Question on "normal" lens

[P3D Dr. George A. Themelis <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

To Dennis' observation:

>> If you measure the diagonal of the 7-perf frame, it's exactly 37mm-thus
>> it's the "normal" length. Like a 50 or 52mm lens on a 35mm SLR.

Paul responded:

>somehow these numbers seem a little off.  7-perf has a 37mm diagonal,
>but 8-perf has a 50 to 52 mm diagonal?  Isn't 8-perf closer to 42mm
>on the diagonal?  

Good point Paul!  Here is an interesting table:

Format     Dimensions   Diagonal
---------------------------------
Realist      24 x 23      33.2     
Belplasca    24 x 29      37.6
Full-frame   24 x 36      43.3

As you can see, the labels "5p", "7p" and "8p" are not accurate
since the Belplasca frame is almost in-between full-frame (7 mm less)
and Realist (6 mm more) but 7 is not between 5 and 8!

The fact that the FL of the lenses in the Belplasca matches the diagonal 
of the film frame to a fraction of a mm, means absolutely nothing to 
anyone, except mathematicians and so called "purists" of the wrong type!  
(now, is that the good old DrT, or what??? ;))

Seriously, the definition of the normal lens is a convention, not an
exact science.  It means little in practical terms .  Anything in the 
range of 40 mm to 55 mm is considered normal for full frame.  The 35 mm 
lenses of the Realist are as much normal for 7p as the 37.5 mm lenses 
of the Belplasca.

George Themelis



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