ground glass hot spots or fresnel?

Discussion in 'Ultra Large Format Cameras and Accessories' started by tim k, Sep 18, 2011.

  1. tim k

    tim k Member

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    Got a question for you guys with big ground glass.

    With a straight ground glass, do you have pretty good view of the overall composition?

    I am still in the process of putting my 11x14 together. For my ground glass, I went down to my local plastic place, and picked up a piece of frosted plastic. It shows a good image for focusing, but the hot spot, or perhaps I should say bright spot is not very large.

    I went down to Staples and picked up one of those 8x10 plastic magnifying fresnel page reader things. It was amazing what it did for the overall view. Its like it bulb was turned on. The combination was 100% usable, but its too small.

    So, do I dump the plastic, and get a piece of glass, or look for a big fresnel?
     
  2. John Jarosz

    John Jarosz Member

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    I don't have a fresnel on my 8x20. I think you need to have a dark cloth that permits you to be far from the glass so you can see the whole image at one time easily. Then you move in close and focus with a loupe after the composition is settled. YMMV.

    john
     
  3. tim k

    tim k Member

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    Thanks John, I'll try moving farther back. Yesterday I was perhaps 2 ft back, and I was still having a pretty major bright spot. I'm starting to think that my easy plastic gg is a little bogus.

    I'm just curious, how far do you have to get back to see the whole 20" picture?
     
  4. John Jarosz

    John Jarosz Member

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    Hmmm.. 2 feet should be far enough. You know, I've been doing this for so long that there's a possibility I don't see the hotspot any more. I need to check.

    john
     
  5. tim k

    tim k Member

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    For what its worth, this weekend I went down the local rock store and got some grinding grit, and picked up some glass. Time to do a little testing.

    I did a small test piece of glass with 200 grit and a piece with a grit that was marked as 400-600f. To be honest, there wasn't much difference in the glass, but there was a huge difference in the size of the grinding compound. It did seem like the 400-600 was a little sharper, maybe a little brighter, but the 200 seemed a little less prone to the hot spot.

    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.

    I then did a 200 grit on real glass. It was sharper, clearer, and maybe a tick brighter. But not enough better in my mind to make up for the "break factor" of glass.

    I may try a 400-600 and see if its better. But for now my 200 grit plastic looks pretty good.

    Just saying.
     
  6. Steve Hamley

    Steve Hamley Member

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    I think what John said in post #2 is the best advice. To me, a fresnel is wonderful for composing, but I find that it is harder to focus critically, even the Maxwell that came in my 8x10. And the coarser the GG is, the easier it is to focus but the harder it is to compose because of the "hot spot". The plastic "glass" I had in my Wehman popped into focus like nothing else, but it was just too dim for me. So it's a balancing act that may not have the same solution for everyone.

    I was chatting with Don Dudenbostel, who has been a commercial and fine art photographer since the 1960s, and he said he never knew of a professional that used a fresnel in a large format camera.

    Cheers, Steve
     
  7. Ian Grant

    Ian Grant Subscriber

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    The secret is to double cut the screen, grind with #400 first and then #600 afterwards that give a screen that's slightly less prone to hot spots. It's a case of finding the right balance.

    Ian
     
  8. tim k

    tim k Member

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    I think I will try that.
    thx
     
  9. Len Middleton

    Len Middleton Member

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    Looking at grinding my own ground glass for a recently acquired 8x20. No sense starting too small... :laugh:

    The existing ground glass is quite dark, and so looking to do something brighter.

    Given the effort involved, is it worth the additional cost of using borosilicate glass rather than soda lime glass?

    I have read the various articles include Ian's on this site and the instructions on the Hopf web site. Any other insights or cautions?

    I did pose this question on another site that focuses upon LF photography, as well.

    Thanks,

    Len
     
  10. Ian Grant

    Ian Grant Subscriber

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    I've found that a quick re-grind usually brings new life to an old dim screen and it'll be as god as a new screen.

    You'll be surprised just how easy it is to make a good glass screen that will match any commercial screen, Hopf included, in terms of brightness and usability.

    Cost wise I think I'd rather make a couple of spare screens and just go for the normal glass unless you can get boro-silicate glass shheets at a reasonable cost. Optically it's additional brightness will be barely noticable/measurable and it can still get broken :D

    Ian
     
  11. SMBooth

    SMBooth Member

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    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.
     
  12. Ian Grant

    Ian Grant Subscriber

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    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
     
    Last edited by a moderator: Oct 22, 2011
  13. SMBooth

    SMBooth Member

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    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.
     
  14. Ian Grant

    Ian Grant Subscriber

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    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
     
  15. SMBooth

    SMBooth Member

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

    Ian Grant Subscriber

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    It should be.

    Ian
     
  17. Andrew O'Neill

    Andrew O'Neill Subscriber

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    Hey Tim,

    Did you rub it with a piece of glass or plexi?
     
  18. Ian Grant

    Ian Grant Subscriber

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    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
     
  19. tim k

    tim k Member

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    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.