ground glass hot spots or fresnel?

[SMBooth]

Re, putting a fresnel behind the ground glass is the focus point still on the ground glass face or the fresnel face, that assuming the ridges of the are contacting the ground side of the glass.

[Ian Grant]

If the fresnel's behind the ground glass it makes no difference, by behind I mean viewing side. If it's on the inside it'll move the screen back focus is always on the ground side of the screen.

So a back is modified if a fresnel is used on the inside between screen and lens, and conversely if a back was designed for an internal fresnel the screen needs shimming to reset the focus if the fresnel is removed.

Ian

[SMBooth]

OK this is the part Ive never fully understood, if there is no fresnel the ground glass is on the same plane as the film when a holder is inserted, now the part I do never have understood, does placing a fresnel between the lens and GG shift the focus point due to moving the GG because you have put the fresnel between the GG and the wood mounting frame physically moving the GG away from the lens the thickness of the fresnel or is it a optical thing changing the focus point of the light because it passes through the fresnel before hitting the GG, and the result is that the GG is at a different plane than the film.

[Ian Grant]

Because a fresnel is a lens there's theoretically a very small shift in the focus, but more critical is the physical thickness of the fresnel screen itself as it shifts the position of the glass. In practice when a fresnel is against the screen you will get no magnification the lenses effect, place a reading fresnel on a newspaper there's no magnification, lift it up it and beomes a magnifying screen.

Ian

[SMBooth]

Thanks Ian, so if i mount the fresnel in a way that does not move the GG the focus point is the same.

[Ian Grant]

Thanks Ian, so if i mount the fresnel in a way that does not move the GG the focus point is the same.

It should be.

Ian

[Andrew O'Neill]

So, I thought, what the heck lets rub some 200 on the plastic and see what happens. It was magic. In ten minutes time I had a usable ground plastic-glass. At 18 to 24 inches I could easily see the entire 16" screen for composition.

Hey Tim,

Did you rub it with a piece of glass or plexi?

[Ian Grant]

It's best to use the same material to grind plastics and also ound off edges and corners first. With glass on plastic you run a continuous risk of scratching.

Ian

[tim k]

Hey Tim,

Did you rub it with a piece of glass or plexi?

Andrew, sorry I did not see this for a while.

I took about 4 pieces of 1/4 plexi and glued them together. Thought it would be easier to hold on to. Ended up with a block about the same size as a deck of cards. I used one side for the glass, and the other side for the plexi.