Sensor size , lens and resulting image  needs explanation   A Lens Is a Lens is a Lens | CineTechnica is written by   a clear explanation to end unfruitful debate.       A lens is a lens is a lens, and it doesn’t know what size sensor or film gate is placed behind it.  A lens projects light and the magnification size of that projection is determined by its focal length. Nevertheless, there has always been a great deal of confusion associated with the focal length of lenses and their relationship to the size of the image area. I’ve read many explanations in various books and on the internet over the years and authored some of them myself. But often written explanations confuse as much as they explain. If a picture is worth a 1000 words, I figure this video is equal to a small novella on the subject. Many people seem to use the various optical terms incorrectly or interchangeably, so I figured showing what happens would help demystify the concepts involved.

Go to the A Lens Is a Lens is a Lens | CineTechnica a beautiful demonstration by 

Because it is not an 85mm, it is a 50mm.  That number is based on a magnification factor and an ANGLE OF VIEW.  The fact that the projection is physically not covering any given format has nothing to do with it. Think of it this way:  why should anything be defined by a 35mm stills format?  Or any other format?  If you took a 50mm designed only to cover APS-C and then a 50mm designed to cover Full Frame 35mm and put them both onto an APS-C camera, the resulting field of view would be identical.  So then it would be rather curious to have them labeled as different focal lengths. A lens has no idea what sensor is placed behind it.  It simply projects light.  The mathmatical relationship of the magnification of light to the size of the aperture defines equal to the distance between the image plane and a pinhole that images distant objects the same size as the lens in question.  To call it anything else would be both a fallacy and lead to vast confusion.  Truly a lens is a lens is a lens


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