written on the side

[Michael Geschlecht]

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Hello Everybody,

 

The NOMINAL focal length of a lens is the distance from the 2nd (Rear) nodal point of that lens to the focal plane. Regardless of what or where that lens is focused on.*

 

No more. No less.

 

It is TRADITIONAL to engrave lenses with the focal length of that lens when that lens is focused at Infinity.

 

When a lens is focused to a point closer than Infinity: The 2nd (Rear) nodal point moves AWAY from the image plane even if there is internal focusing & the lens as a unit physically stays stationary. That means: The nominal (not engraved) focal length gets longer.**

 

Because: Please see * above in this Post.

 

**There are some exceptions to this statement sometimes which are not necessarily relavent to this specific discussion at hand. As an example: The 280mm, F2.8, 1st Apo-Elmarit.

 

Best Regards,

 

Michael

[Exodies]

This tread is very confusing. The posts seem to be a mix of precise and sloppy language and internal contradictions.

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[gyoung]

As with many technical photograhic issues there are a lot of wooly generalisations about things which are quite simple in principle but are confused by a misunderstanding of cause and effect.

In these days of 'blogs' some people delight in inventing jargon to misdescribe things they don't fully understand.

All this is complicated by misunderstanding of language which is sometimes not ones own 'native' one.

The best thing is to buy or borrow a good text book in your own language which should explain the issue,it happens Wikipedia has this matter right in English and I quoted it, but it is not always to be relied on so a traditional text book is best.

 

Gerry

[Peter Branch]

I fear that the question of what do Leica mean when they engrave numbers on lenses is not going to be resolved by looking at the internet.

 

Having just done a trawl through scores of internet sites which include a definition of "Focal Length", (Thanks Google), I have realised that there are many different definition being published.

Some are plain wrong and sow confusion.

 

For my part I fall back on the definitions in the books by Arthur Cox  and  Jenkins & White which were the standard books of reference when I was at University and working in the Kodak Research Laboratory.

 

http://www.amazon.com/Photographic-Optics-Technique-Definition-Photo-Technique/dp/0240508440

 

http://www.osa.org/en-us/about_osa/newsroom/obituaries/arthur_cox/

 

http://www.amazon.co.uk/Fundamentals-Optics-Harvey-E-White/dp/0072561912

 

Some of the internet sites hedge their bets by effectively claiming that there are two main definitions one being the Physics Definition and the other the Photographic Definition.

 

The numbers Leica use on their lenses and in their literature are Physics Definitions. 

 

The Physics Definition is based ultimately on the geometry associated with Image Magnification in the general case where the Object is not at infinity.

In the specific and limiting case where the Object is at infinity then, as I showed in my previous post in this thread, "v" = "f". 

The Focal length in the Physics Definition is "f" but the so called Photographic Definition general seems to be "v". 

 

For anyone with only a rudimentary understanding this can be very confusing particularly as "v" is an important parameter.