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Sensors, Pixels and Image Sizes

Aspect Ratios

 
Here is Hasselblad's 39 Megapixel (36x48mm) sensor (top) compared with an 11 Megapixel full-frame (24x36mm) sensor from a 35mm DSLR. Notice how they differ in both size and aspect ratio. Courtesy of Hasselblad.
Image sensors come in a variety of aspect ratios—the ratio of the sensor's width to height. The ratio of a square is 1:1 (equal width and height) and that of 35mm film is 1.5:1 (1 times wider than it is high). Most image sensors fall in between these extremes. The aspect ratio of a sensor is important because it determines the shape and proportions of the photographs you create. When an image has a different aspect ratio that the device it's displayed or printed on, it has to be cropped or resized to fit. Your choice is to crop part of the image or waste part of the paper or display area. To imagine this better, try printing a square image on a rectangular sheet of paper so either the entire image is printed or the entire paper is filled.
 
Image

35mm film
Computer display
Canon 5D
Canon S3 IS
Photo paper
Printing paper
HDTV
Width x Height

36 x 24
1024 x 768
4368 x 2912
2816 x 2112
4 x 6
8.5 x 11
16 x 9
Aspect Ratio

1.50
1.33
1.50
1.33
1.50
1.29
1.80

To calculate the aspect ratio of any camera, divide the largest number in its resolution by the smallest number. For example, if a sensor has a resolution of 4368 x 2912, divide the former by the later. In this case the aspect ratio is 1.5, the same as 35 mm film but different from an 8.5 x 11 sheet of paper.

If you have ever tried to center a photo on a sheet of paper so there are even borders all around the image, you have been dealing with the concept called "aspect ratios". This is the ratio between the width and height of an image, screen display, paper or any other two dimensional rectangle. To calculate an aspect ratio, divide the largest number in a rectangle's size by the smallest number. The numbers can be mm, inches, pixels or any other unit of measurement. For example, a 35 mm slide or negative is 1.5 inches wide by 1 inch tall so its aspect ratio is 1.5 to 1. A square has an aspect ratio of 1:1. If a camera captures images 3000 x 2000 pixels in size, 3000 divided by 2000 gives an aspect ratio is 1.5, the same as 35mm film.

The aspect ratio of an image sensor determines the shape of your prints.

Aspect ratios are usually expressed in one of three ways:

  • When expressed as 1.5 to 1 or 1.5:1, the actual numbers calculated in the division process are used, even though one has a decimal place.
  • To remove the decimal, the numbers are raised to a new ratio so both numbers are even. In our example, 1.5 to 1 would be raised to 3 to 2. That's what's done with TV screen aspect ratios. The aspect ratio for normal TV is referred to as 4:3 and HDTV as 16:9.
  • In a few cases, where one part of the ratio is assumed to be 1, just the other part is given. For example, a 1.5:1 ratio is expressed as 1.5.

Aspect ratios present a problem when printing or displaying images. Mostcameras don't capture images with the same aspect ratio as the 11 x 8.5 paper we print on—which has an aspect ratio of 1.29 (11 divided by 8.5). Few have the same aspect ratio as the screens we display them on. Even some software that prints contact sheets crops images—greatly lowering their usefulness when you try to evaluate images for printing. When the aspect ratios don't match, here are your options:

  • Crop the image to the desired aspect ratio. Programs such as Photoshop let you crop or select areas of an image using any aspect ratio that you specify. To do it manually you:

1. Determine the aspect ratio you want to use.

2. Determine how high the image needs to be in pixels.

3. Multiply the height by the aspect ratio to determine how wide the image should be in pixels.

When you know the width and want to find the height, divide the aspect ratio's largest number into the smaller and multiply the width by that number. For example if the aspect ratio is 1.5:1 divide 1 by 1.5 to get 0.667. If the image is 3 inches wide, 3 x 0.667 tells you the height is 2 inches.
  • Size the image so it fills the available space in one direction even though some of it extends past the edges in the other dimension. In effect, you are cropping the image.
  • Size the image leaving unequal borders around it. You can then trim it or fix the problem while matting it for framing.

These examples illustrate different aspect ratios. The top image was sized to fill the width of a sheet of 11 x 8.5 paper. This leaves empty bands at the top and bottom of the paper. The bottom image was sized to fill the height of the paper but parts of the image extending past the sides are left unprinted.


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