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Demystifying flash, help requested please.

Discussion in 'Photography & Video' started by Bluelion, 23 Sep 2009.

  1. Bluelion


    Joined: 4 Jan 2008

    Posts: 717

    Location: Southampton

    Hi folks,

    I bought a flashgun about 12 months ago now when I was asked to take some photo's indoors for a newspaper article. In my usual fashion, I equated "best" to "most-expensive" and so ended up with a Nikon SB-900, which has the usual half-inch thick manual with lots of information that I don't understand.

    As a consequence, after said article the flashgun went back into my kit pile never to see the light of day again. Until now...

    Having found a few articles on the web which explain flash usage in English rather than technobabble, I've dug it out for a little play. As an exercise I decided to see if I could "freeze water" by dropping water droplets into a bowl of water in a dim room.

    Please correct me if anything I say here is wrong - I only quarter understand what I'm doing here. So this is what I understand is the principle here:

    a). In a dimly lit room, the flash will be by far the biggest light source, and consequently the shutter can be open for as long as you want within reason - the exposure will principally occur when the flash fires.

    b). Flash doesn't have a brightness value. It is simply on or off - what varies is the length of time it is on for. :confused:
    Question: Could someone confirm this is right please? I would like a definite yes/no answer to this statement. Does the flash brightness vary or just the on-time?

    c). I am using the flash in TTL mode, or Through The Lens. As a consequence I don't set the power of the flash, I simply set an exposure modification value on the flash just like I would on the body, eg. +1ev. At exposure time, the flash fires twice, the first is used by the camera/flash system to determine how long to fire the light for the main exposure.
    Question: why doesn't this pre-flash expose the image? Which should leave me with a ghost of the drops which I don't see.

    d). Normally I stick the camera in Aperture priority mode, as I want to primarily control the depth of field in my pictures. Normally by fixing the aperture, the camera determines the exposure time for me based on the ambient light. However, now I've got two variables - the exposure time AND the light levels which can be varied with the flash.
    I cannot for the life of me work out what the body does in this situation (its a D300), but what I have read is that different bodies do different things when in this mode. Consequently, I'll stick to manual mode where the flash can then choose how long to illuminate the image for.
    Question: Is there anyway you can see how long it decided to flash for? Its not immediate obvious in the EXIF information, only the shutter exposure is present which in this setup is irrelevant.

    So onto the output. To kick off with, I chose to reduce the exposure on the flash (-3ev if my memory serves me correctly), my theory being I wanted to "freeze" the action, and therefore have the flash on for the minimum amount of time. I also used f16 aperture to help with focusing, as I needed a relatively large depth of field.
    (1) F16, 1/80th

    Obviously quite underexposed, as you would expect.

    Next up, trying the orange filter thing which came with the flash. Probably should have read the instructions on how to fit it, I squished it inbetween the pully-out-diffuser thingy.
    (2) F22, 1/80th

    Surprisingly orange! I didn't really expect such a large difference. So I wonder what the green filter does then?
    (3) F22, 1/80th

    Pretty green! Time to play with take the picture at varying points of the droplets impact. A well into oblivion?
    (4) F22, 1/80th

    Could be a water monster from a movie?
    (5) F22, 1/80th

    Impact! Speeding up the shutter exposure time should have no effect if my theory is correct.
    (6) F16, 1/200th

    Enough playing with the green filter, back to basics for a repeat of the impact picture, this time using a large aperture, which should give a faster exposure time and more frozen image? Of course at the expense of depth of field.
    (7) F8, 1/200th

    Must be getting tired, I'm seeing shapes in the images. A women in a dress anyone?
    (8) F13, 1/200th

    ... or how about a vertical snooker cue about to strike the ball?
    (9) F13, 1/200

    Not sure I've moved on too far in understanding flash yet, but its a start. Still got fill-in-flash to get to grips with, and we haven't even mentioned forward or rear curtain sync yet :eek: Flash opens a whole new world of pain :p

    I would appreciate your input, particularly with respect to the three questions above. Thanks in advance.
  2. Gnomey


    Joined: 27 Mar 2009

    Posts: 103

    Some good captures :), I like the crown in 6.

    a) This is right, in a pitch black room, the effective shutter speed will be the duration of the flash.

    b) Flashguns have different levels of power, each has a different duration.

