The facts about camera aspect ratio

Digital cameras usually come with the ability to change the aspect ratio of the image being captured. The aspect ratio has a little to do with the size of the image, but more to do with its shape. The aspect ratio describes the relationship between an image’s width (W) and height (H), and is generally expressed as a ratio W:H (the width always comes first). For example a 24MP sensor with 6000×4000 pixels has an aspect ratio of 3:2.

Choosing a different sized aspect ratio will change the shape of the image, and the number of pixels stored in it. When using a different aspect ratio, the image is effectively cropped with the pixels outside the frame of the aspect ratio thrown away. 

The core forms of aspect ratios.

The four most common examples of aspect ratios are:

  • 4:3
    • Used when photos to be printed are 5×7″, or 8×10″.
    • Quite good for landscape photographs.
    • The standard ratio for MFT sensor cameras.
  • 3:2
    • The closest to the Golden Ratio of 1.618:1, which makes things appear aesthetically pleasing.
    • Corresponds to 4×6″ printed photographs.
    • The default ratio for 35mm cameras, and many digital cameras, e.g FF, APS-C sensors.
  • 16:9
    • Commonly used for panarama’s, or cinematographic purposes.
    • The most common ratio for video formats, e.g. 1920×1080
    • The standard aspect ratio of HDTV and cinema screens.
  • 1:1
    • Used for capturing square images, and to simplify scenes.
    • The standard ratio for many medium-format cameras.
    • Commonly used in social media, e.g. Instagram.

How an aspect ratio appears on a sensor is dependent on the sensors default aspect ratio.

Aspect ratios visualized on different sensors.

Analog 35mm cameras rarely had the ability to change the aspect ratio. One exception to the rule is the Konica Auto-Reflex, a 35mm camera with the ability to switch between full and half-frame (18×24mm) in the middle of a roll of film. It achieved this by moving a set of blinds in to change the size of the exposed area of the film plane to half-frame.

Why are lenses round, and photos rectangular?

Have you ever wondered why lenses are round, and photographs rectilinear? Obviously square lenses would not work, but why not round photographs? Well, lenses do indeed produce a circular image, however the quality of this image with respect to sharpness and brightness is not at all uniform. It is sharpest and brightest near the centre of the lens, becoming progressively less sharp and bright towards the outer edge of the circle. This deterioration is due to factors such as lens aberrations which become more pronounced towards the edges of the image. In terms of the photograph, only the inner, portion of the circular image should be used, hence why photographs are rectangular, or historically more square (before 35mm film).

Basically for lenses on a particular sensor, the diameter of the circle has to be larger than the diagonal of the frame. The example below shows a Full Frame 24mm×36mm sensor and its associated image circle with a diameter of 43.27mm.

This basically means that the image sensor only makes use of roughly 59% of the image circle (the sensor is 864mm², the image circle 1470mm²). Using a circular fisheye lens, or one that is smaller than the sensor, will result in a circular image. For example, using a small 16mm cinematographic lens on a full frame sensor.

In some cases, such in the case of the Leica D-LUX 6, the camera allows swapping between a bunch of aspect ratios: 16:9, 4:3, 3:2, and 1:1. This camera has a 1/1.7″ sensor (crop factor of 4.6). The actual sensor size is 3678 x 2745 pixels.