How to "Focus" at Infinity

Discussion in 'Large Format Cameras and Accessories' started by Gary892, Jan 14, 2009.

  1. Gary892

    Gary892 Member

    Messages:
    250
    Joined:
    Jun 14, 2004
    Location:
    North Orange
    Shooter:
    4x5 Format
    I am embarrassed to bring this up, but, how does one set a 4x5 or any other large format camera to infinity.
    I have been photographing at least 30 years with a 4x5 and larger cameras, usually in the studio but often out in the field.

    I have never given much thought to setting the camera lens to infinity as I have just used swings, tilts, and of course focusing to get the image I wanted or needed. But while I was out and about making photographs a while back, a stranger asked me how to set the focus of a lens to infinity.

    I thought about it a minute and replied, the focal length of the lens determines the amount of bellows draw and when at max bellows draw for a specific lens then that leans is at infinity. So, if one has a 210mm lens the bellows draw would be 8 1/4 inches for infinity.

    The stranger left with their newly gained knowledge and I felt good about helping someone.

    Then I started to think about my answer. I may have given out misinformation, so I searched the internet using Google but could not find a succinct answer.

    So, the folks at APUG now have an opportunity to teach an old dog some new tricks.

    So I humbly ask the question; For a 155mm lens on a 4x5 camera, how does one set it to infinity, or any size lens for that matter.

    I occurs to me that I may have it backwards(dyslexia).

    Thanks
    The Old Dog
    Gary
     
  2. Nick Zentena

    Nick Zentena Member

    Messages:
    4,679
    Joined:
    Nov 21, 2004
    Location:
    Italia
    Shooter:
    Multi Format
    It's not really the focal length it's the flange focal distance. It varies with lens design. Telephotos will need less then the focal length. Some wide angles need more.
     
  3. Frank Szabo

    Frank Szabo Member

    Messages:
    312
    Joined:
    Sep 2, 2007
    Location:
    Broken Arrow
    Shooter:
    8x10 Format
    Just to set at infinity focus requires a bit of prior work.

    Make two marks or locate two points on your camera's standards or whatever you can find on the cam to measure reliably.

    Looking through the groundglass with the camera aiming at something quite some distance off (a mile or so), focus the 155mm or whatever lens you're interested in setting at infinity - when you're pleased with what you did, measure the distance between those aforementioned points and write that number down - don't lose it.

    With the tape measurer or whatever you used to determine that distance, set up your camera and extend the standards - measure off that distance you should have written down.

    VIOLA! Infinity focus. The actual number doesn't matter - just return to the measurement you determined.

    ... or did I misunderstand what you wanted?
     
  4. pelerin

    pelerin Member

    Messages:
    343
    Joined:
    Sep 11, 2004
    Shooter:
    Multi Format
    Hi,
    Yes, bellows draw increases as the point of focus approaches the lens. (easy to test y/self w/ a yard stick) As to a the exact amount of bellows draw being = to the nominal focal length of the lens this is not likely to be true in practice. Your len's actual focal length is likely to differ from the nominal and most probably the nodal point sits in front of where the bellows terminates. Cameras with infinity stops (the little flip-up doo-hickeys on the bed of a Linhoff etc.) have the placed by practical trial. The repairman uses a collimator to provide a reference for infinity and places the stops where appropriate for the lens. I think the short answer here is that your method is working so keep on keeping on... however, ( http://www.trenholm.org/hmmerk/download.html ) if you want to delve into this so you can answer random questions w/ enough detail to make people's eyes go glassy.
    Celac
     
  5. Steve Smith

    Steve Smith Subscriber

    Messages:
    9,083
    Joined:
    May 3, 2006
    Location:
    Ryde, Isle o
    Shooter:
    Medium Format
    What's an oversized violin got to do with it?

    I think you meant VOILA!



    Steve.
     
  6. Pinholemaster

    Pinholemaster Member

    Messages:
    1,504
    Joined:
    Oct 17, 2005
    Location:
    Westminster,
    Shooter:
    8x10 Format
    "when at max bellows draw for a specific lens then that leans (lens) is at infinity"

    Isn't is 'minimum' bellows draw? An 8 1/4" lens is at infinity at 8 1/4". Maximum bellows draw is for macro work.

    Anyway, one measure bellows draw from the nodal point of the lens. Frankly, I don't even worry about it. Put the lens on the camera, and move the back standard to the point where infinity is in focus.
     
