Slow shutter speed Vs flash

Discussion in 'Photography Beginners' Forum' started by k.udhay, Apr 14, 2013.

  1. k.udhay

    k.udhay TPF Noob!

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    Hi,
    I just observed that the when I shoot a picture with flash on, it lasts only for a very short period (fractions of a second), even if I set my shutter to go very slow (say 20s). Its little difficult for me to imagine if the sensor still keeps recording the image even when the flash puts out. Can somebody pl explain what happens in such a situation with the consequences in the picture? Thanks.


     
  2. nonamexx

    nonamexx TPF Noob!

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  3. pgriz

    pgriz Been spending a lot of time on here!

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    With flash, there are two exposures in effect. Aperture+shutter speed determine the amount of ambient light recorded, and Aperture + flash power (+ distance) determine the amount of flash recorded. Let's say you are shooting in a fairly dark area, like an interior. The ambient light exposure for that amount of light could be 1/30 sec at f/5.6. A flash with a guide number of 50 would need to be placed at 9 ft. from the subject to expose correctly at full power and f/5.6. The actual flash duration is very short, 1/5000 of a second or faster, and the only role for the camera shutter is to be open when the flash fires. But in this example, the common element is the aperture, being set to f/5.6. So the sequence of events is as follows:
    1) you press the shutter button,
    2) the camera closes the aperture down to the one chosen (in this case f/5.6),
    3) the shutter opens to start the exposure
    4) the camera sends a signal to the flash to fire (this assumes your camera is set for first-curtain sync).
    5) the flash fires a very brief pulse of light, and its work is done.
    6) the camera counts down the exposure (now it is only the ambient that is being recorded)
    7) the camera shuts the shutter, and the exposure is complete.

    If your camera was set up for second-curtain sync, the camera would fire off the flash just before the shutter would close.

    For most flashes, there is a maximum shutter speed (usually around 1/200 sec) which is called the sync speed. This is the fastest shutter speed you can use on a particular camera with a single-fire flash. The reason this happens is that the shutter is actually made up of two curtains. The first curtain opens, some time elapses,then the second curtain closes. For speeds above 1/200 sec, the second curtain starts to close before the first curtain is fully open, creating a travelling slit which exposes the sensor. The faster the shutter speed, the thinner is the slit. Since the flash pulse is much faster than the shutter, shooting a flash with high shutter speed will result in black bands in the image, which are the shadows of the curtains travelling across the sensor. So the sync speed is the fastest speed at which the sensor is fully exposed (ie, first curtain is completely open, and the second curtain hasn't started closing).

    Some flashes are designed to overcome this sync speed limitation by having a "high-speed sync" mode, in which the flash behaviour changes from a single pulse, to a series of smaller power pulses that fire over a longer period of time. Then, as the shutter slit moves across the sensor (at shutter speeds higher than the sync speed), the flash continues to fire very short pulses, that allow the sensor to become fully illuminated by the flash by the time the travelling slit closes.

    Returning to the issue of flash vs. ambient. It is part of a photographer's bag of tricks to determine how much contribution he/she wants from each source. In the case of using a fill flash on a bright day, the camera exposure is set according to the ambient light. Let's assume we're shooting at ISO 100, and it's a bright sunny day. The sunny-16 rule applies, which says that you will have a proper exposure at 1/100 sec at f/16. You now have to tell the flash (assuming you're doing this manually) how much power to put out, and at what distance (because of the inverse square law that we discussed the other day). Again, assume you have a flash with guide number of 50. The flash needs to be about 3 ft. away from your subject to give a "proper" exposure at f/16. However, you don't want the light to be equal to the ambient, because you want it only to fill in the shadows. Fill is about 2-3 stops below the "normal" exposure. So let's say we want the light to be 2 stops less. A quick calculation tells me that the flash needs to be at 12.5 ft. to give me the right amount of light. Alternatively, I could reduce the flash power from full to 1/4 (2-stops) to get the right exposure. This will therefore give me a properly exposed ambient light photo, AND give me some fill in the shadows.

