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Thread: Fresnel lenses and ground glass focusing screens your worries answered

  1. #1

    Fresnel lenses and ground glass focusing screens your worries answered

    There has been a lot of chat and controversy about the use of fresnel lenses in combination with the ground glass on large format cameras in attempt to brighten the image formed on the screen thus assisting focusing and composition.
    I decided to do a series of experiments ion a 4x5 camera to find out what all the fuss was about.
    My findings are as follows:
    THE FRESNEL LENS
    Most fresnel lenses are plastic and can be bought on ebay. These are designed as reading aids and have a focal length between 5 and 10cm. They can easily be cut to size using scissors or a craft knife.
    PLACING THE FRESNEL LENS
    There has been much speculation on this topic. After much experimentation trying all four possible posiitions,
    1) Camera lens side. Grooves facing back of camera lens smooth side touching screen.
    2) Camera lens side. Grooves touching screen smooth side facing camera lens.
    3) and 4) Same as above but fresnel lens put on viewer side of ground glass screen.
    Because of the machining process in the manufacture of fresnel lenses the lens will not lie completely flat to the ground glass screen and may be found to be bulging slightly in the middle or sides.
    A further series of experiments with the fresnel lens being deliberately distorted and moved up to half a mm from the screen were done to find the effects of this on image an focusing.
    THE RESULTS
    The image using the fresnel lens was noticably brighter in ALL positions.
    The accurate focus of the camera was NOT affected by any of the fresnel lens positions.
    The bulging of the fresnel lens, up to half a mm, had no effect on focusing or image.
    DOWNSIDES
    Some fresnel lens grooves were noticable, under loupe magnification especially on lenses over 180mm focal length.



    Sent from my SM-T210 using Tapatalk

  2. #2

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    Re: Fresnel lenses and ground glass focusing screens your worries answered

    There are much better quality fresnel lenses to be had, than those found on eBay such as what you have used. Some have much finer grooves, like the ones Used on Graflex cameras. There's also integrated fresnels with screens, like the ones made by Maxwell and used by canham. Edmund optics also sells fresnels in various sizes and different focal lengths as well as different groove pitches, which might be worth exploring.

  3. #3
    Randy's Avatar
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    Re: Fresnel lenses and ground glass focusing screens your worries answered

    Quote Originally Posted by john.l.barford View Post
    The accurate focus of the camera was NOT affected by any of the fresnel lens positions.
    This confuses me. The ground-glass (frosted side) has to be located exactly at the film plane in order to achieve exact focus, especially when shooting at wide apertures and close distances. If placing a plastic fresnel lens of 1-2mm thickness under the ground-glass moves the ground-glass 1-2mm from the film plane (because now the fresnel is at the film plane and the ground-glass is moved farther away from the film plane by the thickness if the fresnel), will that not affect the accuracy of the focus? I don't see how it can't. What am I missing?
    https://dl.dropboxusercontent.com/u/52893762/bigger4b.jpg

  4. #4

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    Re: Fresnel lenses and ground glass focusing screens your worries answered

    Quote Originally Posted by Randy View Post
    This confuses me. The ground-glass (frosted side) has to be located exactly at the film plane in order to achieve exact focus, especially when shooting at wide apertures and close distances. If placing a plastic fresnel lens of 1-2mm thickness under the ground-glass moves the ground-glass 1-2mm from the film plane (because now the fresnel is at the film plane and the ground-glass is moved farther away from the film plane by the thickness if the fresnel), will that not affect the accuracy of the focus? I don't see how it can't. What am I missing?
    I was pondering the same thing. The first time I replaced a ground glass/fresnel combo, I got the two bass-ackwards and had to get several out of focus 8x10 negatives before I figured out my stupidity. Switched them around and all was well.

    Well, except for composition, exposure, subject matter and other things that went wrong. But those had nothing to do with the ground glass.
    Michael W. Graves
    Michael's Pub

    If it ain't broke....don't fix it!

