Re: More thoughts on formats

[P3D John Bercovitz <bercov@xxxxxxxxxxx>]

> Why do most 5-P format cameras lack borders between frames? 

Basically it's because they're trying to maximize image size.
5P at 4.75 mm per perf is 23.75 mm.  I think the mask is
nominally 21.4 mm wide.  So there's precious little extra.
They'd have to be very precise in their advance to give you
a border and then there would be no room to make adjustments.
You might get unhappy one day when, if you had had just a little 
more image on one side, you could have gotten rid of something 
nahstie on the other side.

What would really be nice would be if the 5P cameras were
very precise in their advances so that there wouldn't be any 
image overlap.  Then nothing would be wasted.

Going on to 7P, 7 perfs is 33.25 mm but the mask is only 28
so you have tons you can throw away and still allow for uneven
advance and for cropping.


Gratuitous information:

In the standard 5-perf camera progression, there are two frames 
between each frame being exposed.  Each frame is 5-perfs:

         ___________________________________________
        |          |          |          |          |
        |          |          |          |          |
        |          |          |          |          |
        |          |          |          |          |
        |          |          |          |          |
        |__________|__________|__________|__________| 

              |<------- 3 x 5 perfs --------->|


A perf is 4.75 mm.  So 3 x 5 x 4.75 = 71.25 mm which is the
standard spacing of the lenses on a 5-perf camera.  

In the standard 7-perf camera progression, there is one frame
between each frame being exposed.  So 2 x 7 x 4.75 = 66.5 mm
and that is the nominal lens spacing on a 7P camera.  66.5 is 
much closer to the average human eye spacing than 71.25.  So
5P cameras are slightly hyper and therefore make things look 
slightly smaller than life size.  This also means the depth 
cues are slightly stronger in the 5P.

John B


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