Generally speaking, aspect ratio refers to the width of an image to the height of the image. It is always expressed in the form of x:y. For instance, we can always see such numbers like 16:9 or 4:3, which are typical aspect ratio commonly seen and applied in daily life. When used under different circumstances, it is always different accordingly. For example, the aspect ratio of the photography is different from that of television or computer applications. If the aspect ratio is changed in an arbitrary way, then the image quality may be damaged or even the image could be distorted. As a result, it is wise of you to keep in mind that the aspect ratio can not be changed at will. Otherwise, you could lose the opportunity to enjoy the image of high quality and even get your mood spoiled by the distorted image.
OAR, short for original aspect ratio, is the original aspect ratio in which the film is made. The original aspect ratio can be altered in order to make it more convenient for consumers to view in other modes, such as television. It is only possible for you to convert the original aspect ratio by the following method: you are supposed to enlarge the original image to fill the area as well as cut off the excess area. Another way you have a chance to alter the original aspect ratio is to stretch the image in order to fill in the area required by the new aspect ratio.
MAR, refers to the modified aspect ratio, is the aspect ratio which is assigned for the purpose to fit the type of screen. On the other hand, it is also different from the dimensions in which it has been filmed.