[Leica] Dual Markings on Digital lenses

[telyt at earthlink.net (Doug Herr)]

on 12/31/04 2:09 PM, Afterswift@aol.com at Afterswift@aol.com wrote:

> 
> In a message dated 12/31/04 1:29:31 PM, lug-request@leica-users.org writes:
> 
> 
>>> It's possible that in the future new lenses will be dual purpose and
>> carry
>>> dual focal length engravings, one for the 36x24 format and the other for
>>> the smaller digital chip.
>> 
>> The focal length doesn't change.? The angle of view captured by the medium
>> changes.? The angle of view is more important to me than the focal length.
>> -------------------------------------------------------------
> Doug,
> 
> Sure, the focal length of a zoom may not change, but for all practical
> purposes when you zoom, for example, to a 50mm angle of view, the lens 
> barrel
> moves 
> and the 50mm point can be marked on the lens barrel. In fact, on my trusty 
> AF
> 35-70 Nikkor, the angles of view are marked on the zoom ring in bold 
> numbers,
> to wit: 35, 50, 70. The macro range appears in the focus window, which 
> makes
> sense because the macro can be used at any angle of view.
> 
> The double marking to denote the digital field shouldn't be that much of a
> problem. I use the 35-70 Nikkor with my D70 and interpolate the view 
> angles,
> like solving problems on an oral arithmetic quiz that we're all familiar 
> with
> from grade school. We wouldn't have that problem with a full frame digital 
> M
> because I assume we'll all be using or favorite Leica fixed focal length
> lenses. 
> 
> Bob
>

Bob,

The focal length is a measured or calculated value of the lens that is only
partially related to the angular coverage.  It's the distance from the
optical center of the lens ("nodal point") to the film plane when focussed
at infinity.  Since a lens with a shorter focal length has a nodal point
closer to the film plane, it can have a wider angle of view - but only if
the lens is designed to cover the area of the film.  This is something large
format photographer are acutely aware of since they need lenses that cover a
much wider film area to allow for swings and tilts.

If a lens is used with two different film formats the focal length doesn't
change, but as long as the lens can cover the larger format, the angle of
view of that lens changes with the different film formats - but the focal
length does not.  The Leitz 280mm f/4.8 is a good example.  It will cover
the area of 6x6 film, even though it's normally used with 35mm film.  Used
on a Leica it covers 8.8 degrees.  Used on a 6x6 camera, it's nodal point is
still 280mm from the film plane when focussed at infinity, but it covers a
wider angle because of the larger film area.  It's as though the 35mm
negative is cropped from the 6x6 neg, just like the digital sensor is a
smaller area than the 35mm neg.

And a zoom lens does indeed change focal length when you zoom.  What you are
doing when you zoom is changing the nodal point.

Doug Herr
Birdman of Sacramento
http://www.wildlightphoto.com