  5. #5

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    Re: Fresnel lenses and ground glass focusing screens your worries answered

    Quote Originally Posted by john.l.barford View Post
    There has been a lot of chat and controversy about the use of fresnel lenses in combination with the ground glass on large format cameras in attempt to brighten the image formed on the screen thus assisting focusing and composition.
    I decided to do a series of experiments ion a 4x5 camera to find out what all the fuss was about.
    My findings are as follows:
    THE FRESNEL LENS
    Most fresnel lenses are plastic and can be bought on ebay. These are designed as reading aids and have a focal length between 5 and 10cm. They can easily be cut to size using scissors or a craft knife.
    PLACING THE FRESNEL LENS
    There has been much speculation on this topic. After much experimentation trying all four possible posiitions,
    1) Camera lens side. Grooves facing back of camera lens smooth side touching screen.
    2) Camera lens side. Grooves touching screen smooth side facing camera lens.
    3) and 4) Same as above but fresnel lens put on viewer side of ground glass screen.
    Because of the machining process in the manufacture of fresnel lenses the lens will not lie completely flat to the ground glass screen and may be found to be bulging slightly in the middle or sides.
    A further series of experiments with the fresnel lens being deliberately distorted and moved up to half a mm from the screen were done to find the effects of this on image an focusing.
    THE RESULTS
    The image using the fresnel lens was noticably brighter in ALL positions.
    The accurate focus of the camera was NOT affected by any of the fresnel lens positions.
    The bulging of the fresnel lens, up to half a mm, had no effect on focusing or image.
    DOWNSIDES
    Some fresnel lens grooves were noticable, under loupe magnification especially on lenses over 180mm focal length.



    Sent from my SM-T210 using Tapatalk
    Congratulations! You wasted all that time and energy, not to mention expense, doing an absolutely worthless test since you were not testing large format Fresnel lenses.
    Try again but this time buy the proper type of Fresnel for your camera.

  6. #6

    Re: Fresnel lenses and ground glass focusing screens your worries answered

    I absolutely understand that the frosted side of the ground glass screen must be exactly at the film plane. If you place the fresnel lens flat onto this frosted side of the ground glass screen without moving the screen from its position.

    Sent from my SM-T210 using Tapatalk

  7. #7

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    Re: Fresnel lenses and ground glass focusing screens your worries answered

    Quote Originally Posted by john.l.barford View Post
    I absolutely understand that the frosted side of the ground glass screen must be exactly at the film plane. If you place the fresnel lens flat onto this frosted side of the ground glass screen without moving the screen from its position.

    Sent from my SM-T210 using Tapatalk
    Doing this shifts the plane of best focus by ~ 1/3 the fresnel's thickness. Which raises a question. How did you determine that putting a fresnel in front of the GG didn't shift the plane of best focus?

  8. #8

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    Re: Fresnel lenses and ground glass focusing screens your worries answered

    Quote Originally Posted by Bob Salomon View Post
    Congratulations! You wasted all that time and energy, not to mention expense, doing an absolutely worthless test since you were not testing large format Fresnel lenses.
    Try again but this time buy the proper type of Fresnel for your camera.
    Is there some advantage to sarcastic mockery in forum conversations that I'm simply unaware of?

  9. #9

    Re: Fresnel lenses and ground glass focusing screens your worries answered

    I hope this helps to clear up some confusion

    The total cost of this project was about $5 and two hours work. As an added bonus it annoyed the wife

    Sent from my SM-T210 using Tapatalk

  10. #10

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    Re: Fresnel lenses and ground glass focusing screens your worries answered

    Quote Originally Posted by MAubrey View Post
    Is there some advantage to sarcastic mockery in forum conversations that I'm simply unaware of?
    Not really sarcastic. He simply did not do a valid test and presented his findings as if they are valid. He could have easily checked before wasting the .time, cost and effort before the test to verify what Fresnel was necessary.

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