Need help with exposure question

Discussion in 'Photography Beginners' Forum' started by PJM, Oct 26, 2018.

  1. PJM

    PJM No longer a newbie, moving up!

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    I was out playing with my camera today checking out the effects of aperture on depth of field. I ended up with two photos with different exposures and I'm trying to understand why.

    The camera is a Nikon D5600 with a 18-55 mm, f/3.5-5.6 lens.
    Both photos were taken in auto exposure mode and metering mode = pattern.
    Both were also taken in aperture priority mode.
    For the first picture I set ISO = 100, f/4.5. The camera set the shutter speed to 1/640.

    For the second picture I set ISO = 800, f/22. The camera set the shutter speed to 1/200.

    Since the camera was in auto exposure mode I would expect the exposures to be more similar and am trying to understand why the second came out lighter.


     

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  2. Derrel

    Derrel Mr. Rain Cloud

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    I would guess that slight variation is about 4/10 of an EV value different....might just have come down to the JPEG engine's processing of the overall capture. But definitely, the first shot looks a bit nicer to me. How close in time are the frames? A few seconds apart?
     
  3. Ysarex

    Ysarex Been spending a lot of time on here!

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    Another possibility is the minor framing variation (lighter version you titled the camera down a bit compared with the darker version). That's enough for the pattern metering to re-calc a minor difference.

    Joe
     
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  4. AlanKlein

    AlanKlein No longer a newbie, moving up!

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    That's why I bracket exposures on my landscape shots many times and shoot with a tripod.
     
  5. photo1x1.com

    photo1x1.com TPF Supporters Supporting Member

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    Doing the maths, this is the absolute same exposure
    From ISO100 to ISO 800:+ 3EV
    From f4.5 to f22:- 4.67EV
    From 1/640sec to 1/200sec:+ 1.67EV
    0

    So IMO there´s actually two things that could have happened:
    1. The light got brighter (sun cam out)
    2. There is a difference between f-stop (the nominal specs given by the manufacturer of the lens), and the so called t-stop (the real light transmission through your lens).
    These will explain the difference in brightness, but not why your camera didn´t compensate. However, that does happen. In my experience cameras are not that consistent in their metering.
    I didn´t use any automatic or semiautomatic exposure modes for years because I wasn´t happy with the results (I am/was a Canon shooter). That first changed when they introduced the 5D MkIII.
    I think the difference in your case isn´t all that bad.
     
  6. Ysarex

    Ysarex Been spending a lot of time on here!

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    Those are not the absolute same exposures. Doing the math there is a 3 stop difference between the two exposures. And so as Derrel suggested the variation could also be in the camera electronics as it compensated for the exposure difference.

    Joe
     
  7. PJM

    PJM No longer a newbie, moving up!

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    Thanks for all the replies. While I understand the fundamentals of exposure there are lots of nuances that I still have to learn and this all helps.

    To answer the first question, they were taken probably 15 seconds apart, enough time for me to change the settings.

    So for next time, take more pictures (bracketing) and use a tripod.

    Thanks again.
    Pete
     
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  8. photo1x1.com

    photo1x1.com TPF Supporters Supporting Member

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    Joe, you know I value your knowledge, but I wonder why your maths are different. So I recalculated and even took my camera to be on the safe side:

    Let´s start with ISO
    100 0.00
    125 0.33
    160 0.67
    200 1.00
    250 1.33
    320 1.67
    400 2.00
    500 2.33
    640 2.67
    800 3.00
    Next Aperture :
    5 2.67
    5,6 2.33
    6,3 2.00
    7,1 1.67
    8,0 1.33
    9 1.00
    10 0.67
    11 0.33
    13 0.00
    14 -0.33
    16 -0.67
    18 -1.00
    20 -1.33
    22 -1.67
    and finally Shutter Speed
    1/500 -1.33
    1/400 -1.00
    1/320 -0.67
    1/250 -0.33
    1/200 0.00
     
  9. Ysarex

    Ysarex Been spending a lot of time on here!

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    My maths aren't different. ISO is not a determinent of exposure. In the above examples exposure is a function of shutter speed and f/stop. Look at the standard definition of exposure: In photography, exposure is the amount of light per unit area (the image plane illuminance times the exposure time) reaching a photographic film or electronic image sensor, as determined by shutter speed, lens aperture and scene luminance. It matters; and in this case it could matter because the electronic processing the camera does to apply ISO brightening after the exposure could be one of the suspects. Don't let the triangle heads cloud your thinking.

    Joe
     
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  10. photo1x1.com

    photo1x1.com TPF Supporters Supporting Member

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    Thanks for the clarification, of course you are techically correct ;). I remember we had a similar discussion a while ago. I wonder what would be a better term. I tend to use the word brightness, but in this case it would definitely be wrong.
     
  11. PJM

    PJM No longer a newbie, moving up!

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    Correct me if I'm thinking wrong....
    So I need to think of ISO differently from film. Changing ISO on the digital camera does not really change anything about the sensor. Whereas different ISO or ASA films were physically different, changing ISO on the digital camera is about how the camera interprets or processes the light captured by the sensor.

    Pete
     
  12. john.margetts

    john.margetts No longer a newbie, moving up!

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    That is correct.
     

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