Pentax 645 TTL flash ISO range?

Discussion in 'Medium Format Cameras and Accessories' started by f/16, Aug 24, 2013.

  1. f/16

    f/16 Subscriber

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    Does anyone know the ISO range when using TTL flash on this body? I can't find that in the instruction book. I know on many cameras there's a limit such as ISO 400 or 800. What about the P645?
     
  2. lxdude

    lxdude Member

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    I could not find in the manual any hard limitations on the ISO settings.
     
  3. John Koehrer

    John Koehrer Subscriber

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    FWIW, I've not seen limitations on film speed. If it's dedicated to the camera it takes the info from the ISO dial on the camera. The limitation would be shutter speed and aperture. High ISO may have problems at closer distance but will extend your effective range.
     
  4. f/16

    f/16 Subscriber

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    Yes I couldn't either, so I guess there are no limitations??
     
  5. mklw1954

    mklw1954 Member

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    The camera itself can be set up to ISO 6400 but the ISO limit for TTL flash would be that of the flash unit that is compatible with TTL operation. For the Pentax AF280T and AF400T flashes this would be ISO 800.
     
  6. f/16

    f/16 Subscriber

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    Thanks. Where did you get that info?
     
  7. mklw1954

    mklw1954 Member

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    The maximum ISO setting of the camera is not in the manual, I just checked it on the camera using the ISO control. So the flash is the limitation with respect to the highest ISO you can use.
     
  8. lxdude

    lxdude Member

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    The manual for the AF 280T says for TTL operation to first set the ASA speed on the flash unit. So it appears that TTL does not override the ASA set on the flash. It would have been nice if there had been an override setting on the flash, to extend the range and avoid having to set ASA on the flash (along with the possibility of forgetting to do so) when changing film speeds. I have heard the exposure compensation dial on the camera will change flash exposure--on the LX, anyway. I almost never use flash and haven't tried it, so I don't know if it can be used to effectively get a higher film speed.
    The AF080C has no ASA/ISO setting, so the manual's TTL charts of usable f/stops within the flash range assume ASA 100, and say to adjust for other film speeds.
     
  9. mklw1954

    mklw1954 Member

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    TTL has a limited range. For objects beyond the range of TTL, use a flash auto range, where the light to the flash sensor controls the flash duration (3 ranges for the AF400T, 2 ranges for the AF280T). For even more distance, use the flash in Manual mode where you'll have to calculate the correct aperture.

    For TTL, see the camera manual (TTL Auto Flash Operation). With a TTL auto flash for the camera (e.g., Pentax AF280T or AF400T), you set the ISO on the flash and when the flash is charged, the camera automatically goes to the 1/60 synchronization shutter speed. The only variable is then lens aperture and you can manually select this, based on the distance range on the flash, when the camera is set to Aperture Priority AE or Manual mode. Or, the camera will automatically select the aperture based on the camera ISO setting if you set the lens to A and the camera is in Aperture, Shutter, or Program AE mode.

    You can also do TTL outdoor fill flash - set the flash to TTL and the film ISO; set the camera to the film ISO and shutter speed to 1/60; do not use the lens on A. With the flash off, meter the shadows and set the aperture for the correct exposure. Then turn on the flash and change the flash ISO setting by -1, -1.33, or -1.67 stops, depending on how much or little fill flash you want to fill shadows. For example, with 100 speed film, change the flash ISO to 200, 250, or 320 depending on how much fill flash you want (200 provides more fill, 250 and 320 less). Using 400 ISO film, you can only get -1 stop (flash ISO set to 800).
     
    Last edited by a moderator: Sep 9, 2013
  10. mklw1954

    mklw1954 Member

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    TTL has a limited range. For objects beyond the range of TTL, use an Auto range on the flash, where the light reaching the flash sensor controls the flash durantion. For maximum distance, use the flash in Manual where you'll have to calculate the correct aperture.

    For TTL, see the camera manual (TTL Auto Flash Operation) and flash manual. With a TTL auto flash for the camera (e.g., Pentax AF280T or AF400T), you set the ISO on the flash and when the flash is charged, the camera automatically goes to the 1/60 synchronization shutter speed. The only variable is then lens aperture and you can manually select this, based on the distance range on the flash, when the camera is set to Aperture Priority AE or Manual mode. Or, the camera will automatically select the aperture based on the camera ISO setting if you set the lens to A and the camera is in Aperture, Shutter, or Program AE mode.

    You can also do TTL outdoor fill flash - set the flash to TTL and the film ISO; set the camera to the film ISO and shutter speed to 1/60; do not use the lens on A. With the flash off, meter the shadows and set the aperture for the correct exposure. Then turn on the flash and change the flash ISO setting by -1, -1.33, or -1.67 stops, depending on how much or little fill flash you want to fill shadows. For example, with 100 speed film, change the flash ISO to 200, 250, or 320 depending on how much fill flash you want. Using 400 ISO film, you can only get -1 stop (flash ISO set to 800).
     
  11. f/16

    f/16 Subscriber

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    I just looked at my AF280T. The highest ISO setting on it is 800. But I don't think that changes anything electrically. It just shows you your ranges at each f stop. I may burn a test roll and try it at ISO 1600 and see if TTL flash is accurate.