Masking confusion

[T3D john bercovitz <bercov@xxxxxxxxxxx>]

Clew: Title is a 8-)

ANSI PH3.11 standards:

Camera:
Gates are 23.5 wide and 71.25 on center (and 25 high)

Mask:
Apertures 21.1 wide and 61.9 on center (and 23.6 high)
Chips 62.2 OC  

(Note that 5 perfs * 4.75 mm/perf = 23.75 mm so in theory
there is lots of extra film here, both for camera and mask.)


Typical camera:

Gates are 71.2 OC, lenses about 70 OC
So infinity offset is 1.2 for both or 0.6 per aperture 

Lenses are ~35 mm so we have:
1.2 mm change for 35 mm distance
>From which:
70 mm change for (70/1.2)*35 = 2.042 m
giving 2 m window distance

So even if you put the chips' centers on the same centers as 
the mask's apertures, you get a built in window of 2 meters.


My question is, "Why is the chip spacing different from the
mask aperture spacing?"

It seems like pulling the chips an additional 0.3 mm outward 
is some sort of insurance policy against having things 2 m away 
poking through the window if someone gets a little clumsy 
setting the spacing of the transparencies.  In other words, it
gives a hidden or built-in spacing tolerance of -0.3 mm.

What do you think?  Is this a good guess?  Anyone know the 
answer as to why the standard is this way?

The Albion normal masks I have here measure 23.6 wide, 21.4 high,
and 62.2 on center.  The RBT masks I have here measure 21.5 wide,
23 high, and the apertures are 61.8 OC.  So it looks like RBT 
stays closer to the original intent of ANSI PH3.11.  It also looks
like the infinity guide for RBT should be 63.0 mm instead of the 
usual 63.4.  Personally, I _like_ having the window closer and 
have never had a problem with it.  So I can use a 63.4 guide.

Where did the 63.4 guide come from?  It seems like it's 62.2 
aperture spacing plus 1.2 for a 2 m window distance.  But where 
did the 62.2 come from?  A misreading of ANSI PH3.11?

Thanks,
John B


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