Angle of View

This page explains the standard film formats, their usable areas for various standard print sizes, and the method of computing equivalent focal lengths of lenses across formats as measured in terms of how much of a scene they will record from a given position for a given enlargment ratio. There is also an explicit formula for calculating the angle of view of any given lens focal length across any size piece of film.

The Basic Film Sizes

Format Width Height Area Diagonal
35 36mm 24mm 864 sq mm 43.27mm
645 56mm 41.5mm 2324 sq mm 69.70mm
66 56mm 56mm 3136 sq mm 79.20mm
67 69.5mm 56mm 3892 sq mm 89.25mm
69 89.5mm 56mm 5012 sq mm 105.58mm

Print Ratios and Usable Negative Size

A commonly used notion of 'normal' lens is one whose focal length is equivalent to the diagonal of the film used. But it is often more useful to compare sizes of the negative that can actually be used for various print sizes. The rectangles picked out by the above films are not all of the same ratio. The following tables are indexed by print ratio and give the maximum usable film size for that print ratio.

To determine which lenses are 'equivalent' (in terms of coverage) across formats, use the ratio of diagonals in the following tables. Of course, depth of filed will vary. For example, with a 1:1 square print ratio, a 35mm camera with a 34mm lens will capture the same amount of a scene as a 66 (or a 67) camera with a 79mm lens. Use simple ratios to compute equivalent focal lengths. For instance, at a 1.25:1 print ratio (for an 8x10 enlargement), you can determine that a 34mm lens on a 35mm camera will pick out the same amount of a scene as an 89mm lens on a 67 camera. In general, a 67 lens of length X is equivalent to a 35mm lens of length (34/89)*X; similarly, a 35mm lens of length Y will be equivalent to a 67 lens of length (89/34)*Y.

1:1 Prints

The classic square print.
Format Usable Width Usable Height Usable Area Usable Diagonal
35 24mm 24mm 576 sq mm 33.94mm
645 41.5mm 41.5mm 1722 sq mm 58.70mm
66 56mm 56mm 3136 sq mm 79.20mm
67 56mm 56mm 3136 sq mm 79.20mm
69 56mm 56mm 3136 sq mm 79.20mm

1.25:1 Prints

This is the standard ratio print for 8x10 and 16x20 enlargements and roughly the ratio of 11x14 enlargemnts (1.27:1) and 20x24 enlargements (1.2:1). Note that the 67 format is often said to be an 'ideal format' because almost the entire negative area is used in this common print ratio.
Format Usable Width Usable Height Usable Area Usable Diagonal
35 30mm 24mm 720 sq mm 33.94mm
645 51.875mm 41.5mm 2153 sq mm 66.43mm
66 56mm 44.8mm 2509 sq mm 71.71mm
67 69.5mm 55.6mm 3864 sq mm 89.00mm
69 70mm 56mm 3920 sq mm 89.64mm

1.5:1 Prints

This is the standard ratio for 35mm negatives and corresponds directly to print sizes of 4x6 and 8x12 and 20x30, and is fairly close to 5x7 (1.4:1) and 3.5x5 (1.43:1). For this print ratio, 35mm and 69 formats are 'ideal'.
Format Usable Width Usable Height Usable Area Usable Diagonal
35 36mm 24mm 864 sq mm 43.27mm
645 56mm 37.33mm 2090 sq mm 67.30mm
66 56mm 37.33mm 2090 sq mm 67.30mm
67 69.5mm 46.33mm 3220 sq mm 83.53mm
69 89.5mm 56mm 5012 sq mm 105.58mm

Calculating Angle of View

Here's a handy formula that I retrieved from the RIT PhotoForum Lens FAQ, prepared by David Jacobson. This is only one section of the extraordinarily comprehensive RIT PhotoForum FAQ.

For a non-distorting, regular (non-fisheye) lens, the angle of view is given by:

Magnification can usually be approximated as 0 for normal (non-macro) applications.
Also see:
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