How to Mod Mamiya 65mm f/6.3 lens to open to f/4

Discussion in 'Large Format' started by justin_w, Oct 15, 2021.

  1. I recently bought a Mamiya 65mm f/6.3 lens. I've seen a few brief messages online that said it can be "easily" modded to open up to f/4, but I don't know how to do this.

    I just got it back from repair at Nippon camera in NYC, and I asked the technician if he could do this mod, and he shook his head — "Don't do it." It could get stuck in the open position, he claimed. But sometimes those guys just don't want to rock the boat; I've seen it mentioned in several posts that you can indeed remove a plastic tab to get it to open to f/4. I realize the quality won't be great opened up, but that doesn't bother me — I need the extra speed.

    Anyone have any experience/tips doing this?

    Thanks.
     
  2. I presume you are inquiring about the 65mm "Press" lens.

    If you remove that plastic "tab" (it's really a "stop"), the aperture blades could indeed get stuck. The
    mechanical advantage of the movement of the working parts gets reduced to the point where all the moving
    parts are forcing against each other and end up not moving at all - they get stuck.

    That's the technical explanation ... a layman's analogy is your scissor car jack being wound fully to the top
    and then finding it hard to wind back down again, the mechanical advantage is not nearly as much as when
    you first started winding the jack up. The principle of mechanical advantage is the same regardless of devices.

    I would advise you to only plan on opening the blades for half the length of that stop. So if you remove the
    original stop, replace it with a stop that is opprox half the length. I've examined this before on a 65mm and I noticed
    that at f6.3, there would be still enough workable mechanical advantage to open further with safety, but at what
    point would anyone know where f4 is?, that is the question.

    I would strongly advise to forget f4 and just work on the range of "safety" of movement, in other words, don't set
    the new position of the blades too far beyond f6.3 ... one more tip - once the stop has been removed, avoid merrily
    moving the f stop pointer to it's far extent, because that's where the blades could possibly get stuck, do it in slow
    increments and observe the blades opening up further to a guesstimate of f5.6, that would be the safer option,
    which is not far away from f6.3. I would have not hesitation trying for that, rather than getting into the danger zone
    of guessing f4

    PS I have a 90mm Press lens with stuck aperture blades, and it hasn't got a stop that could be removed, the blades are
    stuck on the lens's native f3.5, it doesn't take much. Koni Omega 90mm lenses are bad for it as well, but, another story.
     
  3. Given the commercial advantages of selling an f/4 lens as opposed to an f/6.3 lens, I'd guess there would probably be compelling reasons for the maker to put in the stop.

    kmac has addressed some of these, and that is enough to convince me that it would be best to leave the lens as it is, or at most to follow kmac's limitations.
     
  4. I agree, leave the lens as it is. I don't think it's worth the effort when considering the explanation in this Photo.net post from 2004 .. read bold underlined text.

    After exposing a few films with my 65mm, I dropped any thoughts I had of removing the stop and continued enjoying the lens as originally intended. As you say, the manufacturer knew best, and it seems that if the stop is removed, the lens produces "weird double coma" at night at f4, not my cup of tea. I like the lens the way it is and use the tripod when necessary.

    2004 Photo.net post ....

    "Another interesting tidbit about the 65/6.3--it's really a 65/4!! There's a little limiter
    glued on inside so you can't take the aperture lever past 6.3. If you break it off (requires
    removing the shutter from the helical)
    , and slide the lever all the way over, voila--f4."

    "Now, it's not a very good lens at f4, and at night light sources away from the center have
    huge, weird double coma
    that looks like the ghost of a seagull (totally absent at f6.3), but
    if you're shooting available light on Delta 3200 it's an acceptable trade-off. Also very
    handy if you're trying to focus on the ground glass."

    I can't resist posting a demonstrating of the wide coverage of the 65mm Press lens. It's ideal for small rural towns such as this one. Specs: 6x7, about f11 at 60sec, handheld. A tripod, and utilizing the bellows tilting back on the camera, would have produced a better image quality wise, but the coverage is about 3/8th more than a 90mm on the Press, or 50mm on a 35mm camera, ideal for what I like to do.

    65mm demonstration.jpg
     
    Last edited: Oct 16, 2021
  5. Thanks for your replies. I wasn't able to get more than minimal information elsewhere, so this is really helpful. It sounds like maybe it's not worth it and the repairman was right in just leaving it alone.
     
  6. Good advice to leave the lens alone. It's not limited to f/6.3 for no good reason.
    But why is a medium format lens being discussed in this forum?
     

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