Manual Flash: shutter and aperture

Discussion in 'Beyond the Basics' started by guitarmy, Mar 22, 2007.

  1. guitarmy

    guitarmy TPF Noob!

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    Question for you flashers (hah).

    I'm trying to get away from TTL with my flash unit, so I'm learning manual flash.

    However, I'm kinda lost as to the difference that shutter speed makes on a flash picture as compared to the difference the aperture makes on a flash picture.

    Flash is supposed to freeze motion, right? So is the shutter speed then just to control the ambient light coming in? Does it control flash fall-off as well?

    What about aperture, then? Does it control anything other than DOF? Well, I know that if I stop down there will be less light hitting the sensor, but still. I guess what I'm asking is: in flash photography, what should I use the aperture to control? What should I use the shutter to control?
     
  2. shingfan

    shingfan TPF Noob!

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    i'm also a newbie...but as far as i know

    when using a flash.....aperture affects the expose on flash.....shutter on the other hand doesnt affect the flash exposure much......shutter is used to capture the ambient.....because flash duration is very short compare to the shutter speed.......so depending on whether you want pure flash or ambinet + flash....you would adjust your aperture and shutter accordingly
     
  3. JonR

    JonR TPF Noob!

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    You have two different light sources. Your flash is instantaneous (near enough) blink and you'll miss it. In fact, you'll probably miss it anyway, whereas other lights are continuous, you can see them all the time.

    Because flash is instantaneous , your shutter speed does not affect the exposure of the flash. Hence, if flash was your only source of illumination (as in a studio) you could use any length of exposure from 1/250 (or whatever your highest sync speed is) to 3 minutes and providing your subject stayed still, you would get exactly the same picture.

    When mixing ambient with flash, essentially yes, the aperture controls the flash exposure and the shutter speed controls the ambient exposure. The way I do it is:

    1. Pick aperture for desired DOF
    2. Match flash power so that exposure is correct (by chimping and adjusting)
    3. Set shutter speed to max sync
    4. See what is illuminated by the flash
    5. Bring shutter speed down to adjust ambient exposure. Stop when you like.

    There's loads more here, as I recognise that that was probably the worst description ever: http://strobist.blogspot.com/2006/03/lighting-101-balancing-flash-and.html
     
  4. ksmattfish

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

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    Use aperture to control the exposure of the part of the scene that will be lit by the flash (you can also use flash power settings and distance to adjust this). Use shutter to control the parts of the scene that will not be lit by the flash.

    If you have the flash set for proper exposure of the subject, and you use a high shutter speed, dimly lit backgrounds will be very dark. With a slow shutter speed they will be brighter, but watch out for camera shake if you are going really slow.

    With very bright backgrounds (like bright skies) use flash to light the subject, and set the shutter high to properly expose the sky (assuming flash sync speed doesn't get in your way).
     
  5. guitarmy

    guitarmy TPF Noob!

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    No, that was super helpful. Thanks! I read the Strobist all the time, but as far as I've read I hadn't really found a clearcut, concise description of flash vs. shutter vs. aperture.
     
  6. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    Flash photography is actually capturing two exposures...one is the flash exposure and one is the ambient exposure. Most of the time, we want these two exposure to line up (or for one of them to be completely dominant). It may help you if you think about the two exposures when trying to figure it out.

    Lets say you are taking a photo of a person outside, around dusk. It's getting dark but it's not dark yet. If you don't use flash, it would take a rather long shutter speed to expose them properly...and that would cause blur. So we turn on a flash, the flash fires so quickly that the blur is gone (from the subject)...but the ambient exposure is still there. The more ambient light you get, the more of the ambient exposure will show up...along with the sharp flash exposure. So if you used the same shutter speed that you would have needed for a shot with no flash...but with the flash, you will probably end up with a sharp subject and some 'ghosting', which is the blurriness from the long shutter speed. If the camera and subject are fairly still, the blur won't show up much and the sharp flash exposure will dominate.

    Now, the trick is to get a nice balance...because your flash won't light up the whole background. So you should think about the background as a separate exposure and set your aperture and shutter speed for that (maybe under expose a little bit). This will give you your background exposure. Then set the flash to light up the person...this will depend on the flash power, placement and the aperture. You may want to use less power from the flash, so that the light looks a little more 'normal'.

    So that gives you your two exposures. Depending on the light, you still have to watch out for ghosting...because if there is too much ambient light, the ambient exposure will be more prominent and the flash exposure might not be completely dominant. It's a constant balancing game to get it right...especially when the light is changing.

    I hope that wasn't too much of a ramble.
     
  7. Flash Harry

    Flash Harry TPF Noob!

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    Set the flash to full output/manual the aperture then is opened to cover longer distance at your sync speed, if a subject is close, close down the aperture, far away open it up.

    It all depends too on the model, the guide no. etc but basically this is it.
     
  8. guitarmy

    guitarmy TPF Noob!

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    Great info, peoples. Thanks!
     

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