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SIMION® Ion and Electron Optics Simulator
SIMION Version 8.1 is a software package primarily used to calculate electric fields and the trajectories of charged particles in those fields when given a configuration of electrodes with voltages and particle initial conditions, including optional RF (quasistatic), magnetic field, and collisional effects. In this, SIMION provides extensive supporting functionality in geometry definition, user programming, data recording, and visualization. It is an affordable but versatile platform, widely used for over 30 years to simulate lens, mass spec, and other types of particle optics systems.
science  physics  software  optics  simulator 
7 days ago by RBarnard
What exactly does a fresnel and ground glass screen do? | Photography Forums
On the 4x5 Speed graphic here GG with Fresnel is very accurate; it is what I use to scan artwork with our 35 and 50 Megapixel Phase One scan backs; where never the RF is used.
The GG with Fresnel also works well with the old 178mm F2.5 wide open; a lens that I have used for astro work for about 3+ decades.
On a properly settup Speed Graphic the GG and Fresnel is designed and built from the factory to focus spot on; ie it includes the (N-1)/N shift. This shift was known 400 years ago; thus it would be a total blunder for a person in Optics to not know this and thus compensate for it. The Graflok " frame's reference to seat where the GG/Fresnel combo sits" is different than if only a GG is used.
The only reason a Speed Graphic's GG-Fresnels should be off is if a doofus/Kilroy was in the loop; ie one who screwed up the precision settup. This is actually quite common; folks combine cameras; replace screens and then one gets a focus error; ie the Kilroy/ goober factor.
In my own Many Speed Graphics I have bought some off of Ebay that did have a focus error because somebody just had a GG' or had the GG/Fresnel sandwitch in wrong.

On a modern 4x5 film holder the reference surface to film I believe is 5mm; ie 0.197 inches. The Graflok frame to its reference is something like 1/8" ; 0.125 ( from memory) then one has the Ektalite Fresnel; plus the GG. The Fresnel and GG are roughly 1/16". The Optical stack of (1) machined surface; (2) Fresnel; (3) N-1/N times Fresnel thickness makes the focus be correct on GG; which faces the subject.
If one removes the Fresnel on one of these properly setup GG-Fresnel combos; the GG has to be shimmed out about 1/16 inch.
A common thrown around tolerance is that the film plane to GG error on some LF cameras is can be +/- .007 inches on some folks cameras. On a press camera the; the tolerance really has to be tighter; one is using faster lenses like my 210mm F3.5 Xenar and focusing on the back screen; ie the GG/Fresnel combo; it NOT tiny apertures but big ones.
fresnel  optics  largeformat  groundglass  graflok 
5 weeks ago by bwiese

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