Question about "dragging the shutter" in flash photography

<p>I have done a bit of reading and surfing and searching on the subject, but I have missed something, I think.</p>

<p>If you expose the shot for ambient light, to get the background as part of the shot, and you simply take it that way, then would you get it all well lit, or would the flash power drop off still make the background darker ? If I understand correctly, when you purposely speed the shutter up, the background gets darker and if you slow it down, "drag it", the back ground comes back into play. ( Assuming it is not so far away from the subject, I think. )</p>

<p>When doing all this, ( shutter up or down ) am I still just going with the meter suggested settings ? How does the flash "know" to not blast the room , since the background is part of the scene that the meter is working with ?</p>

<p>I think my missing know how involves understanding how the flash and ISO, aperture, and shutter are linked or not. It seems I recall reading that , " The flash is controlled by the ISO and aperture, and the background is controlled by the shutter speed. " This sounds odd, as I have always thought of the three being linked to get the exposure I want.</p>

<p> </p>

<p>It's not that complicated. I suggest setting iso, aperture, shutter speed and flash power manually and you'll figure it out in no time. Sometimes a little hands on experimentation will be good. Just remember that the flash only burns for a very short time.</p>

 

<p>It is all about proportions of ambient light and flash light contribution.<br>

Shutter speed variation is just a vehicle to help you come up with desired proportions that you want.<br>

There is no magic in the "drugging the shutter", as you are responsible to to know what effect you expect and what results you get.</p>

<p><em>" " The flash is controlled by the ISO and aperture, and the background is controlled by the shutter speed. " This sounds odd" - </em>yes, it is a shortcut of an explanation. ISO and aperture are the vehicles to aid photographer to achieve proper flash light, among other things, perhaps. Shutter speed is a way in this use to cut down the amount of ambient light in a way that does not necessarily alter the flash output, if is withing the X-sync allowed for flash.</p>

<p>Let say it properly, It is the photographer who controls the ambient and flash light contributions/proportions to achieve a desired effect. ISO, aperture or shutter speed are just aids or tools to allow the photographer do the job. There is nothing special in the "dragging shutter speed" - just a shortcut explanation that perhaps does more harm if not explained. Since you are 10 years of photo.net user, this post is live example that the term "dragging shutter speed" did not do good to you.</p>

<p>"<em>How does the flash "know" to not blast the room</em>" - if you want to include ambient light to the flash, then use flash iTTL/Balanced Nikon mode, that will preserve correct exposures for the ambient camera light as much as possible, and will adjust flash light to the lighting level comparable with the ambient light, usually reducing the flash output, but all depends...</p>

<p>If you do not care about the ambient light presence, you could just use iTTL flash mode, and blast the room to achieve proper exposure, but if the ambient light is very high, then the difference between balanced and regular iTTL flash may be small.</p>

<p>Classical balance would be to measure the ambient light level, then to know and reduce the flash output (manually), so both light levels are in acceptable vicinity, totalling to correct and "not blasted" exposure.</p>

Let me try to explain in another way.

When you use flash, the duration of the

flash is in milli seconds. So changing

the shutter speed will not affect how

much light will fall on the sensor. Then

the next question is normally, then why

is there a flash sync speed. The

camera has two curtains. When the

exposure starts the first curtain starts

to open and once enough light is

received by the open position, the

second curtain starts to close. At low

speeds there is some time that the

sensor/film is fully exposed. Flash

needs this to expose the whole frame.

For very high speeds, the curtains are

both moving together more like a slit.

So if you flash there, then it won't

expose the whole picture. Try it.

 

So shutter speed does not affect the

exposure for flash. What does is the

brightness of the most reflective items

in the scene. In ttl modes normally an ir

light is used to take a reading and the

flash output is set to ensure that the

highlights are not blown out.

 

But as we all know regular exposure

uses shutter speed also. So when you

are dragging the shutter what you are

doing is to combine the flash exposure

for the highlights with the ambient

exposure for the backgrounds. TTL will

not blast the room if you change the

shutter speed since flash calculations

are based on highlights and not based

on shutter speed.

<p>Lets turn the approach around backward to the current way you are thinking. In a former life shooting dozens of weddings a year, one of the important technical things in exposure was to have both the groups and the sanctuary properly exposed in the same frame.<br>

Obviously, the people were the more important, so flash exposure on them was the basis for proper exposure of the scene.<br>

However, to rely on that exposure combination of f stop (control of flash brightness) and shutter speed (option for background brightness) would often leave the background deeply in shadow or totally black. So the ambient light on the background would ALWAYS be metered separately with a hand held incident meter such as a Luna Pro to determine the base exposure combinations(f stop and corresponding shutter speed) for the background to be exposed by ambient light (no effect from the flash and I'll get to that shortly). At that point, it is a simple matter to match the f stop needed for the flash exposure to the same on the meter and use the corresponding metered shutter speed which would guarantee proper exposure on the background. For esthetic reasons, I would opt for one speed faster than the optimal choice so that the groups would stand out from a slightly deeper background tonality. </p>

<p>This was on the Hasselblads that I used in those days, so with today's digital cameras, I would use exactly the same technique but using the camera in total manual settings.</p>

