RE: [Leica] Rangefinder/Superwide lens + Question

[Buzz Hausner <Buzz@marianmanor.org>]

Good clarification, Christoph, thanks.  My point was that I believe that the
actual focal length of lenses varies however slightly from one lens to
another.  The little double digits next to the meters marking on the
focusing ring of lenses 50 mm and longer indicates that lens's departure
from the stated focal length.  At least these markings are on my lenses
which all date from the last, not present, generation.

At the end of the day, though, I don't actually think this is of any
consequence.  Interesting, but inconsequential.

	Buzz Hausner

> -----Original Message-----
> From:	c.blaue@bmsg.de [SMTP:c.blaue@bmsg.de]
> Sent:	Monday, March 22, 1999 7:35 AM
> To:	leica-users@mejac.palo-alto.ca.us
> Subject:	Re: [Leica] Rangefinder/Superwide lens + Question
> 
> >> Buzz Hausner <Buzz@marianmanor.org> writes:
> 
>     >> Can somebody tell me if the coverage specified for the lenses (e.g.
> 92°
>     >> for 21 mm) refers to the diagonal or to the coverage from straight
> left
>     >> to straigt right?
>     >> 
>      > 	I believe diagonal.  However, I don't know how accurate this
> stated
>      > angle is from lens to lens...Erwin?
> 
>      > 		Buzz
> 
> A more precise value is 91.702105 for a 21mm lens. However this angle
> depends
> on the real focal length and it holds only for the infinity setting of the
> lens. If you set it to some other distance (usually less than infinity)
> the
> distance between lens and film becomes bigger and the angle of view
> becomes
> narrower.
> 
> The formula for the above value is derived as follows:
> 
> 1. negative size is 24 x 36
> 2. The distances from the middle of the neg to the borders (not the
> corner!)
>    are 12 x 18
> 3. the distance from the middle to the corner is by usage of the
> Pythagoras
>    theorem sqrt(12*12 + 18*18).
> 4. the tangens of the half angle of view is then this radius divided by
> the
>    focal length.
> 
> christoph
>