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Thread: Focusing technique for very wide angle lenses

  1. #11

    Focusing technique for very wide angle lenses

    Re: Rob Tucher

    "Then with a lupe I focus on close and then far light pinpoints, getting a feel for where each point is on the rack-and-pinion, and then settle at the one-third/two-thirds point."

    The Rodenstock calculator suggests that you pick the point along the focus track which is exactly half way between the near and far focus points. I've always assumed that this point actually represents the 1/3 in front - 2/3 behind point. When I feel the need to be extremely accurate, I use a slide caliper to measure the relative displacement of the rear standard along the track. Is this half-way point method indeed correct?

    The flashlight trick sounds very promising.

  2. #12

    Join Date
    Mar 1998
    Posts
    1,973

    Focusing technique for very wide angle lenses

    Through trial and mostly error I have also found that the "halfway in between point on the focus track" above mentioned method works best.

  3. #13

    Join Date
    May 1998
    Posts
    218

    Focusing technique for very wide angle lenses

    Larry: yes, exactly halfway is the usual technique for getting equal unsharpness for the two points. To be precise, it isn't exactly halfway (you would have to look at a diagram to see why), but it is close enough.

    The one-third/two-thirds rule-of-thumb is for subject distances, and only applies to 'normal' distances, i.e. neither infinity nor macro. The half-way rule applies to the lens-to-film distance, and is the reason why DoF markings on lenses are symmetrical.

  4. #14

    Focusing technique for very wide angle lenses

    Alan:

    re: "Larry: yes, exactly halfway is the usual technique for getting equal unsharpness for the two points."

    In fact I just completed an emperical experiment on this issue. My experimental setup is as follows: Shot Type 55 (4x5) with a 210mm Rodenstock Sironar-S. Test target was an 8.5 x 11 sheet of paper with repeating text (Times-Roman 12pt) from top to bottom (46 lines). Target was positioned on a stand with an angle of 39 degrees. The furthest ways lines (lines 1, 2, 3 etc.) were at the top, and the closest lines were at the bottom (lines 44, 45, 46). Reproduction ratio was about 1:1.5. All exposures taken with strobe at 1/125 sec. Camera leveled, and pointing squarely at the target (to a good approximation). Type 55 negs examined with a 10x hastings loupe. All focusing done with rear standard. Camera was a Canham DLC45.

    Since I seem to be having endless critical focus issues, I first focused on line 24 and shot it at f=5.6. I wanted to see if there is some problem with my 545i holder. Test negative was right on the money. Only line 24 was sharp.

    Next experiment was to use the two point method mentioned in previous posts. First I focused on line 13, and then line 27, measured the rail displacement (14.7 mm) with a caliper, and set the rear standard half-way. Incidentally, line 20 was in focus without the lens stopped down at this rail position. I then consulted my handy Rodenstock calculator, and determined that f=45 would be required (even though the experimental setup is a perfect situation to use front tilt, I didn't tilt because I wanted to limit the experiment to a smaller set of variables). The negative from this setup was interesting. Under the 10x loupe, I wouldn't rate either lines 13 or 27 as being in good focus. The range of good focus was closer to lines 16-17 at the far end, and lines 24-25 on the near end. Obviously, for my taste the Rodenstock calculator must assume a C of C larger than I'd find acceptable. To Allan's point, you could easily rate line 20 as being the sharpest, and everything on either side was to some degree a compromise in sharpness. I knew this was the theory, but was suprised that I could actually observe this fact. Doing some simple trig it also appeared that the zone of sharp focus was pretty much symmetrical around the center point (not following the 1/3 - 2/3 rule).

    As an aside, I put a scrap Fuji Quickload in the 545i and pulled the darkslide. It appears that the 545i keeps the film flat everywhere except on edge near the rollers. The design of the 545i (and 545) doesn't appear great in that respect. I haven't used the Fuji mechanism, so don't know if this issue is only when using the 545i.

    Now, if I could only solve my overall focusing problem. When using the two point method and the Rodenstock calculator, more often than not, my focus is excellent in the foreground and slightly out of focus in the background. Typically, the range of focus I'm trying to achieve is fairly large (requiring f=22 to f=45). I've checked the position of my Boss screen and film holders, and everything seems OK.

  5. #15

    Focusing technique for very wide angle lenses

    To those who read this far back. I use a 80mm enlarging lens as a loupe to check my focus on ground glass. Theyn are inexpensive ($15) and short. They can be held at an angle to check for focus at the edges of the ground glass and still be sharp, unlike the expensive schnieders and toyos. Try one and you'll dump your expensive glass.

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