Adobe Photoshop 7 Tutorials
Productivity / Image Size
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Subtitles of the Movie (Adobe Photoshop 7 / Productivity / Image Size)
You can learn a lot about an image by opening it up, and going to the image menu and choosing image size. And this will give me some very important information, namely the exact pixel count as well as its width and height and resolution. Now you know that a pixel based raster image is made from a grid of pixels, and the more pixels there are in the image the larger the image file. This picture (which came from my digital camera) has 1600 pixels along the width and 1200 pixels along the height. Its document size is been programmed to be 5.3 inches by 4 inches at a resolution of 300-pixels. These three values, width and height times resolution, gives me this file size 5.5 megabytes because it's an RGB image. The other way to get to this value is multiplying these pixel dimensions. 1600-pixels by 1200-pixels in RGB will give me of course the same information. Because this information is this information down here. Well one reason why I might bring this up, this dialogue box up that is, is to do something I often do. That is, down res this type of image to make it small enough to be easily emailed to somebody, a friend say, as a JPEG file. My general rule of thumb is I don't like to send pictures, which are larger than 100 kilobytes in size. So what I might do is stick with this dimension, 5.3 by 4 but for resolution I'm going to type in 72-pixels, and making sure that resample image is enabled - because if it's not, it won't throw out any pixels. But I've told it to reduce the resolution from 300-pixels to 72. And now if I look at the top of my image size dialogue box, I can see my new pixel dimension when I click ok will be 324k, much smaller file and of course that's because I'm throwing out quite a lot of pixels. I'll click ok to that, and you can see my files shrinks down considerably. I'm going to double click on my zoom tool to see my image one to one. That is 1 image pixel for 1 screen pixel, so that's exactly how it will look on a computer monitor at 100% resolution. You may have noticed that it did not quite shrink down to 100k or less. Well the next stage if I were planning to send this out as an email attachment would be to save for web, and choose the JPEG format, and choose one of these resolution or compression options. So that it essentially looked good, so there weren't too many JPEG artefacts, but made it under the 100 kilobyte limit. And I could see that even at the highest compression my kilobytes are only 62.5 kilobytes in size. So this will be compressed down nicely to be emailed to a friend of mine. I'm going to undo that, so I'm back to my original resolution, and talk a little bit more about image resolution. Just as you can shrink down your file by changing the resolution here, I could also increase my file size. So if I typed in 600 here, my file will balloon from 5 1/2 megabytes to 22 megabytes. Now what Photoshop would be doing is adding pixels to my image. And what it's going to do - it's going to resample by adding pixels to my image and re-sampling up, or that's also known as resing up or up-resing. Another fun and interesting feature you can use this way is to actually choose or disable constrain proportions. Notice when constrain proportions is enabled, there is this chain icon here, indicating that if I make a change on one of these dimensions a change will be made on other dimension because my proportions have been constrained. So if I make my height 5 inches, my width will increase. And conversely if I decrease my width to 3 inches my height will decrease. So the document's height to width ratio is not changed at all, just the amount of pixels that make up the image. Well one option that you have of course is to disable constrain proportions and create an interesting effect. So I'm going to choose width and make this a square image. So now my width will be compressed. And for what it's worth, it is indeed throwing out some pixels. So my file size will be a little bit smaller. I'll zoom-out to see that effect, I'm going to undo that, and just talk a little bit more about some rules that you should consider when doing things with file size or image size. Generally it's much safer to throw out pixels by down resing. So if I were to reduce the resolution to 150 pixels per inch, I would be throwing out every other pixel. And if I were to have down res this a lot, it'd still be ok. However if I start off with a small file like this and then upres it, the general rule of thumb is do not upres more than 50% of your original image size. And for good reason, because it's very much like taking a very small negative and making a gigantic print. You'll immediately begin to see image resolution problems. Because there wasn't enough data to start with, so if I take my new small image and res it up say 300%, I'll just go up here to my percentage area and type in 300%, what I begin to get is a soft and fuzzy picture - because my image didn't have enough pixel information to start with. So you can res up a little bit or res down a fair amount. But essentially if you are making your images more than 50% larger than the original image, you should probably start with a better scan. Or a larger image from the very beginning, because there just isn't enough pixel information to create a decent output. So that's the general rules of thumb with image size.
Tutorial Information
| Course: | Adobe Photoshop 7 |
| Author: | Andrew J. Hathaway |
| SKU: | 33329 |
| ISBN: | 1889347272 |
| Release Date: | 2002-09-05 |
| Duration: | 11 hrs / 152 lessons |
| Work Files: |
Yes |
| Captions: | No |
| Compatibility: |
Vista/XP/2000, OS X, Linux QuickTime 7, Flash 8 |
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