When traditional paper size is used for printing digital photos, the final result is either cropped or some area of the paper remains blank. The main reason for this is the aspect ratio. Traditional paper sizes are designed for film which has an aspect ratio that differs from that of digital photographs. To understand this difference first it is required to know the relationship of aspect ratio to Digital Photo Printing.

The relation between the horizontal size and the vertical size of a photo is its aspect ratio. The width is divided by its height and aspect ratio is calculated. This aspect ratio may also be applied to video picture, paintings or any two dimensional image. In case of a digital camera, its sensors which take place of traditional film have physical attributes of height and width. Film has an aspect ratio of 3:2 for which paper size of 4×6 is perfect for final prints and is most commonly used. Other paper sizes for traditional film are 5×7, 8×11 and more. However this same paper might not be suitable for digital photo printing. If the aspect ratio of the printing paper is not similar to that of the sensors of the digital camera, the picture may appear cropped, stretched or blank spaces may appear on the print. The image can be adjusted into the paper size but it may again appear distorted.

Sensor sizes in pixels can be summarized with width, height and aspect ratio as follows:

2Mega Pixel -1600×1200 4:3

3Mega Pixel -2048×1536 4:3

4Mega Pixel -2272×1704 4:3

5Mega Pixel -2592×1944 4:3

6Mega Pixel -2816×2112 4:3

8Mega Pixel -3200×2400 4:3

The above table tells us that no matter what the resolution is of a digital camera it aspect ratio always remains 4:3. Thus for digital photo printing the paper size becomes 4×5 1/3 which is the closest to the 4×6 paper size. This new paper size used in Digital Photo Printing is called as 4×D where 'D' stands for digital. Introduction of the 4×D paper size was done to print digital images without any distortion and to achieve proper prints. The 4×D paper is compatible for digital photo printing as it is compatible towards the aspect ratio of digital camera sensors. Paper size 5×D is also available which means it is 5×7 and is corrected to aspect ratio of 4:3.

With the introduction of digital cameras, printing of digital photos will become even more popular and will introduce many more paper sizes compatible to them.