Whitehead Institute BaRC

Updated 7/9/08


Inside WI > BaRC > Graphics > Scanning > Resolution


Basics ........

Resolution is determined by the size of the units of information representing an image. A pixel is a unit of information displayed on a monitor. Each pixel holds a defined amount of information stored on your disk. An image of a given area will become more detailed as more pixels are used to describe it.

Resolution can be measured in many ways:

1. Samples per inch (spi, scanners)

2. Pixels per inch ( ppi monitors)

3. Dots per inch (dpi, printers)


More pixels in a given area will give you a smoother, more detailed image but it will also give you a much larger file.

Fortunately, as the pixels become smaller and smaller, the difference between high resolution and extremely high resolution are not discernible by the human eye, and are not reproducible on our printers (although a professional printing press is capable of much higher resolution than our printers here).

The key is to find the point at which you no longer get a benefit from additional resolution. For output to Whitehead devices, that point is between 300 and 400 ppi.


We often use dpi as a generic term, but that isn't accurate. Sometimes a printer (such as laser printers) will use several dots of different colors to make the color of one pixel of an image. This is why even though a printer may have a 600 dpi capability, it doesn't mean the same thing as 600 ppi resolution.


Scaling........back to top

So far resolution has been easy, but scaling is where you can ruin an image if you aren't careful.

Because pixels make up an image, it makes sense to enlarge or reduce an image simply by making the pixels bigger or smaller.



Photoshop has a dialog box called "Image Size" (Image > Image size).


To scale by enlarging pixels, make your changes in the Image Size dialog box and make sure the "resample image" box is not checked. Notice the linking symbol to the right of the width, height, and resolution boxes. This indicates that changes in width and height will be accomplished by altering the resolution (size of pixels), or changes in resolution will be made through changing the width and height.

Although I said that the resolution will be changed, as long as it doesn't drop below the minimum threshold of roughly 300-400 ppi, you won't be able to detect the difference in output.

This is why planning ahead and beginning with enough resolution is essential.

Making changes this way will not permanently alter your image. You can change it back and forth all day without losing any information.


ResamplingBackbback to top

Resampling can be dangerous because instead of only enlarging or reducing the size of pixels, it actually adds or removes pixels in your image. It is not reversible (except by undo). However, it can be very useful if used carefully. There are two reasons to resample:

1. You want to change the resolution without affecting the dimensions

2. You want to change the dimensions without affecting the resolution (you don't have any extra resolution to use for enlarging an image).


Reducing resolution by resampling is called "downsampling". A mathematical equation is used to determine how pixels should be combined to achieve the desired number of pixels per inch. The same process is used to increase resolution and is called "upsampling". As you can see from the example above, upsampling doesn't really give more detail to your image, it just takes up more space on your disk. (Although it can reduce graininess).


Downsampling: In the example below, reducing the number of pixels per inch from 320 to 10, the image becomes very pixelated.


However, from 600 dpi to 400 dpi, as shown below, you aren't likely to see a difference, because our best printer has a limit of 400 dpi. The benefit of downsampling is that you get a much smaller file size that is easier to store and work with. But be careful! If you resample to lower resolutions than 300, your image will start to look pixelated.


How to do it: Remember that extreme scaling by resampling may cause your image to look pixelated.


Scaling by resampling Backback to top

If you have no extra resolution to use for enlarging an image (see scaling), then you have no choice but to resample. You can do this by checking the resample button in the Image Size dialog box, or you can select the image and hit "command-T " (for transform), to manually stretch the image. Holding shift while scaling will keep the proportions constant.


Changing resolution by resampling Back

If you want to change the resolution of your file (because it is above 400, if you need to save disk space, or if you want the resolution of one image match another), Go to the Image > Image Size dialog box and check the "resample" button. Now change the resolution. This is not reversible (except with the history palette or "undo"). Repeatedly changing the resolution by resampling will cause pixelation of your image.

Remember, adding resolution to your file will not increase the level of detail, but will increase your file size exponentially.


Guidelines for choosing resolution Backback to top

For prints and slide output, your final images should be somewhere between 300 and 400 ppi.

For the web, you only need 72 ppi.

When scanning slides, keep in mind that you will want to scale the image later. Give yourself plenty of extra resolution to do that, and then when the size is final, discard any resolution above 400 as described above.