  7. John Koehrer

    John Koehrer Subscriber

    Messages:
    6,378
    Joined:
    Apr 3, 2004
    Location:
    Montgomery,
    Shooter:
    Multi Format
    So the OP is correct. At infinity the film plane to nodal point is 8 1/4" approximately. It's not going to be precise but is a good answer.
    Consider it a rule of thumb.
     
  8. Ralph Javins

    Ralph Javins Member

    Messages:
    832
    Joined:
    Jul 4, 2008
    Location:
    Latte Land,
    Shooter:
    Multi Format
    Good morning;

    I agree with PinHoleMaster; the minimum bellows draw should be small enough or short enough to allow the lens to come back close enough to the ground glasss to allow the image of the object/subject a very long distance away ("infinity") to be "in sharp focus" on the ground glass.

    The bellows extension out along the rail would allow the lens to move out away from the ground glass to allow an object/subject very close to the camera to be "in sharp focus." This is what we do when we are trying for a large image on the film or "close focusing" or "macro" imaging with our camera. As I recall, for a lens to be called a "macro lens" it had to be able to bring the image on the film plane up to a 1:1 size ratio. If it did not do that, it was only a "close focusing lens." I think "micro" lens labeling did not begin until around a 10:1 or 10X size ratio. The optics section in physics class was a long time ago.
     
  9. Frank Szabo

    Frank Szabo Member

    Messages:
    312
    Joined:
    Sep 2, 2007
    Location:
    Broken Arrow
    Shooter:
    8x10 Format
    Picky, picky, picky. How much fly poo did you get out of the pepper? <grin>

    You're right - tried to type too fast.

    They are a bit easier for me to play, though, with the arthritis and all. My fiddle is giving me problems anymore - painful to play.
     
    Last edited by a moderator: Jan 14, 2009
  10. CPorter

    CPorter Member

    Messages:
    1,662
    Joined:
    Feb 2, 2004
    Location:
    West KY
    Shooter:
    4x5 Format
     
  11. Q.G.

    Q.G. Inactive

    Messages:
    5,682
    Joined:
    Jul 23, 2007
    Location:
    Netherlands
    Shooter:
    Medium Format
    The "aperture plane"? You mean the one the exit pupil is in? Or the one the entrance pupil is in? Or where the physical diaphragm is?
    None of the three is the correct thing to measure from. :wink:

    You need to measure from the rear principal plane. Where that is located (and where the entrance and exit pupils are) depends on the lens design. When data are provided by the manufacturer, they usually state the position relative to other bits of the lens, like the last lens vertex. So you need to do some calculations (involving the exact focal length, not just the nominal focal length) to find a number you can use.

    But that's all too technical. Not practical.
    Cameras have a groundglass screen. Use your eyes.
    Reduce the lens to film distance/bellows draw, until everything starts getting out of focus. Go back a bit, "et voila"! Infinity focus! :wink:
     
  12. Steve Smith

    Steve Smith Subscriber

    Messages:
    9,083
    Joined:
    May 3, 2006
    Location:
    Ryde, Isle o
    Shooter:
    Medium Format
     
  13. CPorter

    CPorter Member

    Messages:
    1,662
    Joined:
    Feb 2, 2004
    Location:
    West KY
    Shooter:
    4x5 Format
    Well, I'm not going to pretend to know the ends and outs of lens construction and lingo--it's more than I care to think about. I can only tell you that I measure from where the aperture is and it works beautifully.
     
  14. CPorter

    CPorter Member

    Messages:
    1,662
    Joined:
    Feb 2, 2004
    Location:
    West KY
    Shooter:
    4x5 Format
     
  15. EASmithV

    EASmithV Member

    Messages:
    1,925
    Joined:
    Aug 22, 2008
    Location:
    Maryland
    Shooter:
    Multi Format
    When I wanted to find out where my lens was when focused at infinity, I took it outside at night and focused on some stars...
     
  16. CPorter

    CPorter Member

    Messages:
    1,662
    Joined:
    Feb 2, 2004
    Location:
    West KY
    Shooter:
    4x5 Format
    I simply found the distance from lens to film plane by measuring with a ruler just once, then noted the distance in mm from inside to inside of the front and back standards. Anytime I want infinity, I simply separate the standards by 126mm inside to inside on the monorail------very quick.