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Thread: Lens Not Focusing, What is the Cause?

  1. #11

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    Re: Lens Not Focusing, What is the Cause?

    Quote Originally Posted by domaz View Post
    Take the lens off the camera, open the shutter. Position it so it's pointing at an open window with a wall behind it. You should physically see the image the lens projects. If it doesn't focus on the distant objects out the window at the expected focal length as measured from location of the aperture something is wrong with the lens.

    You can also focus "aerially" by putting a loupe up to your eye and positioning the lens in front of you. You can actually see the aerial image of the lens in the loupe if you do it right. It's quite tricky to do but is a good way to check coverage.
    Close.
    Darken a room, point the lens out the window with the aperture at 5.6 and the shutter open. While pointing it out at a distant object hold a mount board or a shirt cardboard behind the lens and move the board towards or away from the lens till you see a sharp image. That is the extension needed to use the lens on your camera at infinity.
    The measurement will be appx. Since you would not be using a loupe to focus the image sharply, but it will be close.

  2. #12

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    Re: Lens Not Focusing, What is the Cause?

    Quote Originally Posted by Bob Salomon View Post
    Close.
    Darken a room, point the lens out the window with the aperture at 5.6 and the shutter open. While pointing it out at a distant object hold a mount board or a shirt cardboard behind the lens and move the board towards or away from the lens till you see a sharp image. That is the extension needed to use the lens on your camera at infinity.
    The measurement will be appx. Since you would not be using a loupe to focus the image sharply, but it will be close.
    Thanks Bob and Domaz. I did this and achieved a sharp image at about 130mm distance. Next I remounted the lens on a Horseman rail camera and was able to focus on a bunch of flowers. It may be that the Gandolfi didn't have enough compression on the bellows for the distance I was at which was only about 1m. When I tried to use it on the Gandolfi it was for the first time and I moved the lens close and far, I should have been able to get a sharp image but for some reason I wasn't, I'll blame me and the bellows at this stage.

  3. #13
    Corran's Avatar
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    Re: Lens Not Focusing, What is the Cause?

    Just to be sure, I will mention that to focus closer, you must extend the bellows out. If 130mm extension gives you infinity focus, 260mm would give you a "1:1" magnification (life-size). Your 1-meter focus distance will be somewhere in between. There's a formula but it doesn't really matter at the moment. Set up your camera/lens again and start racking the focus out until you achieve focus.
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  4. #14

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    Re: Lens Not Focusing, What is the Cause?

    I'm with Corran on that. By definition, a 120mm lens focal length lens focused at infinity is 120mm. Focusing on anything closer than that requires you to increase not decrease the bellows so something doesn't sound right. Here's some formulae you might want to look at on Kenneth Lee's web page.

    http://www.kennethleegallery.com/html/tech/bellows.php

    Using fl = 120mm and 1000mm focusing distance would result in a bellows draw of around 135mm if I'm doing the calculations right.

    Hope this helps
    Dave

    (I'm assuming we're talking 4x5 cameras here. Getting full coverage on larger cameras with a 120mm lens might be an issue on some cameras)

  5. #15

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    Re: Lens Not Focusing, What is the Cause?

    Quote Originally Posted by DHodson View Post
    I'm with Corran on that. By definition, a 120mm lens focal length lens focused at infinity is 120mm. Focusing on anything closer than that requires you to increase not decrease the bellows so something doesn't sound right. Here's some formulae you might want to look at on Kenneth Lee's web page.

    http://www.kennethleegallery.com/html/tech/bellows.php

    Using fl = 120mm and 1000mm focusing distance would result in a bellows draw of around 135mm if I'm doing the calculations right.

    Hope this helps
    Dave

    (I'm assuming we're talking 4x5 cameras here. Getting full coverage on larger cameras with a 120mm lens might be an issue on some cameras)
    Depends on the flange focal length which isnít necessarily the same as the focal length.

  6. #16

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    Re: Lens Not Focusing, What is the Cause?

    Or set your extension to approximately 250mm (plus minus a bit doesn't matter - do it by eye) and then move the subject toward and away from the camera and see if you get it in focus. This is just a rough test after all.

  7. #17

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    Re: Lens Not Focusing, What is the Cause?

    Quote Originally Posted by Corran View Post
    Just to be sure, I will mention that to focus closer, you must extend the bellows out. If 130mm extension gives you infinity focus, 260mm would give you a "1:1" magnification (life-size). Your 1-meter focus distance will be somewhere in between. There's a formula but it doesn't really matter at the moment. Set up your camera/lens again and start racking the focus out until you achieve focus.
    This did cross my mind as well but it was late and I ran out of time when I did the simple test. At 1m I was not getting the entire flower arrangement on the screen so I'll do more comprehensive testing in the next couple of days, mostly further from the subject until I have the entire set up on the screen and then see if I can focus normally.

    When I did a quick comparison with the 135mm on the Gandolfi at the same distance they looked to be about what you would expect with the 120mm image slightly smaller which I took as a good sign, but I still have the nagging doubt about not being able to focus when I originally tried.

  8. #18

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    Re: Lens Not Focusing, What is the Cause?

    Quote Originally Posted by DHodson View Post
    Here's some formulae you might want to look at on Kenneth Lee's web page.

    http://www.kennethleegallery.com/html/tech/bellows.php

    Using fl = 120mm and 1000mm focusing distance would result in a bellows draw of around 135mm if I'm doing the calculations right.

    Hope this helps
    Dave

    (I'm assuming we're talking 4x5 cameras here. Getting full coverage on larger cameras with a 120mm lens might be an issue on some cameras)
    Quote Originally Posted by Bob Salomon View Post
    Depends on the flange focal length which isn’t necessarily the same as the focal length.
    Yes it is on a 4x5.

    The 1m test was just a test to see if I could get a clear image. In reality I will have the camera further back so the entire subject is on the screen, I will test this in the next couple of day.

  9. #19

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    Re: Lens Not Focusing, What is the Cause?

    Quote Originally Posted by DHodson View Post
    Here's some formulae you might want to look at on Kenneth Lee's web page.

    http://www.kennethleegallery.com/html/tech/bellows.php

    Using fl = 120mm and 1000mm focusing distance would result in a bellows draw of around 135mm if I'm doing the calculations right.

    Hope this helps
    Dave

    (I'm assuming we're talking 4x5 cameras here. Getting full coverage on larger cameras with a 120mm lens might be an issue on some cameras)
    Not quite right. There are two solutions, not one, unless magnification is 1:1. Otherwise there are two magnifications for every film-to-subject distance.

    If film-to-subject distance is 1000mm and focal length is 120 mm, the possible magnifications and lens rear node-to-film distances are 0.1627:1 and 139 mm and 6.15:1 and 858 mm. These are extreme examples. Even so, they're the reason why the best way to focus close up is to set extension to give the desired magnification and then move camera + lens or subject to focus. If necessary once the final setup is more-or-less reached focus can be fine tuned by adjusting extension.

    Dave, image circle grows with magnification. Extension required, measured in focal lengths, grows with magnification. This is why in general the higher the magnification desired the shorter the lens used.

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