Are Dahua vari-focal lenses constant aperture?

Discussion in 'Dahua' started by Mr_D, Feb 6, 2018.

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  1. Mr_D

    Mr_D Getting comfortable

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    Specifically the HDW5231R-Z

    The specifications say it has an f/1.4 lens, but is it still f/1.4 when zoomed to 12 mm?

    I am an amateur photographer and know that cheaper zoom lenses become slower (smaller aperture) at longer focal lengths, requiring a combination of higher ISO or slower shutter speed to maintain the same exposure.

    If the aperture does change, does anyone know what it is at 6 or 12 mm?
     
  2. pcunite

    pcunite Young grasshopper

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    Good question. The Max. Aperture value in the datasheet reads F1.4. For non-constant aperture zooms, it will show something like F1.4~F4.4. So, from that ... I would say it is constant.
     
  3. cam235

    cam235 Pulling my weight

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    You are correct that with SLR camera lenses, it is traditional to show the aperture range whenever it is not constant. However with security camera lenses, I would not make that assumption, particularly when maximum f/number is a selling point, and it is much more expensive to make a constant-aperture zoom. If they can claim "Max Aperture" means maximum at SOME focal length (and not every focal length), they probably will.
     
  4. bp2008

    bp2008 Staff Member

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    Yeah, don't trust everything you read on a spec sheet. Just like the light sensitivity, they will print the number that looks best on paper, not necessarily the number you'll be working with in practice.
     
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  5. John316

    John316 n3wb

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    I don't think they're constant aperture. The review of HDW5831R-ZE estimates the aperture at telephoto at F2.0 and the datasheet for this camera lists only "F1.4" on the spec sheet. F1.4 must only apply to the wide end of the lens.

    I'd like to know how the aperture changes for the HFW5231E-Z5 and the HFW5231E-Z12 throughout their zoom ranges if anyone has tested it.
     
  6. dragan11

    dragan11 n3wb

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    +1, wondering the same thing. If so, perhaps fixed lens version of 5231 has an advantage in comparable zoomed in dark scene, it would be interesting to see the results. Is reducing the shutter speed a proper way to compensate for smaller aperture?
     
  7. Mr_D

    Mr_D Getting comfortable

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    Reducing the shutter speed will allow the same exposure, but will have more motion blur, particularly at night where the difference between 1/60 and 1/20 is huge.
     
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  8. alastairstevenson

    alastairstevenson Staff Member

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    I believe they do have a fixed aperture.
    Only lenses with an 'auto-iris' (ie electronically controllable) will change the aperture, to optimise exposure and depth of focus with light level.
    These are the sort of lenses that are fitted to higher-end professional cameras such as Axis, Bosch, Pelco etc.
     
  9. dragan11

    dragan11 n3wb

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    Now that i think about it, i believe there is auto-iris on/off button in 5231 settings. I'll double check it night but maybe that is the answer, if there is auto adjust option that would imply it's variable? i wonder if fixed versions have this option in camera settings, if they do maybe it's not necessarily an Aperture control.
     
  10. alastairstevenson

    alastairstevenson Staff Member

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    I've seen this when the firmware covers a range of models, some of which may have a controllable iris.
     
  11. dragan11

    dragan11 n3wb

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    that could very well be the case.