Look at the "Image Size..." dialog in the "image" menu. Note that there are several different issues:
FILE SIZE is how many megabytes the file takes up on your disk.
The DOCUMENT SIZE is the physical size that the image would be if you printed it out. If you're using inches, then this is how many inches tall and how many inches wide the picture will print.
What Photoshop calls the PIXEL DIMENSIONS is how many pixels wide and how many pixels tall the image is.
The RESOLUTION is the number of pixels per inch (if you're using inches - this could also be mm or picas or any unit you choose). The more pixels per inch, the more detail there will appear to be at the printed size.
Any change you make to any one of these values will always change that value plus one other value, because they are all related to each other. Changing the document size also changes the pixel dimensions (or vice versa), but won't change the resolution; changing the resolution changes the pixel dimensions but doesn't change the document size.
If you click off the "res&le image" check box, you will lock the pixel dimensions so you can only change the resolution and document size (and changing one will change the other).
Note that you can't directly change the file size, but it will change if there is any change to the pixel dimensions.
Say you have a document that is 10 inches x 5 inches and is 1000 pixels x 500 pixels. This document will have 100 pixels per inch (1000 pixels tall divided by 10 inches = 100 pixels per inch, or 500 pixels wide divided by 5 inches also =100 pixels per inch).
1000 x 500 pixels (pixel dimensions) 10 x 5 inches (document size) 100 pixels per inch (resolution)
If you only made the document size twice as big, with "res&le image" checked, you'd get this:
2000 x 1000 pixels (pixel dimensions) 20 x 10 inches (document size) 100 pixels per inch (resolution)
Photoshop leaves the resolution the same, so it must increase the pixel dimensions because you doubled the document size. This is not a good thing; the file size is now four times what it was (2000x1000=2,000,000; 1000x500=500,000), which means that the document now consists of two million fake pixels, made from the half-million original pixels.
It's often necessary to change settings in several steps; you cam mess around as much as you want and only hit 'OK' when you've got the setting you want.
If you made the document size change above, but then changed the pixel dimensions back to the original value, you'd get this:
1000 x 500 pixels (pixel dimensions) 20 x 10 inches (document size) 50 pixels per inch (resolution)
Since you have forced Photoshop to keep the same number of pixels, it must now change the resolution. This leaves you with the same number of pixels to cover twice the distance - 1000 pixels over 20 inches = 50 pixels per inch. Again, two values must change if any one changes, while the third (pixel dimensions) stayed the same.
You could have done this in one step by un-checking "res&le image" and simply changing the document size to 20 x 10. This would force Photoshop to change the resolution from 100 to 50 pixels per inch.
If you went back to the original image and halved the pixel dimensions from 1000x500 to 500x250, you'd get this:
500 x 250 pixels (pixel dimensions) 5 x 2.5 inches (document size) 100 pixels per inch (resolution)
Again, notice that you change one value, but two of the three values change and one value (resolution) stays the same. Notice that this file will be one-quarter of the original file size, because it has one-quarter of the number of pixels (500 x 250 = 125,000; 1000 x 5000 = 500,000). If you double the document size, you'll quadruple the file size; if you halve the document size, the file size will be one-quarter of what it was before.
If you went back to the original and changed the resolution from 100 pixels per inch to 50 pixels per inch, you'd get this:
500 x 250 pixels (pixel dimensions) 10 x 5 inches (document size) 50 pixels per inch (resolution)
This is the same number of pixels as the previous ex&le, but the document size stayed the same; since this stayed the same, and the resolution changed, the pixel dimensions also had to change.
I hope this isn't making it more confusing. Just think about the relationships of the things I list at the top of this message and how changing one forces another one to change.
One important thing - watch the file size that's displayed at the top of the "Image Size..." dialog box. Pay attention to whether you're making the file larger or smaller. Normally, you will care most about the document size and resolution - obviously, you want it to be a certain size for printing purposes, and a certain resolution as well. You normally don't care about the pixel dimensions, as long as you get the size and resolution you want. But watch to see that you don't grow the image larger than its original size.
If you un-check "res&le image", you can change the resolution or document size without creating fake pixels or throwing away real ones (because you cannot change the pixel dimensions when the box is un-checked). You can then change the document size or resolution to what you want - this would be a sensible thing to do when dealing with your original 1.5 x 1 inch scan, and again, it doesn't change any actual pixels to do this. Then you can check the box again it again if you want to change the document size or resolution in relation to each other.
It makes sense to make any corrections to the original scan at its full resolution and then save it, before you decrease the number of pixels, so that you have a full-size master image. If you do all the work on a small version, then decide you want it bigger, you'll need to re-do it if you didn't save the full-sized one.
Just an note about the fact that doubling the document size quadruples the number of pixels - a lot of marketing hype is based on the number of pixels that different camera sensors are capable of capturing. If you remember that you're dealing with surface area, and that doubling the document size requires four times the number of pixels, then you'll see that the differences between 4 and 5 megapixels, or 5 and 6 megapixels, is smaller than is immediately obvious - even a 1.5 megapixel camera will give you a file that is one-half the size of a 6 megapixel camera, if both files are at the same resolution.
Hope this helps.
- Paul
