SB-600 stop equivalent

Discussion in 'Photography Beginners' Forum' started by jands, May 10, 2011.

  1. jands
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    jands New Member

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    Using an SB-600 in manual, what increment of power is equivalent to one stop? For example if I am at 1/4 power, take a picture, then stop down my aperture by one stop, what is the next "stop" on the flash to compensate?
  2. Derrel
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    Derrel Mr. Rain Cloud

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    Full, 1/2; 1/4; 1/8; 1/16; 1/32--this is commonly referred to as a "five f/stop range" of variable power. Some flashes also offer 1/64 and 1/128 power settings for a seven-stop range.
  3. jands
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    jands New Member

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    Thanks.
  4. jands
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    jands New Member

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    Incidentally, Derrel, I'm asking because I'm playing with the 105mm AI you recommended.
  5. KmH
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    KmH Helping photographers learn to fish

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    Except lens aperture, divide by 2, or times 2, is a stop.

    ISO 100, to ISO 400 - is 2 stops. 100 x2 = 200 x2 = 400

    Or ISO 1600, to ISO 400 - is 2 stops going the other direction 1600 / 2= 800 /2 = 400

    Shutter speed is the same 1/200, to 1/800 - is 2 stops as is 1/1600, to 1/400.

    For lens aperture the area of a circle (imperfect) is involved, rather than the aperture diameter, and we have to use the square root of 2 instead; 1.4142 to define a full stop. f/2, to f/4, is 2 stops - f/2, f/2.8, f/4.

    Conversely, going the other way f/5.6 is to stops larger than f/11 - F/11, f/8, f/5.6

    I love math!
    Last edited: May 11, 2011
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  6. jands
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    jands New Member

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    Wow, thanks KmH. I think I got it. I'm still in the middle of letting it sink in.

    I didn't even notice (until now) flash power worked the same way (x2 /2). Now it makes sense why the flash jumps from 1/2 to to full.
  7. KmH
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    KmH Helping photographers learn to fish

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    Since the thread started based on a flash unit, don't forget the Inverse Square Law.

    If you don't change the flash power, the amount of light on a subject is a function of the square of the distance, if the distance is changed.
    If the light is moved twice as far away, only 1/4 as much light hits the subject. If you double the distance again, only 1/16th as much light hits the subject.
  8. jands
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    jands New Member

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    Funny that you mention that, I just was re-reading about it. It didn't quite make sense until now. A lot of things have been clicking lately... no pun intended.

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