[LargeFormat] 12" Commercial Ektar - lens board extension

[Les Newcomer]

thanks to two prime directives of my life--never throw anything away and
It's all on my desk, I can provide some usefull data


Kodak Professional data book 0-18 "Camera Technique fro Pro photogs" 
states under the commercial ektar page

"The four f6.3 lenses described here cover at full aperture an angle of
53° and at small stops an angle of 64°"

Now I'm going to make some asumptions

First I"m going to assume the focal point is at the back edge of the
lensboard. We know its actually somewhere behind the lens board, but
this gives room for error. I"m also assuming Kodak was talking about the
covering angle and not the angle of view.


with that imagine an angle coming out with a centerline bisecting the
angle. the Half angle will be half of the 64° since I suspect Mr. f32
wants to stop down. If we measure out from the vertex 2,3 or even 5"
that will be the length of the centerline. With the angle B and length
of leg C known we can find the short leg b which will be the radius of
the image circle for a given length.


Accroding to the 16th edtion of Machinery's Handbook, b=c x tan B

 Mr. TI 30 says the TAN of 32 is .6248

2" long tube  x .6248= 1.249 or 3" dia

3        "    x .6248= 1.874" or 3.75" dia

4" long tube  x .6248= 2.50 or 5" dia

5" long tube  x .6248 =3.12" or 6.25" dia



Without showing my work

a 12" lens at  14" will focus at 7 feet

           at  16"     """""""   4feet
          
           at  18"    ''''''''   3 feet


formula:  object distance=  bellows draw  x focal length/ bellows draw -
focal length

So an extension box that will fit a 4" lens board that's 3" long will
allow you to focus to about 5 feet without vignetting.


Les Newcomer



Clive Warren wrote:
> 
> Here's a chance for all you peops with the good mathematical brains
> to help me out!
> 
> Have decided to put a 12" f6.3 Commercial Ektar on a 4x4 board for
> use with my MPP 4x5 outfit in the absence of a reasonably priced f9
> 300mm lens.
> 
> The MPP bellows extend to nearly 12" - I am thinking about making or
> having made an extension tube integral with the lens board to allow
> closer focusing and to make sure that it does focus at infinity. If
> the extension tube is parallel to the lens and is say, 2 or 3" even
> 5" how much danger is there of mechanical vignetting from the
> parallel extension tube?
> 
> Cheers,
>         Clive
> 
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