What exactly does a fresnel and ground glass screen do?

Discussion in 'Medium Format' started by thomas_w|2, May 12, 2010.

  1. What does a fresnel focus screen do exactly? And why do some cameras only have a ground glass screen (Rolleicords) and others have ground glass and a fresnel (Yashica)? Just trying to understand more about cameras. Thanks.
     
  2. A ground glass provides a translucent surface on which a real image can be projected and viewed. The diffusion is not perfect, so much of the light from the lens tends to continue in a straight line, away from the person's viewing eye, making the edges appear darker than the center. A Fresnel lens bends the light so that it converges on the eye, making the corners appear as bright as the center. To this extent, a Fresnel lens acts like a simple, convex lens at the surface of the ground glass, but is much thinner and lighter.
    Some viewing screens have both a Fresnel and convex lens. That's one reason the Nikon F3 has such an extraordinarily clear viewfinder. The ground surface of an Hasselblad "Acute-Matte" screen is actually composed of microscopic prisms angled to redirect light toward the center, in addition to a conventional Fresnel lens. Screens for autofocus cameras (e.g., Nikon) are bright because they are ground less aggressively (in addition to a Fresnel lens), which passes more light but makes them somewhat hard to use for manual focusing.
     
  3. Edward, I have always thought the same as you, but...
    On my LF cameras I tend to think that the coarser the groundglass the more difficulties for critical focusing. In fact, I have removed all kind of "brightscreens" (with their grittier surfaces) to use just plain, soft grounded glass. Also, the coarser GG, the brighter. I only use fresnels with a Sinar (just because it`s detachable!).
    If my stament above is true, how could be explained that current DSLRs are brighter with their soft grounded surfaces? (They are REALLY much brighter). Also, I`m not sure the coarser F3 standard screen is easier to focus than the F6 one.
    I`m confused. Probably I should check it more carefully, or perhaps it should be considered as a whole (GG, fresnel, pentaprism) and not by spare parts.
     
  4. Jose,
    The 'grain' size of the screen will obscure fine detail, so a coarse screen can indeed make focussing difficult.
    But i don't think you're right believing that the coarser a screen the brighter it is. The brightness of a screen depends on (as Edward said) how much light is scattered away vs towards your eye. A simple ground glass screen favours no particular direction, and it makes no difference whether coarse or fine.
    The Fresnel lens is (also as Edward said) the bit that is responsible for directing light towards the eye. It bends rays coming from the lens that are 'off target' towards your eye. But it can't do much about the scattering by the screen.
    A trick employed by some screens (like the Acute Matte screens) is to not use a scattering structure, but instead use an array of very small optical elements that each direct the light hitting it towards one point. Instead of scattering light in all directions, most then ends up going straight towards your eye. (The way it works is very much the same as how projection screens are made to reflect more light towards the viewing audience, and how 'cat's eye' road markings work)
    And that's the only way to make a screen brighter: making sure most of the light hitting it goes in a direction where it will be seen. The coarseness (or fineness) of a scattering surface would make no difference.
    There is a problem though with the optical tricks (and those include using a Fresnel lens): because being directional, they have a focus, and assume your eye will be in a certain place. And not only that: they also assume that the exit pupil of the lens is in a certain place (that's why some manufacturers supplied different focussing screens for use with long lenses).
    Furthermore, as with anything that has a focus, there is a depth of focus. The result is that there is not just one plane but a range, stretching from a small distance behind to a small distance in front of the plane in which the true image lies, in which the image can be seen to be sharp. So making a screen brighter the only way you can (i.e. by bending light towards one point: your eye) also makes it a bit more difficult to focus using such a screen.
    The thing to do is find a compromise, make a screen that combines even illumination (the thing the Fresnel lens provides) with a fine grain structure making fine focus possible (both dependent on Fresnel lens and screen), a way to stop light scattering in 'unproductive' directions thus making the image appear as bright as possible, and still a 'sufficiently definite' plane of focus.
    (The first generation Acute Matte screens Hasselblad used, for instance, had too little of the last, so the micro-optical structures were changed to scatter light a bit more again. A little less bright, but a bit easier to focus.)
    So i don't think your "the coarser the GG, the brighter" is correct.
     
  5. Thanks, Q.G. I`m now thinking that I`m probably into this assumption because time ago, I used to use some screens from Beattie; these were one-piece plastic fresnel+groundglass. I remember to have this ones installed on a 4x5" and on the F3 as well.
    I try to remember that coarser grainier surface, with extremely bright grains on the LF camera. Probably, is that brightness (from the fresnel) what gets me wrong about the grain size issue. On the F3, I try to remember that it was not as noticeable, perhaps a slightly bit brighter than the standard red point? screen. Anyway, I must have stopped using them after a short time.
    I now also recall that I had to switch to lower magnification loupes for easier focusing.
     