    Alternatively, you may want the flash to be the main source of light, and the ambient exposure to be 2 stops less. This is known as "killing the sun". Assuming we are still working with a flash with a guide number of 50, you'd have to locate the flash about 8 inches from your subject to get that amount on light on the subject. This is not really ideal, so photographers use higher-powered flash units to locate the flash at a reasonable distance from the subject, and still provided enough light to overpower the sun.
     
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  4. Josh66

    Josh66 Been spending a lot of time on here!

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    Not true.

    There are plenty of times you would want to use flash with a long exposure, and it is certainly possible.
     
  5. MK3Brent

    MK3Brent No longer a newbie, moving up!

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    Wrong.

    There's plenty of uses for long shutter speeds with flash(es).
     
  6. nonamexx

    nonamexx TPF Noob!

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  7. pgriz

    pgriz Been spending a lot of time on here!

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    There is a difference between stating an opinion (we all have them ;) ), and making a factual statement. I had an idea of what you had in mind when you wrote your post, but you didn't consider all the other possibilities. There is no need to be offended, as we all make mistakes, and more experienced members point out the error we make, in the interest of informing the community. When I first joined the forum, I make enough mistakes, and people were kind enough to point them out to me. I still make mistakes, just not the same ones. In fact, it's a good thing, in my opinion, in that (at least for me) it makes me review my assumptions and my factual knowledge. I'd suggest you stick around, as the benefits greatly outweigh the occasional bruises.
     
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  8. Josh66

    Josh66 Been spending a lot of time on here!

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    "It appears that even small mistakes are pounced upon by the experts here and the person making the error is named and shamed in public. After years of being online, I've taken enough of this kind of sh1t and don't want any more...?

    LOL, don't take everything so seriously.

    edit
    There is no reason to be ashamed at making a mistake, but I think it's also unrealistic to expect the mistake to go uncorrected. PMing you to give you a chance to edit your post honestly never even occurred to me. The way you stated it, it seemed to me like you were pretty firm in your belief of it anyway. Deleting all of your posts now is just childish.
     
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  9. David444

    David444 TPF Noob!

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    .
     
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  10. ratssass

    ratssass TPF Supporters Supporting Member

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    ...have a SNICKERS ;)

    ...one thing i'm having trouble wrapping my head around is flash duration,and how it relates to exposure/shutter speed/inverse square,etc.
     
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  11. pgriz

    pgriz Been spending a lot of time on here!

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    My understanding is that studio flashes modulate the intensity by the amount of current they force through the flashtube, but a studio flash has a rather "long" duration (see Studio Flash Duration Chart). In contrast, the speedlights tend to regulate the amount of light emitted by the duration of the pulse. A full-power speedlight 580EX II flash has a duration of about 1/250 sec (with the main part being around 1/1000 sec), and much faster at the 1/128 power end (see Andy Gock : Newcastle Photographer Actual Measured Flash Durations of Small Speedlight Strobes). Simply put, the flash pulse duration is usually much shorter than the comparable shutter speed in use.

    As for the inverse-square law, it applies very well (but it is a mathematical approximation based on a point source). The difference between flashes and the sun, is that the sun (or clouds, for that matter) are much further away from us than is the flash, and difference in distance from the sun is negligible for all the things on earth (even the moon!), whereas the distance between a foreground and background object illuminated by a flash is quite substantial.
     
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  12. Josh66

    Josh66 Been spending a lot of time on here!

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    The flash duration wouldn't really be a factor in exposure, and the inverse square law wouldn't be any different than if we were talking about a desk lamp - the same rules apply, nothing special happens just because of the flash duration.

    As far as shutter speed, that going to be whatever your camera is set to - BUT, if all of the exposure is from the flash, the flash duration effectively becomes the shutter speed. If you're really trying to freeze motion, use the lowest flash power you can get away with (TTL modes will tend to go with the highest power it can get away with).

    My camera has a minimum shutter speed of 1/8000, and x-sync of 1/250... But my flash will blow that away if I can get the ISO high enough and the aperture large enough to allow me to use a very low power on my flash.
     
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