Aperture impact on light picture

Discussion in 'The Black & White Gallery' started by patav, Oct 23, 2007.

  1. patav

    patav TPF Noob!

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    Good day, I wanted to take these pictures in order to get some feed back on some of my questions ( I have a Canon 40d with a 50mm 1.8):

    1- I would of expected to have picture # 1 looking like picture # 2 in normal mode...why is the camera putting the aperture to 5

    2- I would expected to have picture # 1 looking like picture # 2 when having big aperture due to the fact that depth of field would be close.

    Let me know what you think...and how I should interpret this. Any links would be appreciated. regards


    Picture 1:Mode: Normal ; Speed: 1/320 ; f5 ; FL: 50 mm ; ISO: 500 ; Exposure: Normal
    [​IMG]



    Picture 2:Mode: Manual ; Speed: 1/400; f1.8 ; FL: 50 mm ; ISO: 500 ; Exposure: Normal

    [​IMG]
     
  2. Alpha

    Alpha Troll Extraordinaire

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    They're different because f1.8 is more than a full stop faster than f5, whereas 1/400 is not more than a full stop faster than 1/320. In fact, it's less than half a stop faster. Therefore, it's no wonder more light was let in and #2 came out brighter. That's very screwy.
     
  3. patav

    patav TPF Noob!

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    I agree but why would the "auto" mode set the f stop to 5.

    regards
     
  4. Helen B

    Helen B TPF Noob!

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    Number 1 was shot in program mode, ie autoexposure. The lights are affecting the exposure - the metering system tries to set an exposure that would suit a middle grey scene. Number 2 was taken in full manual mode, with just over two stops more exposure (as Max says, -2/3 stop because of the shutter speed difference, +3 stops because of the aperture difference)*.

    Number 2 has less depth of field than number 1, because of the wider aperture.

    *You can also calculate this using the TV and AV data in the EXIF.
    For Number 1 TV + AV = 8.38 + 4.63 = 13.01
    For Number 2 TV + AV = 8.63 + 1.63 = 10.26
    Difference = 2.75 stops (there are mismatches between the exact TV, AV, shutter speed and aperture numbers, probably because of rounding)

    Best,
    Helen
     
  5. patav

    patav TPF Noob!

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    Thank you very much for all your help.
     

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