A few scanning tips

Photo editor programs vary in the name they call the resampling and scaling options. They don't make it very clear, and sometimes it's a real puzzle knowing what the options actually do. They seem to try to protect us by hiding the details, but for sure we don't know what to do then. Help files often ignore the subject, with changes" size" as the full depth of the description.

Resampling changes the size of the image. The dimensions in pixels changes to be a smaller or a larger image. If you resample to 1/4 the previous size, then 4x4 pixels (16 pixels) are reduced to one pixel of an averaged pixel size.

Scaling is just a fancy word to mean "First, don't do any of that". No pixel is disturbed in any way by scaling. Scaling merely changes the one value dpi (stored separately in the image file, decided at whim of some humans choice at the time), to a different dpi number (dpi meaning these pixels spaced at this "pixels per inch" printed on paper). Dpi is NOT a property of the image pixels, but it is a property of printing those pixels. Changing dpi (called scaling) does not change the image pixels at all, Not in any way, but it will scale to print the image at a different size on paper (if the printing software should even bother to look at this dpi value). For example, if you go into a one hour printing shop and request an 8x10 print, then you will get an 8x10 print regardless what the current dpi number might have said (they will instead scale it to print 8x10). But your home printer will look at the dpi, unless you tell it to use an offered way to scale it instead.

Scaling is the same word as the "scale of a map", like perhaps one inch is 1000 feet, or maybe one inch is 20 miles (another scale factor). Of course, its scaling does not change the countryside, it merely prints the map at different size. Other maps might be scaled to other sizes. In the case of printing images, if you had a 2400×3000 pixel image, and you told the printer to print it at 300 dpi, it would print exactly 8×10 inches (2400/300 = 8, 3000/300 = 10). That is what setting the dpi number tells, to print 300 pixels per inch, but you can scale the dpi number to print it any size you please instead.

The normal convention is to print photos at 300 dpi, to show all the detail the color printer can print, and all of the detail that the human eye might perceive. Printing photos at more than 300 dpi won't help the printer or the eye, so more is wasted effort. However a big exception is line art images (only two colors, white paper or black ink) which can be printed fairly well at 300 dpi, but 600 and 1200 dpi will be better, or even 2400 dpi too in commercial printing presses. When printing presses say 2400 dpi, it means possible printing locations of single dots of a primary color, but what they print in color is a halftone array, where maybe an 8×8 array of dots (64 dots) print one color of one pixel. Our usual digital work in computers is 256 colors of each of the three primary colors. It gets to be a long story, but we do try to print most color photos at 300 dpi.

Dpi means pixels per inch, as spaced on printed paper. It means pixels per inch, as spaced on paper. Some people have started calling it ppi instead of dpi, and it does mean exactly that, and it does avoid confusing newcomers with "ink drops per inch" which are also called dpi by inkjet printers. But it has been long time established conventional practice to call resolution of printing images as dpi, which might just be jargon, but I'm of the old school, so it is dpi to me. You can call it either way you please, and people do need to understand hearing either way.

Photo editors normally will also offer either method, resampling or scaling. Both are in the same Resize dialog, but Resampling changes the actual image pixels to be a different size of image, perhaps very drastically changed (and there is no going back then), but Scaling merely prints the unchanged image at another size on paper.

The dpi number in the photo image file does absolutely nothing, UNLESS the printing job chooses to look at it and print it to that scale, such as 300 pixels per inch of paper. But then you can choose to immediately print it again at 200 pixels per inch. The image pixels themselves are Not affected in any way by just printing it scaled at another print size.

Several programs use a "Constrain File Size" or "Maintain File Size" option to switch the same "resize" dialog box between Resample mode and Scaling mode. This is the good way to do it. In the same way, the Photoshop or Elements resize dialog has a check box that directly says Resample, On or Off, and Off means "Scaling".

Scaling

Scaling mode disables all options that allow changing the image size in pixels. The scaling option may be described as "Lets you change the image dimension or resolution but keep the file size the same". They don't ever say they are speaking only of printed inches, about the printed size and printed resolution, but they obviously are speaking only of printing size when file size and pixel dimensions stay the same.

Selecting "Constrain File Size" does not allow any change of image size in pixels. Therefore, it changes only the future printed resolution and size in inches. This is scaling. Change the resolution ppi and you will see the calculated size of printed inches change. Or usually you can change the printed dimension in inches, and it will then calculate the necessary resolution ppi to produce that printed size. The resolution ppi is the only value that is changed and stored, and the printed inches will track or follow on paper. Nothing at all visible happens until the image is printed at that size.

Repeating yet again (it is truly essential to understand this): Scaling does not change one single image pixel in any way whatsoever, and the file size therefore stays the same. What is changed is only the one resolution "number" that is saved away someplace, that indicates what resolution ppi should be used to space the pixels on paper when the image is printed. If printed with a lower number for ppi, the printed image size will increase in inches for example. This is because the number of pixels stayed the same, and ppi is "pixels per inch".

Resampling

Resampling always changes the image size in pixels, and you see this new image size on the video screen immediately. The file size must change too, in order to store the different number of pixels. So to enter this resampling mode, we cannot use "Maintain File Size", we must turn that off so the file size can vary. We are allowed to change the image size in pixels, either directly, or by percentage. If we change the dimensions to 50%, it will be half size on the screen. This will also change printed size too, because if we reduce the pixel dimension to 50%, but still print those pixels at the same ppi, then it will print half size. We could change either the printing resolution or the printed size in inches, and since the other was not changed, the size in pixels must change to accommodate this relationship (We normally would not do that to print it, we would scale instead). This is resampling, and we see the pixel dimension change, the memory size change, and the file size change.

The resampling option normally provides a way to maintain or change Aspect Ratio, which is the ratio of image width and height. Sometimes this option is called Proportion or Distortion (meaning size distortion of proportion). Aspect Ratio normally should always be maintained constant (unchanged) when resampling photos, to prevent proportion distortion, but graphics are not always so concerned with constant aspect ratio. My point here is that this option to allow changing aspect ratio is disabled in Scaling mode, because it does not have meaning there, the pixels do not change.

If image size in pixels or size in bytes changes, then it is resampling. If not, it is scaling. Previous pages of this section discuss these modes.

Image programs vary. No one program does everything best, they each have different strengths and weaknesses, and you will likely use more than one. Each one handles printing differently. Sometimes the tools are rather strange, and hard to interpret. Simple programs typically do not offer scaling, but better image programs do provide a menu to set printing resolution. But it is not always clear.

This is the Adobe Image Size box:

Resample Image selected - is Resampling, the size changes to resample the image to create the necessary pixels for the print size specified, for example, "the 6x4 inches at 300 dpi" will resample whatever pixels were there to now be 1800x1200 pixels. Or, the pixel dimensions can be specified directly, and then it will show you the new printed size at the specified dpi.

Resample Image NOT being selected is Scaling, not resampling. The pixels are not affected in any way. Instead, new print size in inches is simply scaled from the existing pixels to match printed size to the dpi specified.

Constrain Proportions - Images have two dimensions, width and height. Constrain means enter one dimension, and the other will automatically change to maintain the same aspect ratio, which is normally very desirable, otherwise disproportionate changes can distort the image shape. Instead, Not Constrained means the image ratio (width×height) will change, which will distort the shapes in the image (for example, circles, maybe like tires on a car in the photo, may be elliptical instead of round anymore). If you do not want shape distortion, then always choose Constrain Proportions. See more.