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Dragging the shutter, and flash for dummies (by a dummy)

William Au , Sep 04, 2004; 02:10 a.m.

There have been a couple of questions about dark backgrounds and dragging the shutter lately, so I thought I'd take a shot at a "weekly wedding theme" - DRAGGING THE SHUTTER. (I'm no technical or lighting wizard, so if any of the master photographers in here want to chime in/correct me/put me in my place, please feel free to do so). And sorry in advance for the length...


Unless you're at a disco, most light sources around us (the sun, a lamp, a candle) tend to emit light at a relatively constant rate. A candle puts out a constant, very low level of light. The sun pours forth an avalanche of light onto the Earth in the middle of the day. The challenge, when taking a picture, is to make sure you get "just enough" light onto your film/sensor. Your trusty camera and lens let you do this by

  1. controlling how much light gets in (aperture); and,
  2. controlling how long the light gets in (shutter).
So when using available light, you set your shutter and aperture appropriately, bang, and voila!... there's your perfect, unposed, photojournalistic picture of the bride and groom with the perfect expressions on their face.

Problem with weddings is the sun is not usually an invited or reliable guest, and the church is lit by lamps/windows as dim as candles or candles themselves. "Gosh, getting that perfect shot of the bride sure is hard when you have to use ISO 50000 film and a shutter speed of 15 seconds!" Well, no problem. Let's bring our own portable sun into the church: your trusty flash! Slap it on, set your shutter, aperture, and flash appropriately, and bang! There's your perfectly exposed image indoors. Magical, mystical devices are these flashes.

An important difference between your flash and the sun is that your flash does not pour light out constantly. If you're looking for lights with constant output, there are hot lights that you can use - but these require you to plug them into the wall, they tend to be really hot, and are pretty dim for their size - not always ideal for weddings. The portable flash gets around all of these problems:

  • it's small
  • it runs on batteries, and
  • instead of pouring out a bunch of dim light for a long time, it packs a big punch of light into a short burst
How does your flash know how much light to put out? Let's just assume you're using an auto or TTL flash. What happens is when you press your shutter, your camera sends an electric current to the flash which tells it to GO! Your flash opens the flood gates and light goes pouring out of it. This light strikes your subject(s), and some of it bounces back to your camera. Now the sensor on your flash (auto) or in your camera (TTL) measures this light coming back, and when it figures enough of it has come back to make a proper exposure, it yells at your flash to STOP! If you have a good sensor, just the right amount of light is flashed out and you will get a good exposure.

Most flash bursts are very short - they pack in all the light you need into a very small window of time. One of the benefits of this is that to a large degree, your shutter speed no longer affects the exposure of your subject anymore. Whether you shoot at 1/1000th of a second or 1/60th of a second, the flash puts out the same amount of light. The only thing that affects your subject's exposure is now aperture, and if you set your lens and flash accordingly, you should get the proper exposure of your subject regardless of shutter speed. Here's where numbers and a highly simplified example may help:

Let's say to get a proper exposure, you need 1000 units of light to reach your film. Now let's say you're in a dark church, and when you focus on the bride and set your aperture at f8, 100 units of light reach the film every 1/10th of a second. In order to get a proper exposure of the bride with this available light, you'd have to leave your shutter open for 1 second - not really great for handholding or freezing motion!

Instead, you pull out your flash and it packs 1000 units of light into 1/1000th of a second. So instead of having to leave your shutter open for 1 second, you can fire your camera at a much faster shutter speed - thus freezing motion and avoiding camera shake. Life is good (until you realize your shots look flat and boring from on-camera frontal flash - but that's another thread...).

Now the only trick your camera has to do is make sure that the shutter is open at the same time as when the flash burst goes off. Shooting at 1/1000th of a second with flash would often be nice, but most cameras can't reliably synchronize the shutter with the flash at that speed (I'm not going to touch on the new high-speed sync mechanisms here). Most cameras therefore have a maximum sync speed, which is the fastest shutter speed at which it will reliably time the flash burst so that it occurs when the shutter is fully open. On my old manual cameras, this is 1/60th of a second. Some newer models go up to 1/250th of a second and beyond.

The great thing about a faster sync speed is that it lets you do fill flash outdoors more easily. When I?m shooting with my old cameras, setting the shutter speed at 1/60th of a second on a bright sunny day often requires setting a tiny aperture to get the right exposure. If you like everything for miles to be in focus, this may be your thing, but I prefer to shoot at wide apertures. A faster sync speed lets me shoot at wider apertures without blowing out the exposure.

Getting back to indoor flash exposure... it's dark inside, but hey, you've got your flash that puts out 1000 units of light in a heartbeat. You set your camera at its max sync speed to make sure your camera can catch the flash burst, shoot flash all day long, and everything else being equal, your subjects will come out properly exposed. There's the kicker though: your subjects. You get your pictures back, and lo', your bride and groom look great, but you don't recall them getting married in a church that looked like a dark cave. What happened??

