size it up” to an 8 x 10 and 240.  Any time you change any value that causes the file size to get bigger, you’re asking Photoshop to “interpolate” the pixels (a fancy way of saying invent extra pixels to fill in the blanks). If you’re in the Image Size dialog box and you change the numbers so that the files size is bigger – stop. Although technically there is nothing to stop you from increasing the resolution through interpolation, you will not be satisfied with the results! Invented pixels are never as good as “real” pixels.

So remember, when you’re shooting images, it’s probably better to capture at the highest quality possible. You can always lower the resolution or dimensions to fit your needs, but you can’t (or at least shouldn’t) increase the file size.

Dave Cross is Senior Developer, Education & Curriculum for the National Association of Photoshop Professionals (NAPP). To learn more about Dave Cross visit www.dcross.com, or check our NAPP’s web site www.photoshopuser.com.

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Great stuff for anewie Tks. Posted by: ke6iqs Jul 6, 2007 @ 1:36 PM EST

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