  6. In a true ground glass, the coarser the grit, the brighter the screen, and the harder to see and focus on fine detail. The finest screens in "the day" were etched with HF vapor, rather than sandblasted (or molded), making their diffusion nearly complete.
    For whatever reason Graphlex put the Fresnel on the lens side of the screen, where it affects the focusing distance. Acute-Matte screens are the same, but presumably with compensation for the effect on focusing. My Sinar has a Fresnel on the viewing side, and can be easily removed (not so easily stowed) for critical focusing.
     
  7. Very interesting. Certainly I never know what to do with the fresnel while focusing on the Sinar. Thanks God it`s made in plastic.
    Coming back to Thomas question, Rolleicords could be much older cameras than Yashicas, and I bet there are different categories.
    I`m not fond of TLRs but I suspect older/cheaper models could not have fresnels factory installed.
    Also, it could be that Yashicas come with a fresnel already installed, and with Rollei it`s an accessory.
    Shorter lenses needs fresnels for better viewing, longer lenses could be used without them. Depending on the lens availability, the manufacturer could offer different solutions. As far as I know, Yashicas are fixed lens cameras, then it`s a closed camera system. If on Rolleis lenses are interchangeable, the user could choose one or another configuration.
    Whatever the camera you choose, you`ll probably find a fresnel for it. There are several third party fresnel manufacturers that cut fresnels for different cameras.
     
  8. Putting the Fresnel lens on the side of the lens isn't really creating a big problem. And being on the other side instead of between you and the screen itself, it does make inspecting the screen image easier.
    Rolleiflex TLRs are fixed lens cameras too. Early Rolleis probably have one as a separate accessory, simply because they didn't think of building one into the viewing system when they first made the thingies. It came, as it were, as an afterthought.
    You may be able to find Fresnel lenses to fit view cameras, but it will be difficult, if possible at all, to find ones for SLRs and TLRs. In large part, because their viewing systems/focussing screens already are equiped with Fresnel lenses. ;-)
    I'm not sure i agree, Edward, that coarser screens are brighter. In fact, i don't . ;-)
     
  9. My 1947 Speed Graphic has a Fresnel screen and Ground Glass on a Graflok 4x5 back; came new that way. On this camera the fresnel is towards the front; the ground glass at the back. Kodak Ektalite fresnels rings fact the subject; the Ground glass faces the subject. The grakflok is machined to have the GG in the correct place. If some Goober guy gets them in the wrong place; you get a focus error. An unknown ebay 4x5 or TLR can have this issue; Goober or Gomer assumes wrong and rebuilds wrong
     
  10. Anything between the GG and the lens affects the focus. Just by refraction, the effective distance is increased by about 1/3rd of the thickness, and the Fresnel refraction adds to that. When focusing a view camera, hundredths (perhaps thousandths) of an inch matter at the focal plane.
    I suspect the GG on Speed Graphics was either window dressing, or used only at tiny apertures. Most of the time, the photographer would use a rangefinder for focusing and an optical or sports finder on top of the camera.
     
  11. Edward;
    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.
    One too has the issue that some bent or weak springs has ones focus panel not completely seating too
    On Ebay Speed Graphics mine are really two piles; perfect GG/Fresnel setups; OR ones with a focus error ; due to the Kilroy factor ( GG and Fresnel in backward; or only GG in a GG/Fresnel type focus panel)
    The Speed here has been used for many decades for astro stuff; where the focus was done with the GG/Fresnel combo; with lenses wide open; often big fast ones. On a properly non messed with speed the back panel is very accurate. One does have a slight focus shift on some lenses if one stops down ; one with the scan backs can do a minor tweak if wanted.
     
  12. No Fresnel Focus errors.
    These bags were shot for some Attorney Work with the 50 megapixel scan back on the 4x5 Speed Graphic; where the focus is by the GG/Fresnel .
    Then one slips in the scan back under the Graflok; then does a prescan; then a scan. With only one 500 watt lamp the lens was not stopped down much; ie about F8 on a 127mm Ektar. On a full scan one can see the fibers in the bags paper.
    Many of these bigger bags had boilerplate text that recorded poorly with a high end dslr. The bags were to thick to go through our 36" RGB scanner. They are court evidence. The big scan back records the fine type.
    A shot wide open at F4.7 is sharp at the center; the bag is only about 6 ft from the camera/scanner; thus here one has little if any Frensel issues.
    It is the same camera I have used for 30+ years.
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