Please tell me I am correct & the pro Is wrong???

Its ALWAYS been this way. A 150mm lens on a 4x5 camera is a moderate wide-angle. On a 35mm camera its a medium telephoto. On a mobile phone its an extreme telephoto.

Film/sensor size must be known to determine how a particular lens works on that camera. Focal lengths, by convention, are referenced to 35mm (digital full-frame) size; APS-C is approximately 2/3 of the 35mm/FF size which is why you get about a 1.5 crop factor (inverse of 2/3 is 3/2 or 1.5.)

Image circle is a separate issue. For example, I have a Sony 35mm f/1.8 lens, for crop bodies. The 35mm/FF EQUIVALENT is 52mm but regardless, its a 35mm lens. The image circle is too small for full-frame so it is intended for crop Sony bodies.

If you adapted, say, one of my Mamiya lenses to a Sony A mount and stuck it on my A850, the image circle would be much larger than needed but the effective FOV would be that of 35mm/FF, not 645.

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David M. Converse
Lumigraphics
http://www.lumigraphics.com
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