Your trusty sensor in your flash/camera is single-minded: it lets your flash keep pouring out light until it thinks enough light has bounced back to make the proper exposure. If you're shooting a bride and groom at the front of the church, and they are standing 30 feet away from the background, a lot more light will bounce back from the couple than from the far away background. Once enough light has bounced off the couple back to the camera, your flash will shut down ? this results in the couple getting enough light to look right, but your background sure hasn't had enough light bouncing off of it for it to be properly exposed. This = good looking couple & poorly exposed background, which ends up looking like a dark cave.

This may be a good thing if your background is really ugly/distracting. But a lot of church backgrounds aren't that bad, and people get sick of looking at cave shots after awhile. Now remember that your flash has packed all the light it needs to expose your subject properly into a very short burst. Using our example, within 1/1000th of a second, your flash has already exposed your couple properly. You set your camera at its top sync speed at which it can synchronize the flash burst and shutter and everything is hunky dory. But dang, that background is still dark?

But wait, even though your flash puts out a lot of light, there is still ambient light in your church. If you want to get your background better exposed, you can take advantage of this ambient light: set your shutter at a speed that is slower than the maximum sync speed, so that you let in some of the light from the background. This is termed dragging the shutter. Instead of shooting at 1/60th of a second, you can set your camera to 1/8th of a second - that's 3 stops more light that you're letting in from the background. In some cases, this may be enough to render the background bright enough to look decent/really nice. The ambient light, for the most part, will have very little effect on your subjects. The exposure for your subjects is from the flash, while the exposure for your background is from the ambient light - you drag the shutter long enough to give this ambient light a chance to burn in the background.

The trade-off with dragging the shutter is that you're often back to using really slow shutter speeds, meaning camera shake and subject movement can become factors again. The good thing about the flash burst is that it freezes your subjects at the position they were at during the flash burst. Sometimes ghosting can appear around the frozen image, which can either add or detract from the image. And with practice, you can get pretty steady at reliably handholding your camera at slower shutter speeds - that or you can use a tripod.


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William Au , Sep 04, 2004; 02:14 a.m.

Example without dragging the shutter

It's not the greatest example, but here is a shot in my house where I didn't drag the shutter - moose in chair in dark cave.

ISO 400, 1/125th of a second @ f5.6

William Au , Sep 04, 2004; 02:16 a.m.

Example dragging the shutter

Here I used a slower shutter speed (1/4 second - handheld), and the background is far more visible. Now just imagine the moose is a bride and my dining room is a church... heh, that sounds funny - I need to sleep...

dragged shutter at 1/4 second @ f5.6

Rodolfo Negrete , Sep 04, 2004; 03:47 a.m.

I thank you very much for this. this is one of those things that I wanted to have more knowlege about.

asoon as a get more time I am definitly going to study it and put it in practis.

Nikos Peri , Sep 04, 2004; 06:05 a.m.

This dummy thanks you too, from the cavernous depths of his center of worship - the dining room.

Steve Levine , Sep 04, 2004; 08:49 a.m.

Think of each flash picture you take as a double exposure.The shutter opens and closes,and allows the ambient light in.When the shutter is completely opened,our flash fires,hence a double exposure.Flash is either the main light,or the fill light,depending on how much ambient light there is.Always including the ambient level into our flash calculations,is the desired way to balance things.And balance between ambient & flash exposures,is where its at.The closer they are balanced,the more natural the lighting appears.

Shun Cheung , Sep 04, 2004; 09:00 a.m.

In real wedding situations, I probably wouldn't use 1/4 sec though. Unless that is the effect you want, when there are people in the background, they'll probably be blur due to movements. Camera shake will also be an issue at 1/4 hand held. Something like 1/15 is probably more appropriate.

Shane Gilbert , Sep 04, 2004; 09:40 a.m.

Good show ol' chap.

Jon Austin , Sep 04, 2004; 12:14 p.m.

William: Excellent write-up. I'd just like to add that when dragging the shutter, the subject(s) will often move after he/she/they see the flash, believing the capture is complete. This is one of the causes of the ghosting you've described, which happens when the subject moves while the shutter is still open to capture the background.

To avoid this, I'll set my camera to second curtain sync, so that the ambient light is captured first, and the flash doesn't fire until just before the second shutter curtain closes. The only issue here is if the flash fires a pre-capture burst to meter the exposure; then the subject gets confused/annoyed by the double flash. Infrared metering capability is much better/more desirable.

Finally, I've learned to use Manual mode almost exclusively for indoor / dark flash applications. I set the aperture and the shutter speed I want (f/4 to f/8 and 1/60 to 1/90), based on the depth of field and ambient exposure desired, turn on the flash and fire away. When metering, the camera almost always "panics" that there isn't enough light to expose the subject properly, but I consistently get the green "good exposure" confirmation light and well exposed images.

Ralph Wilson , Sep 04, 2004; 01:42 p.m.

Good job William, It's great to see this forum used as tool for learning and teaching. Keep it up.

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