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  1. #21
    PeterB's Avatar
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    Quote Originally Posted by Ryuji View Post
    Depending on the screen, the sharpest point doesn't look that sharp. Mine is C3 with old fashioned ground glass, which is easy to reach the optimum focus point, but the image is rather dark and coarse.
    I totally agree Ryujii. However even though the image on my viewing screen isn't razor sharp, it is very easy to determine the point that it is the sharpest it can be (as one turns the focus knob). That point by definition is when the object is in focus.

    My problem isn't that the image in the viewing screen (via the viewing lens) isn't sufficiently sharp. My problem is when that image is as sharp as it gets, the corresponding image in the film plane (via the taking lens) isn't as sharp as it can get. I have to move the main lens assembly back and forth by about 0.6mm (focus knob turns about 2mm circumferentially) as I switch between looking at the film plane and the viewing screen to ensure each is at its sharpest.
    I'm impressed that TLR camera manufacturers were able to manufacture their cameras to sufficiently high precision to ensure appropriate lens and image alignments in the first place.

  2. #22

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    PeterB, when your viewfinder is in focus, does the focus of taking lens come near or far from the point in focus in the viewfinder?

  3. #23
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    Quote Originally Posted by Ryuji View Post
    PeterB, when your viewfinder is in focus, does the focus of taking lens come near or far from the point in focus in the viewfinder?
    Good question Ryujii as I'm sure the place to look for the problem will differ depending on the answer. I've switched back and forth between lens plane and viewfinder so many times that I can't remember which direction was positive or negative. I can find out in about 5 hours when I test it again. Let me know if there's anything else I should check for while I'm at it.

    Peter

  4. #24
    MattKing's Avatar
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    Seen as we are making suggestions about what you should check for ....

    How does the focus discrepancy manifest itself at different subject distances.
    Matt

    “Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”

    Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2

  5. #25
    PeterB's Avatar
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    Quote Originally Posted by MattKing View Post
    How does the focus discrepancy manifest itself at different subject distances.
    From about 1.5m to infinity it is the same (within limits of repeatbility) - about 0.6mm linear movement of the lens assembly (perpendicular to the film plane). I haven't tested sub 1.5m for a few days after making the changes to the viewfinder foam and swapping back the front halves of the viewing and taking lenses.

    Peter

  6. #26

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    To PeterB - I wasn't addressing your problem,but the Original posters.His image didn't look properly sharp when correctly focused. Are you seriously suggesting that a magnifying lens can be used for critical focusing even when not focused on the ground glass?
    That is equivalent to dialling out a viewfinder dioptre adjustment, or using a focusing loupe set wrongly.
    You can do it,but WHY?

  7. #27
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    Quote Originally Posted by Smudger View Post
    To PeterB - I wasn't addressing your problem,but the Original posters.His image didn't look properly sharp when correctly focused. Are you seriously suggesting that a magnifying lens can be used for critical focusing even when not focused on the ground glass?
    That is equivalent to dialling out a viewfinder dioptre adjustment, or using a focusing loupe set wrongly.
    You can do it,but WHY?
    Hi Smudger, thanks for clarifying your response, apologies for suggesting your premise was false as you weren't answering my question but the OP's.

    I'm certainly not suggesting the magnifying lens remains out of focus at all ! And if it is out of focus, by all means fix it. In my case I have a bigger problem to fix first (the discrepancy btwn WLF focus and film plane focus). My magnifying lens may or may not be optimised but to my eyes it is good enough for now while I solve the main problem.

    My measurement results are becoming more and more confusing. Last night I repeated the testing inside to discover that focussing on objects at 3m and 15m did not give the discrepancy (but focussing at objects 60cm away did). Whereas a couple of days earlier when testing outside (on different objects), the discrepancy existed at 3m and infinity (I didn't test 60cm outside). In all cases when the discrepancy exists it is approx. the same magnitude and direction.

    Something must be changing between my measurements but I can't work out what. Each time I do a set of measurements I repeat the focus at least 3-4 times with near identical results to ensure it isn't just a simple repeatability issue (e.g. caused by the limits of the system or my ability).

    I'm attaching some pics to show
    1. The tracing paper screen I set up behind a perspex/plexiglass pressure plate (held in place with blu-tack). Paper and plate were all correctly dimensioned so they rest on the appropriate set of silver rails.
      Click image for larger version. 

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    2. Two pen marks on the focus knob showing an example of the two focal positions equating to a linear displacement of about 0.6mm. The mark at 11:58 o'clock is when the WLF screen is in focus, the one at 12:00 o'clock is when the film plane is in focus.
      Click image for larger version. 

Name:	IMAG0861_focus_knob_position_deviation_between_Mamiya_C330f_screen_and_film_plane.jpg 
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    thanks
    Peter

  8. #28
    clayne's Avatar
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    Stupid question, but in all cases the focus cases under testing were all for straight-on subjects directly in the horizontal plane, right? I.e. no funky close-ups with aggressive angles, or things of that nature? What's the actual test subject?
    Stop worrying about grain, resolution, sharpness, and everything else that doesn't have a damn thing to do with substance.

    http://www.flickr.com/kediwah

  9. #29
    PeterB's Avatar
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    Quote Originally Posted by clayne View Post
    Stupid question, but in all cases the focus cases under testing were all for straight-on subjects directly in the horizontal plane, right? I.e. no funky close-ups with aggressive angles, or things of that nature? What's the actual test subject?
    The test subjects vary and are basically whatever happens to be in my line of sight with a sharp line on them (if up close) like a window sill, the edge of a desk lamp, the brake cable of a bicycle, the characters/text of a system menu (i.e. sharp text) on the screen of Television set, a tree off in the distance. In all cases I focus on exactly the same point on the object when switching my view between between the WLF and the film plane (using a loupe).

  10. #30
    clayne's Avatar
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    However, in those cases, you don't move/shift the body to have the taking lens be directly where the viewing lens once, was right? This will affect things if a. subject is up-close, and b. angle is not straight on, or c. subject is irregular in form.


    |_\ .... <-- c330
    |=|: ------------------------------ *
    |=|: ----------------------------- ** <--- xmas tree / pile of cannon balls
    .-- _____________________ ****______________________________

    I'm throwing out this example above ^^.

    However, seeing as you're already there with a loupe and are familiar with the focus of the film plane, this might be entirely too elementary and already discarded by you as a possibility. :-)
    Stop worrying about grain, resolution, sharpness, and everything else that doesn't have a damn thing to do with substance.

    http://www.flickr.com/kediwah

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