questions about flash compensation

Discussion in 'Photography Beginners' Forum' started by fwellers, Dec 20, 2008.

  1. fwellers

    fwellers TPF Noob!

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    Just have some beginning type questions about using flash on my pretty new dslr.

    1) Just bought an sb600 speedlight for my Nikon D90. It has been my understanding, when using the onboard flash, that if I set the flash compensation value to some negative number, it in effect makes the flash less bright, and can take some of the harshness out of the shot.
    Is that right ?

    2) And if it is, I see that there is also a flash compensation setting on the sb-600. If I'm shooting with the speedlight in either TTL or even in Command mode, is that setting slaved to the onboard flash setting, or are each of them doing their own thing ? IN other words, can I set the flash cv to -3 on the body, and to some other value on the speedlight, and they will both fire according to their individual settings ?

    3) Can the sb600 be forced to slave off of the cv that is set on the body ?

    On another note, maybe I need to do some more practicing, but it seems that when if I use flash, my exposure indicator goes off the charts to the right ( overexposed ). Even when shooting on a gray day, I thought to turn the flash on for 'fill lighting' but it seems like I have to dial the shutter real fast or the aperture real narrow to get the meter to be zero.
    Does that sound right or am i doing somethign wrong/missing something ?

    Thanks for your advice.

    Floyd
     
  2. tirediron

    tirediron Watch the Birdy! Staff Member Supporting Member

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    1. Yes.

    2. Yes.

    3. No.

    4. Don't forget that your sync speed (The fastest speed at which the shutter is totally open when the flash fires) is 1/250 of a second. Any shutter speed above that is having an adverse affect on your exposure. The way you're describing the problem almost makes me think that your ISO may be set either very high or to 'Auto'.
     
  3. fwellers

    fwellers TPF Noob!

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    Thanks Tirediron.
    I'm glad I seem to have a bit of a handle on the flash compensation.
    I am however having a hard time wrapping my brain around the sync speed thing.
    You mentioned 1/250. That is my fastest shutter speed I believe. I know the speedlight can fire faster than that. But the relationship between those two speeds and what they mean to exposure, is evading me at the moment.
     
  4. tirediron

    tirediron Watch the Birdy! Staff Member Supporting Member

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    Flash sync speed is the fastest speed at which your camera's shutter is completely open. At speeds faster than the sync speed, the shutter isn't completely open, the top blade will already have completed it's travel while the lower ones are still open or partly open.

    If you fired the flash at this point (The duration of which is something like 1/10,000 sec) only part of the scene would be illuminated since the part of the shutter which was already closed would prevent the flash return from striking the sensor.
     
  5. fwellers

    fwellers TPF Noob!

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    Two questions about that may help me understand it better.
    1) is flash sync speed a setting on the camera, the speedlight, or just a term to describe something ?
    2) I would think that the flash, if set manually to a fast value, would wait until the shutter was open completely, then fire.

    Well one more.
    To get the flash to fire super fast, do you have to put the sb600 in manual mode and tell it how fast to fire ? And if it's in ttl mode I would assume it just works in conjunction with whatever shutter speed your camera ( or you ) have set.


    Thanks in advance again.
    Floyd
     
  6. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    Flash comp, as well as most other exposure considerations, is measured in stops. A stop is half or twice as much light.

    Normally the camera wants to make everything middle gray tone. If you set flash comp to +1 you are telling the flash to make the subject twice as bright as middle gray tone. If you set the flash comp to -1 half as bright as middle gray tone. For instance when I shoot a wedding and I'm photographing a bride in white I usually dial in +1 flash comp: I want the bride's dress to look bright, not middle gray. On the other hand when photographing the groom in black I dial in -1, because I want the tux dark, not middle gray. When the flash is any sort of auto mode remember that it doesn't care what your subject is, but it wants to make it middle gray.

    Unless Nikon is doing something I'm not aware of, your in-camera meter that is displayed to you is not taking the flash into consideration. It is only metering for ambient light, even when the flash is on. So you don't look to the in-camera meter to tell you what's going on with flash exposure. To meter the amount of light coming from the flash you would need to use a flash meter.

