Adjusting the Resolution and the Size of the Digital Camera Image

When copying a photograph from the memory of a digital camera an opened in Photoshop we will notice that in most cases the image's resolution is very low (72 ppi) and the physical dimensions are very large. In other cases we have an image of high resolution but who's size in MB is too large to see on the screen, to place on a web page or send by e-mail. It may also happen that we need to enlarge the dimensions of an image to print it in a greater format. With any change in image size we shouldn't forget that we can conserve or reduce the quantity of information but not create new.

The changes in resolution or image size are made in the submenu Image > Image size , were a dialog box appear with which we control all the variables.

In the first box is the image data: number of pixels and file size. The second box indicates the physical dimensions and the resolution. In the bottom part is the operation options. The restrict proportions box is usually full, since it indicates any increase in the dimensions will proportionally affect the other (both are tied with a chain symbol). We will only leave that box blank (the tie disappears) in case we want to stretch or flatten the image, meaning, modifying it's dimensions separately. The resample the image box is the most important because it determines how the quantity of information is going to vary: when the box is blank the quantity of information and the file size in number of pixels and in MB doesn't vary and this is symbolized by a chain which unites the physical dimensions and the resolution; when the resample box is full we can introduce changes in the resolution or in the physical dimensions independently (the chain symbol disappears).

With these variables we can make any change in the image:

• Convert an image or large dimensions and low resolution into one of smaller dimensions and high resolution, as happens when we open a recently copied image. When opening the dialog box we see that the resolution is 72 ppi and the physical dimensions are huge. To obtain a high resolution image with smaller dimensions without loosing information we have two options: set a resolution in the dialog box, for example 300 ppi, or printing dimensions. In both cases the resample image box has to be in blank. We introduce the resolution value or the dimensions wanted and we observe that the variable which we haven't changed adjusts automatically for the new value; also, in the superior box we don't see any change, since the image still has the same number of pixels. The image in the screen doesn't vary either because the dimensions of an image on the screen depend on the number of pixels and the zoom factor (magnifying glass). Since the number of pixels hasn't varied and we haven't changed the zoom, the aspect is constant.

• Reducing the size of an image and maintaining the resolution or vice versa. In both cases a loss of information is produced. There may be a case when we want to print an image at a smaller size with a satisfactory resolution (300 ppi); or maintain certain dimensions and reduce the resolution, maybe because the photo is going to be seen only on the monitor or because we are going to e-mail it. In this case we will fill in the resample image box and apply the reduction in the variable we want. We will see two things: the variable which we haven't altered doesn't adjust automatically to the new values any more, it stays set and the size in number of pixels and in MB is reduced; the image on the screen shrinks because now it has less pixels ad to see it at the size we saw it before we would have to increase the zoom factor.

• Increasing the image size. Photoshop can increase the dimensions of a digital image or it's resolution with a mechanism called interpolation , also used by cameras and scanners, which consists of adding new pixels between the existing ones based on the values of the adjacent pixels. The different modes of interpolation appear in the dialog box of image size and are: bicubic (the most recommended), bilinear and neighborhood . Until now we have considered that the resolution is a synonym of image sharpness. Nonetheless, it is also true that we can't get more information were there is none. When we increase the dimensions or the resolution of an image we don't add detail. Like a printed image of larger dimensions is also seen from further away, the interpolation can be considered a method of maintaining the original sharpness and avoiding increasing the size of the pixels. In high resolution images it provides acceptable results, specially if the increase isn't excessive. If the detail of the image is poor, the interpolation can do little to maintain a sensation of sharpness. Therefore it should be used only for moderate increases in the dimensions of the image, as long as this is an acceptable original quality. With the resample image box full and the interpolation selector on bicubic, we introduce in the height box or width box a new value. In the above box we will see an increase in the number of pixels and in file size, as an increase in the image size onscreen. An increase in resolution itself doesn't have much sense, since we will only increase the file size. Nonetheless, it is necessary when we displace an image of set dimensions and another of a greater resolution: a cutout of an image of 150 ppi transferred onto an image of 300 ppi resolution becomes half the size, since 150 linear pixels (one inch) in the original image only covers half an inch in the destination image. To maintain the dimensions of the transferred image it is necessary for both to have the same resolution. With the resample box full we proceed to change the resolution of the original image in the corresponding box, making it equal to that of the destination image.