<p>Now, as to why the flash has no effect on the background. You have to know your physics of light in that the brightness falls off by the square of the distance from the source. It simply means that the flash will be set for the distance from it's source to the subject and the camera will be set for that correct f stop. If the flash is properly exposed at ten feet from the subject, the light it produces will have spread so much by the time it gets to ten feet behind, you will be twice as dim in brightness and at twenty feet, it will be four times as dim. The flash is of no real effect on a background unless that background is almost on top of the subject which is why the dragging of the shutter speed is used to build up the ambient light in the background to match or compliment the flash exposure.</p>

<p>The technique is actually quite simple, but you have to take command of the camera functions and not let them be in command of your thinking.</p>

<p>Or you could bounce your flash, getting a much more even light all round and a far softer lighting on your subject. Custom white balance means you don't even need a pure white surface to bounce off these days.</p>

<p>The central issues here are:<br /><br />1) The distance between the subject (say, a person in front of the camera) and the background behind that subject. The flash power falls off at the square of the distance, and quickly becomes insignificant (to the background) in a large room, with a relatively up-close subject.<br /><br />2) A lot of this depends on whether you're asking the camera's metering, flash system, and distance-aware lens data to impact the power of the flash. If you're setting the exposure <em>and</em> the flash power manually, you'll very quickly get a feel for what's going on. But if you manually meter for the ambient light, and then fill with flash <em>while letting the camera's software decide on the flash power</em>, then this can seem (and is) a lot more capricious. The good news is that onboard camera software is <em>really</em> smart these days, and this works surprisingly well a lot of the time.<br /><br />When you're just getting your head wrapped around this stuff, it goes a lot faster if you set ISO, aperture, shutter <em>and</em> flash power manually. The relationships between those things and your subject, room, and ambient light become very clear, very quickly. <br /><br />When I'm dragging the shutter, my usual habit (with Nikon's iTTL system in play) is to set exposure manually (usually starting with the camera's own metering of the ambient light as a kick-off point), then use the speedlight in TTL mode. Becuase it's so easy to chimp the results, I can take a quick look, and then use the flash power compensation to push the flash up or down a stop or so as needed. That way the camera is helping me out with varying distances between me and the foreground subject.<br /><br />It's important to note that - with Nikon, anyway - part of the math it does is based on getting a read from the lens on the distance at which it's focused. This isn't always an option with older lenses, but anything made recently will provide that info to the camera's computer, and thus the system does all the heavy lifting for you. Works really well under most circumstances.</p>

<p><em>If you expose the shot for ambient light, to get the background as part of the shot, and you simply take it that way, then would you get it all well lit, or would the flash power drop off still make the background darker ?</em></p>

<p>The background would be well lit. If the subject and the background are both in the same or similar level of light, the subject may become overexposed because you've added another layer of light on the subject (ambient plus flash--light is additive), and you correct for it via aperture (making the aperture smaller), the background will appear to be darker, since you have changed the balance of exposure between the subject and background.</p>

<p><em>If I understand correctly, when you purposely speed the shutter up, the background gets darker and if you slow it down, "drag it", the back ground comes back into play.</em></p>

<p>This is true. However, sometimes you have very bright backgrounds and depending upon the balance of exposure of the background to subject, you could be dragging the shutter using 1/250th. Don't get stuck on specific shutter speeds. The important thing is to understand the relationship between background and subject exposure.</p>

<p><em>When doing all this, ( shutter up or down ) am I still just going with the meter suggested settings ?</em></p>

<p>Depends how you metered. Using automated modes (aperture priority for instance) will not allow you to control that balance between background and subject. If you metered the background and subject separately, and then figured out that balance yourself, you would be controlling that balance. In practice, some use of automated modes can still get you what you want, but you should understand what the automated mode is doing and understand how to use that mode to get what you want.</p>

<p><em>How does the flash "know" to not blast the room , since the background is part of the scene that the meter is working with ?</em></p>

<p>Depends on the metering system your flash uses. With ETTL, for instance, part of the flash metering algorithym includes separating out background and subject (or attempting to).</p>

<p><em>I think my missing know how involves understanding how the flash and ISO, aperture, and shutter are linked or not.</em></p>

<p>I suggest you go to Neil van Niekerk's website/blog and look for the tutorials about using on camera flash. Specifically search for 'dragging the shutter'.</p>

<p>Dragging the shutter basically makes the flash be the shutter shooting in a somewhat dark/dim environment. If ambient light as in the background is too bright, you may get an odd double exposure effect that only shows up along light/dark edges of objects in the foreground where the flash and ambient light overlap.</p>

<p>You may not see this double edge artifact until you view the image zoomed in on your computer or zoomed in on the camera's LCD. This is why you want to shoot manual on this technique so you can adjust. </p>

<p>Just an FYI I discovered experimenting with this cool exposure technique stopping a spinning fan blade dead in its tracks in a dark room at 1/2's shutter speed. I'm not an expert on this technique mainly because I don't find myself needing to shoot in dark environments using flash, but it's an interesting study on light and motion which if you want to stop motion shooting in the dark, this is the technique to use. </p>