Quote:
Originally Posted by TrevorW
Based on the original formula (using my calcualtions) the slit width for an 80ED f/7 should be roughly 3.75mm. The original post of the 100ED mask from Houghy rescaled to 83% produces slit sizes of this width whereas the recent post produces signifcantly smaller slit widths which are not easily workable.
Cheers
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Hi Trevor
The “standard” formula is Focal Length/N(range150 to 200)=(Bar Width + Bar Gap). For shorter focal lengths we multiply the Focal Length by a factor of 3 giving:
3xFL/N=(Width+Gap)
An 80mm F7 has a FL of 560mm.
So, using the denominator at max value of 200, I get a 4.2mm Bar Width.
560x3=1680
1680/200=8.4
8.4/2=4.2mm
And, using the denominator at min value of 150, I get a 5.6mm Bar Width.
560x3=1680
1680/150=11.2
11.2/2=5.6mm
The x3 multiplier for focal lengths of say, less than 800mm means that you don’t have to cut the slits too thin.
Cheers
Dennis