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Forum: Pentax SLR Lens Discussion
11-26-2017, 01:21 PM
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The DA40XS was designed to cover the APSc sensor. Regardless of whether it ‘works’ acceptably on a FF sensor, It is not truly FF compatible.
The FA50 is a film-era lens designed to cover a 35mm (film or digital) sensor. It is, therefore, FF compatible.
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Forum: Pentax SLR Lens Discussion
11-26-2017, 01:11 PM
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40mm is 40mm - it is a property of the lens and has noting to do with the sensor size. We express lens focal length in real terms for each lens, regardless of the sensor size the lens was designed for.
The Field of View of the lens (the portion of the entire vista in front of the lens projected onto the sensor) is a function of the sensor size. We use ‘35mm equivalent’ as a shorthand to describe the Field of View a lens/sensor combination will capture, expressed in 35mm film terms, only because 35mm film has been accepted as a shared standard most users will understand. When 35mm film became popular, 120 film users (usually used in TLR’s) scoffed at the small film. 645 and 6x7 medium format film, 8x10 sheet film, glass plates - these are also ‘sensor sizes,’ but the lenses are still the focal lengths they were designed, regardless. 40mm is a common TLR taking lens focal length.
Pentax / Sony APSc has a factor of 1:1.5, .i.e. the Field of View of a lens on APSc is reduced such that for any given lens on an APSc sensor, a lens 1.5x the focal length would capture the same Field of View on a 35mm sensor. Thus a DA40XS has a 60mm equivalent FoV in FF terms.
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