M42 Lenses

Discussion in 'Classic Manual Cameras' started by greg_nixon|2, Sep 14, 2018.

  1. I have a Praktica Nova camera, with a Sun Optical Zoom attached. Changing the aperture on the lens doesn't have any effect on the metering in the camera. The lens has a stop down pin which works ok, and it has an aperture tab which is functional. There doesn't seem to be anywhere in the camera body to where the aperture tab is coupled. Is this normal behaviour for M42 Lenses?
  2. AJG


    M 42 lenses went through a lot of changes over the years--the first ones had no automation at all, followed by preset diaphragms, semi auto diaphragms that had to be manually reopened after each exposure and then full auto diaphragms (which your Sun lens probably has). If your Praktica has TTL metering it should have a DOF preview button that would stop down the lens for metering, since that was the most common way for M 42 cameras to meter in the 60's. One possibility is that the auto diaphragm pin on the lens isn't lining up with the diaphragm actuating lever in the camera body. I have seen a lens or two over time where this was the case. It is also possible that the diaphragm actuating lever is no longer working on the camera since you say that the lens mechanism seems to be working properly.
  3. m42dave

    m42dave Dave E.

    The Praktica Nova has an uncoupled, above-the lens selenium meter, and other than actuating the stop-down pin, there is no other connection to the lens. The Nova also has a a small button inside the camera above the stop-down lever that can be switched to disconnect the auto diaphragm mechanism when using older manual diaphragm lenses. If the stop-down pin on the lens works, but not when mounted on the camera, that may be the reason why.

    As AJG mentioned the lens may have a DOF preview button or auto/manual switch. There were some lenses with a "tab" on the rear of the aperture ring to couple with open-aperture metering on certain cameras (such as Fujica and Mamiya) but third-party lenses and adapters for them are scarce.
    Last edited: Sep 14, 2018
  4. m42dave

    m42dave Dave E.

    This is the button I was referring to on the Nova. When switched to the right, it will disengage the camera's diaphragm mechanism.

    Nova IB.JPG
  5. "There doesn't seem to be anywhere in the camera body to where the aperture tab is coupled."

    What you seem to be describing is meter coupling, that is, a linkage which "tells" the camera what aperture the lens is set on, to enable open aperture metering. That's hard to achieve with screw fit lenses, because there's no positive stop to fix the lens's orientation when tightened - because of wear and manufacturing tolerances. Only a few M42 systems achieved this, by adding extra stops and levers, for example the SMC lenses on Pentax Spotmatic F. Praktica had a different approach, with the Pentacon Electric lenses they used electrical contacts to communicate with the appropriate body.
  6. Thanks all for your replies.
    AJG: There is a stop down pin on the lens which is coupled to the camera, and the lens will stop down to the set aperture. Useful for DOF preview at light shutter button actions.
    Dave E: I see that button which inhibits the semi circular piece under it from coming forward and stopping the lens down. It works as it should. Yes the meter is an uncoupled selenium meter, not TTL metering.
    John: That exactly what I was hoping to see, but there isn't one.

    Which begs the question, how do you meter with the uncoupled meter and take the aperture setting into account?
  7. m42dave

    m42dave Dave E.

    Greg, with this camera you would take a meter reading by pointing the selenium meter at the subject, then turning the exposure calculator dial on top of the camera until the meter needle matches the pointer, then manually transferring the appropriate shutter speed/aperture settings indicated on the calculator dial to the camera. Just like using a handheld meter, really.
    Last edited: Sep 14, 2018
    John Farrell likes this.
  8. The meter is uncoupled, meaning there is no communication at all between the aperture set on the lens and the meter. The meter also has no through-the-lens ability. So stopping down the lens won't affect the meter in the slightest.

    Basically, it's a separate meter stuck on top of the camera. Not only that, but it's a wide-area selenium reflectance meter, and therefore about as much use as a chocolate teapot!

    My advice would be to just forget that the camera has a meter, and buy a handheld meter with incident reading ability. Or use a light-metering app for a smartphone.
  9. m42dave

    m42dave Dave E.

    I find the meter on my Nova IB to be surprisingly accurate, and linear, too.
  10. I've shot a few frames with the inbuilt meter. I guess I'll know when I develop the film if it's in the ballpark. I have the iPhone light meter app, I just never seem too use it. Thanks Joe for your comments and Dave for yours on your metering experience.

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