New Way to Make a Focus Panel?

[Mike1234]

Okay folks... we have GG and parafin sandiches and combinations with fresnels. The parafin variety seems to be very popular making for better viewing but the parafin melts easily. Has anyone tried a hot glue sandwich?? Sort of an emulation of the parafin version.

1. Make a bubble-free ball of white-ish hot glue and center it on the bottom glass (low temp hot plate slowly brought up to temp?)

2. Slowly heat the glass to begin the melt process.

3. Center the top glass and work it gently to mash the melting glue evenly between the sheets working it gently outward toward the edges and ensuring no bubbles form.

4. Continue working the glue outward until it bleeds past the edges of the glass and forms a thin uniform layer of glue.

5. Use calipers on all edges/corners to ensure even distribution of glue and adjust accordingly by lightly squeezing.

6. Remove from heat and let slowly cool on a thick towel.

7. Use razer blades to trim the excess from the edges and remove any spill over on the exterior surfaces.

Worth a try?? It would certainly be less prone to heat problems.

[Richard Wasserman]

Please let us know how it works out! :)

[Mike1234]

I'll need to buy a bigger stand-alone stove burner and a thick sheet of very flat iron... might need to have it machined to absolute flatness. Then I may give this a try. OR... maybe I'll try the milky epoxy method outlined in my other thread. :)

[Robert Hughes]

I just made one with a sheet of waxed paper in front of a fresnel - it seems to work fine. :)

[Jack Dahlgren]

How about drymount sheets? They are thin and flat already.

Hot glue is pretty viscous and thin glass is flexible so I think you would have issues getting a thin even layer. But give it a try.

Making ground glass is pretty much the easiest option - that is why it is most commonly used.

A fresnel is "better" if you want maximum light gathering.

I don't see what would be gained with a bit of hot glue which would be more difficult than gg and would transmit less light.

[Glenn Thoreson]

If you're wanting to make your own the easy way, get a can of NoVue window spray at the paint or hardware store. A few very, very light coats and you have a ground glass. Not the best you'll find, but it works. There are auction sellers who have ground glass. I would rather just buy one and be done with it. That happens when you get old. :D

[Mike1234]

I'm trying to emulate/simulate the Boss screen but eliminate the worry of melting the parafin. I live in south Texas.

[Jack Dahlgren]

Why emulate the BOS? It would seem that having the focus plane offset from the front of the glass would require reworking whatever is holding the glass so it would be at proper focal distance. The ideal is to have a focal surface as thin as possible which will drop in so it is positioned exactly where the front face of the existing ground glass is. There are ways to accomodate this by milling or making the front cover sheet of glass smaller so that only the rear sheet contacts the frame, but why bother?

There are also self-adhesive window films which could be used. Or perhaps break open an old laptop display and grab the diffusion sheets and/or fresnels out of it and see if they would work for you.

I just think you are going to have trouble making a thin layer of some translucent liquid and getting it positioned correctly.

[Mike1234]

Regarding a diffusion sheet: Intriguing but how would I adhere it to the front of the glass permanently, evenly, with no trapped bubbles, and without compromising its translucence?

[Jack Dahlgren]

You were going to sandwich the glue, just sandwich the diffusion sheet. Or get the self-adhesive type of sheet and put it on the front side using some care it is not difficult to avoid bubbles. If you get a bubble you can move it to the side or worst case, pop it with a needle. Or use any sort of thin slow-drying adhesive.

Pop open an old film holder, cut out the back and slip a sheet of the diffusion material where the film should go. Remove the ground glass entirely and pop in your portable focus screen. That way you can experiment with different things or have a wide fresnel and a long fresnel. Your darkslide will protect the GG when not in use.

There are tons of possibilities and experiments to be done.

[bobwysiwyg]

Along these lines, I recall posts where folks were grinding their own GG's. Just curious, how do folks decide which grit to use? It would 'seem' the finer the grit the brighter/sharper the image, but there must be some practical limit. Anyone know what that is?

[RichardRitter]

The way Sinars and Satin Snows grinds their glass is a trade secret. But the finer the grit the smother the focusing surface.

[Mike1234]

Along these lines, I recall posts where folks were grinding their own GG's. Just curious, how do folks decide which grit to use? It would 'seem' the finer the grit the brighter/sharper the image, but there must be some practical limit. Anyone know what that is?

I can't remember but there's a current thread on this and the guy is offering to make a few GG for free. I believe he "starts" with 60 grit and moves finer in steps. But I'm looking for something with no scratches nor pits of any kind.

I previously posted about using acid etched glass which I may still do.

[bobwysiwyg]

Hmm, seems almost counterintuitive to take a smooth piece of glass and have at it with a course grit, which 60 seems, and then work down to fine. Why not go straight to the fine even though it could take a while??

[r_a_feldman]

Re: making your own ground glass: http://www.dokasphotos.com/techniques/ground_glass/

[bobwysiwyg]

Re: making your own ground glass: http://www.dokasphotos.com/techniques/ground_glass/

Very interesting, thanks. I think I have another thing to try some time. ;)

[Nathan Potter]

Along these lines, I recall posts where folks were grinding their own GG's. Just curious, how do folks decide which grit to use? It would 'seem' the finer the grit the brighter/sharper the image, but there must be some practical limit. Anyone know what that is?

There is a tradeoff between the fineness of the grit used, the brightness of the screen and the resolution of the screen. The principle here is that you need a light scattering surface to generate a real image. Anything that will scatter light will do the job hence the many suggestions. In emergency situations I've used strips of frosted Scotch tape - pretty good actually - and you can leave a bit of space between each strip to focus precisely on the aerial image there.

There is not any particular grit size commonly used but the range from coarse to fine screens is made from about 180 grit (76 um average) for a coarse screen to perhaps 800 grit (12 um average) for a fine screen. During the grinding operation the grit, flying freely in water is crushed against the glass causing fractures in the surface of the glass. These fractures are what scatter the light. The scattering from each fracture is at different and random angles as a function of the lapping conditions, such as force applied, lapping speed and the material of the lapping block. These conditions, as well as grit size, have an effect on the brightness of the screen. In general as the grit gets smaller the screen is less bright.

When the grit size is too small say less than 2 to 5 um the damage to the glass is reduced to the point where the grit acts as a polish, with light enough pressure, and no scattering occurs and no real image is formed.

As Richard implies above, all screen producers have their own little tricks for maximizing brightness and resolution at the same time, but there are definite limits to what can be done, so we employ Fresnel screens and a few other tricks to improve brightness. :) :)

Nate Potter, Austin TX.

[Jim C.]

I'll add my .02 here, you're going to have a huge problem using hot glue, it's formulated to be a certain viscosity
at it's melting temperature, the only way to change that is to start messing with flow modifiers
and adding waxes, I know that because I worked on an animated commercial that required a translucent
hand prop, we wound up using 3M Jet Melt hot glue and we added wax and flow modifiers to get it to cast
properly. On top of that depending on the glass thickness and type I would worry about thermal shock
putting a room temp piece of glass on top of hot glue that may be past 200ºF may crack it or the glue
will form flow lines from contact with the cooler glass as it spreads out.

An oven would be better, heat both pieces of glass at the same time as well as your mixture.

I'm curious as to what makes the Boss <sp?> focusing screens so great, any pictorial examples comparing it
to fresnel/GG and plain GG ?