    The SB900 has the following durations:

    * 1/880 sec. at M1/1 (full) output
    * 1/1100 sec. at M1/2 output
    * 1/2550 sec. at M1/4 output
    * 1/5000 sec. at M1/8 output
    * 1/10,000 sec. at M1/16 output
    * 1/20,000 sec. at M1/32 output
    * 1/35,700 sec. at M1/64 output
    * 1/38,500 sec. at M1/128 output

    Taken from Nikon USA

    c) Preflash is fired before the shutter is open.

    d) I don't think this is in EXIF.
  3. benneh


    Joined: 9 Apr 2004

    Posts: 9,170

    Location: Nr. brumijum

    a). I'm not totally sure what you mean by this. Yes the flash will probably create an area within the photo that is brighter than its surroundings, unless you're bouncing flash (off the walls/ceiling etc). The correct exposure will be the one that exposes for the lit subject. The flash can fire either at the beginning or towards the end of the exposure depending on the setting (1st+2nd curtain flash, front/rear).

    b). The output of your flash unit can be changed, not sure how that goes, with my canon it's 1/64 thru 1/1 (full power), if in manual mode. Power = Brightness. The brightness can vary. Afaik the on time is constant (being as it's a process of stored energy being discharged from a capacitor to a pulse transformer and into the tube, I would assume it is a constant duration, I could be wrong). <-- I was wrong lol.

    c). I have no idea. I would go manual and play around, but that doesn't answer your question.

    d). No idea.
    Last edited: 23 Sep 2009
  4. Snapshot


    Joined: 11 Oct 2006

    Posts: 4,102

    Location: Wiltshire

    b. Not entirely true as the flash does have a rise and fall time but it's not usually significant. The flash duration in auto mode may be more flexible than the figures quoted by gnomey suggest as those are the manual settings.

    d. The camera only deals with light, it doesn't care if it's ambient or flash. So if you have a relatively large amount of ambient light, it'll just reduce the duration of the flash to give the correct overall exposure. It sorts this out with the pre-flash.
    Last edited: 23 Sep 2009
  5. Jotun


    Joined: 11 Jun 2005

    Posts: 3,606

    Location: Liverpool

    There's lots of information on the strobist website - http://strobist.blogspot.com/2006/02/welcome-to-strobist.html

    Or have a look at the one light dvd, although it's more aimed at portrait photography. There is also a strobist dvd set, if you can borrow a copy of either it'll be very informative.

    I'd recommend setting everything to manual whilst playing around.

    Th flash duration is very short, and if you are using manual then it will always be the same. Thus, shutter speed controls the ambient light hitting the sensor and you use aperture setting to control the light from the flash. ISO effects both so it's a bit of a juggling game to get it all spot on. In addition you can adjust the flash power or distance from subject to the flash.
  6. Bluelion


    Joined: 4 Jan 2008

    Posts: 717

    Location: Southampton

    Thanks for all the advice guys, its cleared up things nicely. I'll go and peruse through the strobist site Jotun, and switch to manual to get a better feel for what's going on.
    Cheers all!
  7. balls

    Wise Guy

    Joined: 20 May 2007

    Posts: 1,188

    Location: Bath, Somerset

    Shutter speed still the same 1/200
  8. Bluelion


    Joined: 4 Jan 2008

    Posts: 717

    Location: Southampton

    But as I was using TTL exposure compensation, a larger aperture should give a shorter burst of flash light, the effective "exposure" time in my setup no?
  9. robmiller


    Joined: 26 Dec 2003

    Posts: 16,522

    Location: London

    Yep, that's right, although you're unlikely to be able to tell the difference with most subjects: we're talking a range between 1/880th of a second at full power vs. 1/38,500th of a second at 1/128th power, all of which should freeze most motion.