Re: More thoughts on formats

[P3D John Bercovitz <bercov@xxxxxxxxxx>]

> 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.
 
Nice goin', Bozo!  :*)  Sorry about that.  That's the spacing of
the images on the film, not the spacing of the lenses.  The lenses
have to be closer together than the images to create the built-in 
window.  The amount generally is a thirtieth of the lens focal
length or 1.2 mm.  This means the 5P would have a lens separation
of 71.2 - 1.2 or around 70.  Most (all?) 7P cameras are not the
real thing.  There's just too much extra film going to waste.  So
they often cut the film gate spacing down to around 64 and the lens
spacing down to around 63.  Roughly.  There's quite a bit of variation.
It's not so critical in 7P because of all the extra film in the gate.
Mutter mutter...  There was something bothering me about the earlier 
figures - I knew the numbers weren't that "good". (That close to the 
65+ average IPD of humans.)  Argghhh!!
 
John B


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