01 Focal length and Angle of View

These images were taken at Brighton beach, which is situated across the bay from the Melbourne Central Business District (CBD) and these colourful beach huts make great subjects.

Equipment Used: Canon G1X and a tripod. The lens is a built in 15.1mm – 60.4mm zoom (28-112mm equivalent for a full frame 35mm camera, the 1.5 inch sensor having a 1.85x factor when converting to 35mm ‘full frame’ thinking)

As this camera has a built-in powered zoom lens I am limited between the focal lengths listed above. For this exercise I will use 15.1, 27 & 60.4mm, respectively these are 28, 50 & 112mm in 35mm equivalent focal lengths with similar angles of view. By assigning the front dial on the camera to alter focal length of the lens I am able to set the lens to specific lengths.

Image 1 15.1mm (28mm 35mm equivalent)

Image 2 27mm (50mm 35mm equivalent)

Image 3 60.4mm (112mm 25mm equivalent)

With this camera the ‘standard’ focal length with a similar angle of view to that of a 50mm lens on a 35mm camera is stated as 27mm. This is due to the relationship between the size of the senor/film and the angle of view produced by different focal lengths. In this case the sensor is smaller than a typical 35mm image and therefore the focal length is shorter to produce a similar angle of view.

The angle of view is most often quoted from the diagonal of the image. The diagonal measurement across the film or sensor is also considered to give the standard focal length. For example with a 35mm still frame the normal dimensions are 24mm x 36mm which gives a diagonal measurement of 43.3mm and a lens with a focal length of 45mm produces an angle of view of 51.4 degrees measured across the diagonal. Much of this information can be taken from lens charts produced by the manufacturers. Interestingly 50mm is the typical ‘standard’ lens found for most 35mm and so called ‘full frame’ digital cameras giving about a ~45 degree angle of view. Only a few manufacturers produced 45mm versions as a standard lens option, one of which was Minolta now Sony in the digital age.

Relationship of focal length, angle of and film/sensor size. As with my 1.5inch sensor 27mm would be considered to be the ‘standard’ focal length for my Canon G1X camera and is roughly the diagonal measurement across the sensor producing a ~45 degree angle of view. For a 6x6cm format film camera the standard focal length is considered to be 80mm and 5x4inches is 150mm, this of course is due to the considerably larger diagonal measurement in these formats.

Returning to with the photographs was an interesting experience. The standard print, taken at 27mm (50mm) could be easily held and compared to the original scene, the wider angle of view print taken at 15.1mm (28mm) needed to be held much closer, to the point of obscuring the view. The picture with the narrower angle of view taken at 60.4mm (112mm) could not be held far enough away and had to be propped up over a meter or more away before it appeared to be the same size.


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