When you print digital photos on traditional paper sizes photos are either cropped or areas of the paper are left blank. The reason is that tradition paper sizes were designed for film and have different aspect ratio than digital photos. Digital paper sizes solve this problem and are offered by many here is how. Understanding aspect ratio Before you can understand why digital paper sizes are different than traditional ones you need to understand what aspect ratio is. Aspect ratio is the relation between the horizontal and vertical sizes of the photo. It can also be applied to video, paintings or any two dimensional rectangle. Aspect ratio is calculated by dividing the width of a photo by its height. For example TV in normal mode has an aspect ratio of 4:3 or in other words if we were to divide the width of the screen by its height the result would be 4:3. Widescreen TV on the other hand is stretched like a movie theater screen and has an aspect ratio of 16:9. The camera's sensor which replaces the traditional film is a rectangle and has physical attributes such as width, height and resolution (number of pixels). As such the CCD also has an inherent aspect ratio its width divided by its height. What does aspect ratio has to do with paper sizes? The paper photos are printed on has a width and a height and also an aspect ratio. When a photo is printed the aspect ratio of the paper must equal to the aspect ratio of the photo (or actually of the sensor that the photo was taken with). If the aspect ratios are different the results might be: a blank area left on the paper, a cropped photo or a stretched photo that fits the paper but distorts the objects in it. Here is why: to simplify the explanation lets assume a camera's sensor that is 6 inches wide and 4 inches high (real sensors are much smaller). Lets also assume that we are trying to print a photo taken with that sensor on a paper that is 4 inches high. If the paper width would be less than 6 inches we will have to crop part of the photo since there won't be enough space available on the paper. If the paper width is more than 6 inches we will have to leave a blank area on the paper since we do not have enough photo" to print on more than 6 inches. If we want our photo to fit the page we can stretch or shrink it horizontally to whatever the paper size is but doing so would distort the objects making them look more fat or thin as they are in real life. Printing with tradition paper sizes Common traditional paper sizes are 4X6, 5X7, 8X11 and more. Lets take 4X6 for example which is the most common photo paper size. Traditional film has an aspect ratio of 3:2 and thus 4X6 paper was a perfect for those prints. Digital cameras however use sensor with different aspect ratios than traditional film. The following table summarizes popular sensor sizes in pixels width, height and aspect ratio:
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