    A flash normally occurs in 1/10000th a sec or even faster, so shutter speed doesn't effect flash exposure at all. Except that at very high shutter speeds the shutter never fully opens. High shutter speeds are created by moving a slit across the film or sensor. Flash sync speed is the fastest shutter speed where the shutter is all the way open. At faster shutter speeds it never opens all the way, and if your camera would allow you to take a photo only a portion of it would be exposed by the flash (assuming it's not in some sort of high sync mode). High sync flash extends the duration of the flash, usually with some loss of power. Flash sync speed is a characteristic of the camera body.

    Use your in camera meter to determine exposure for the parts of your scene that won't be exposed by the flash, and then use exposure comp (or adjust the flash manually) to adjust the flash. For instance if I'm photographing someone against a bright blue sky I set the shutter and aperture for the sky exposure I want, and in a perfect world the flash would light my subject up to match. This will look funny to our brains because we know there should be some shadow on a back-lit subject, so many folks like to dial in -1 flash comp so that the flash lights it up a little less. But it's rarely a perfect world, and the camera/flash may have a hard time telling the difference between sky and subject. If the sky is really bright, or takes up a lot of the image, the flash in auto may already be lessening it's power, so you may have to take a test shot or two to dial in the balance you prefer.
     
  7. Captain IK

    Captain IK TPF Noob!

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    Excellent explanation KS...very informative.
     
  8. fwellers

    fwellers TPF Noob!

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    Thanks for that explanation KS.
    Can't say I understand it all. Maybe it'll sink in someday.
    I'm pretty sure my camera is taking the flash into account for the metering, because the way I remember it, I had the meter set to center, then turned the flash on, and immediately the meter gage moved all the way over to the right.

    So the flash sync speed thing. Does that mean if my flash sync speed is 1/250, that if I the speed light, any shutter speed that is slower than 1/250 will mean the flash doesn't open fully and therfore the picture won't be exposed as well ?

    Thanks,
    Floyd
     
  9. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    Then you've got a heck of a fancy camera, because I've never seen one that does. Normally the flash communicates with the meter, and then adjusts it's power for correct exposure.

    No. If your flash sync speed is 1/250th of a second that means that's the fastest shutter speed in which your shutter opens all the way. If you are using a flash that doesn't have a high speed sync feature you have to use a shutter speed of 1/250th or slower. If you shoot at 1/320 there will be a small sliver of your photo that doesn't get flashed. The faster you go the smaller the portion of the photo that gets flashed is. At your highest shutter speeds the shutter never opens more than a slit, and that slit moves across the entire sensor, but the flash occurs in a fraction of the highest shutter speed you have, so where ever the slit is when the flash goes off gets exposed, and the rest is usually dark.

    Go to this wiki page and there is an example photo (it's a Pentax 35mm SLR) that shows what using a shutter speed higher than the flash sync speed would look like. Your camera probably won't let you make that mistake, and it will automatically adjust the shutter to the flash sync speed even if you try to go beyond it.

    http://en.wikipedia.org/wiki/Flash_synchronization

    Google flash sync, and you'll probably find lots of examples. This was a much more common problem with mechanical cameras.
     
  10. AlexColeman

    AlexColeman TPF Noob!

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    Good, this helped me too.
     
  11. fwellers

    fwellers TPF Noob!

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    I'm sure you're right. I guess the camera compensates for it. All I know is that when I turn the flash on, the meter moves all the way to the right.

    Thanks again for the explanation. I think I'm getting it.
     
  12. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    I'm shooting Canon, so it's possible Nikon is doing something I'm unaware of.

    The thing is I could almost understand the meter changing if the flash was in manual power mode, but in auto it adjusts it's power to what the meter is telling it. Normally the camera settings don't adjust to work with the flash, the flash adjusts power to work with the given camera settings.

    Here's how to figure flash exposure without a meter (running the flash in manual power mode). Figure out your guide number (GN). If you don't have your flash manual handy, just look it up at anyplace that sells your flash. GN is a number used to describe max flash power. GN is usually given in feet and/or meters for ISO 100, and the formula is:

    GN/distance to subject = aperture for "correct" exposure

    So if the GN is 100 (in feet), and your subject is 10' away, then at full flash power the correct aperture would be f/10 at ISO 100.

    When you half or double the ISO number it's a stop. When you half or double the flash power it's a stop. When you double the distance from flash to subject it's 2 stops.
     

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