[Newtophotos]

When a lens is listed as 70-200mm or 55-300mm what distance within the lens is specifically being measured? A quick good search of focal length shows various points being measured?

[normhead]

Optical center to focal plane.

On a complex lens, the optical centre may not be apparent looking at the diagram.

[pathdoc]

I've always wondered about this, especially in regard to some of the great big monsters compared to Pentax's tiny pancakes, where I think "How the hell can they get a 70mm or even a 40mm focal length into that tiny thing, and why is Sigma's 35mm f/1.4 wonderprime so much longer than my pancake?" Now all is substantially clearer - or at least considerably less murky!!!

[sterretje]

I've always wondered about this, especially in regard to some of the great big monsters compared to Pentax's tiny pancakes, where I think "How the hell can they get a 70mm or even a 40mm focal length into that tiny thing, and why is Sigma's 35mm f/1.4 wonderprime so much longer than my pancake?" Now all is substantially clearer - or at least considerably less murky!!!

It really becomes interesting when you start thinking about lenses with a focal length that is shorter than the registration distance (e.g. the DA15Ltd as a relative extreme example); the optical centre is inside the camera Telelenses are boring from that perspective

[stevebrot]

Optical center to focal plane.

With lens focused to infinity.


Steve

[glasbak]

It really becomes interesting when you start thinking about lenses with a focal length that is shorter than the registration distance (e.g. the DA15Ltd as a relative extreme example); the optical centre is inside the camera Telelenses are boring from that perspective

Telelenses are exactly the same, except that the focal length is in front of the lens i.s.o. behind the lens.

[stevebrot]

It really becomes interesting when you start thinking about lenses with a focal length that is shorter than the registration distance

Yep! Oh the wonders of retro-telephoto design! We owe so much to Pierre Angenieux who pioneered this approach in the early 1950s.


Steve

[stevebrot]

Telelenses are exactly the same, except that the focal length is in front of the lens i.s.o. behind the lens.

So true! What's more, many of the lenses we refer to as "tele", aren't! Telephoto refers to design, not focal length.


Steve

(...has a 210mm view camera lens that would be so much easier to work with were it a telephoto design...)

[normhead]

With lens focused to infinity.


Steve

Oh ya, I forgot that part...