# Multiply, substract and add...



## shashinkaman (Jan 23, 2013)

Taking the risk of launching a topic which has been treated before: Why do makers insist on using '35mm' (read: FF) focal length numbers for lenses which were specifically designed for use on their aps-c (read: crop) cameras? 
For example: Canon's EF-S lenses are not supposed to be used on anything else but Canon's aps-c cameras, so why do we have to multiply EF-S xxmm by 1,6x to get to the 'real' focal length. Wouldn't it be easier to for example call the EF-S 60mm f/2.8 Macro an EF-S '97mm' f/2.8 Macro?
Even Olympus, who with their "digital from the ground up" 4/3 system, insists on having us multiply what ever lens of theirs we buy by 2 to get to the real/actual focal length... Who tackles this one for me? Thanks!!


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## Random Orbits (Jan 23, 2013)

They don't. Focal length is the distance between the effective focal point of the lens and the image sensor plane in the camera, and that is the number printed on the lens. The angle of view depends on the sensor size. Most people are more familiar with the 35mm relationship between sensor size and angle of view.


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## neuroanatomist (Jan 23, 2013)

Exactly. Focal length is an intrinsic property of the lens, independent of whatever sensor is behind that lens. If a manufacturer were to label a lens with a 'crop factor adjusted' focal length of some sort, that would be a lie.


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