Aps-c lens

Started 2 months ago | Questions thread
A Marcus Forum Member • Posts: 50
Re: Aps-c lens

Lenses are fitted to cameras based on the size (height & width) of the format. The full frame film camera 35mm film size measures 24mm height by 36mm length. We measure the corner to corner distance of the rectangle (diagonal measure), this comes out to 43 ½ mm. If we fit a 43.5mm lens on this camera and hold it horizontal (landscape view), the angle of view will be approximately 45⁰. This view is considered by the industry to be a “normal” view. Now a 43.5mm focal length lens is somewhat uncommon, thus the industry as elected to round this “normal” lens up to 50mm. Such a lash-up gives slightly more magnification and the angle of view (horizontal) 40⁰. Thus a 50mm focal length mounted on a full frame (film or digital) is the accepted “normal” lens for this format.

A wide-angle lens starts at about 70% of “normal” or shorter. Thus a wide-angle for the FF format is 35mm or shorter. A telephoto is about 200% of “normal” or longer. Thus for the full frame, 100mm or longer is considered telephoto.

The compact digital (Dx) is a spin-off or a film camera size introduced in about 1990. This camera used 24mm wide film, the format size is 16mm height by 24mm length. The diagonal measure of this rectangle is 30mm. If you mount a 30mm focal length lens on a compact digital (APS classic rectangular format), this lash-up delivers a 45⁰ angle of view, camera held horizontal. Using the 70% rule-of-thumb, a wide-angle for this format is 20mm or shorter. A telephoto for the APS-C is 60mm or longer.
The 35mm film format has been around for almost 100 years. Its popularity is great so most of us gray hairs and used this format and thus we are quite familiar with how it images using the various lenses available. When we switch to a different format, likely smaller, we might benefit is there was a simple math method that we could use to make a comparison.

There is! We divide the diagonal measures of the two formats to get a magnification factor, more often called a crop factor. Thus comparing the Fx to FF we divide 50mm ÷ 30mm = 1.6. This is the so called crop factor. If a 50mm is mounted on a FF we divide 50mm by 1.6 = 30mm (rounded). Thus a 30mm on a Fx delivers a similar image to a FF sporting a 50mm.

Mount a 50mm on a Fx and the equivalent on a FF is 50 X 1.6 = 80mm

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