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Flash and Shutter Speeds
Old 04-21-2007, 08:35 PM   #1 (permalink)
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I just have a "newbie" question. I understand the concept behind using the flash sync speed on the camera. However...

Since my camera has a sync speed of 250, what (if any) benefit would there be if I used a shutter speed of say 60? From what I have read, the flash is so fast, that it doesn't matter what I set the camera to. On the other hand, wouldn't a slow speed let more ambient light in? (or would the flash wash it out anyways?)

Thanks!
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Re: Flash and Shutter Speeds
Old 04-21-2007, 09:02 PM   #2 (permalink)
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Well, your shutter speed will still be a factor with regard to the ambient light (if any). For example, if you took a shot outside at dusk. You want to capture the light coming of the buildings, sky, signs, whatever. This is where you would set a lower shutter speed. Say you want to take a picture of someone and behind them is a busy street. Setting the shutter speed to something slow would blur any fast moving cars or people. This is called dragging the shutter. hope this helps
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Re: Flash and Shutter Speeds
Old 04-21-2007, 09:10 PM   #3 (permalink)
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Quote:
Originally Posted by dighost View Post
From what I have read, the flash is so fast, that it doesn't matter what I set the camera to.
It matters a great deal. It doesn't matter how fast the actual burst is. If the flash fires at any time that the shutter is not fully open (film/sensor not fully uncovered) then the flash will only light part of the image. At shutter speeds faster than the camera's sync speed, the shutter never opens fully. It acts as a slit that travels across the frame.

Quote:
On the other hand, wouldn't a slow speed let more ambient light in?
Yes.
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Re: Flash and Shutter Speeds
Old 04-21-2007, 09:31 PM   #4 (permalink)
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Quote:
Originally Posted by dighost View Post
I just have a "newbie" question. I understand the concept behind using the flash sync speed on the camera. However...

Since my camera has a sync speed of 250, what (if any) benefit would there be if I used a shutter speed of say 60? From what I have read, the flash is so fast, that it doesn't matter what I set the camera to. On the other hand, wouldn't a slow speed let more ambient light in? (or would the flash wash it out anyways?)

Thanks!
The faster sync speed can be useful when shooting outdoors using flash. Suppose you have a model in the shade and you use flash to light her. The camera is set at f2.8 because you want to blur the background. But you want the background to be properly exposed. If there is bright sun outside the shade and that will be part of the background, then you need the highest shutter speed you can get to properly expose the background. Even at 1/250this may not be possible. So you would have to stop down to get the background right. That means changing the power of the flash accordingly.

If shooting in low light, then dragging the shutter, already mentioned comes into play.

The basic rule is always this: Determine the fstop needed for the flash at a given distance from the subject. Then if you want to properly exposed areas lit by ambient light, meter them at the fstop already selected to get the shutter speed at that fstop for proper ambient lit exposure. There you have it.

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Re: Flash and Shutter Speeds 


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Re: Flash and Shutter Speeds
Old 04-22-2007, 08:52 AM   #5 (permalink)
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Quote:
Originally Posted by dighost View Post
--snip--
Since my camera has a sync speed of 250, what (if any) benefit would there be if I used a shutter speed of say 60? From what I have read, the flash is so fast, that it doesn't matter what I set the camera to.
--snip--
Thanks!
Your maximum camera sync speed is really only applicable when using smaller flash units like a shoe-mounted flash. These small units emit all of their light very quickly, typically in the range of 1/2000 to 1/100,000 of a second, so the flash duration is extremely short.

The story changes with studio lighting gear (monolights or generator/head systems). Perusing the specs, you'll find a number for flash duration. But unless you specifically read otherwise, that number is the amount of time it takes for the flash to discharge 50% of it's full power. It's known as the "t 0.5" value. (From a standards perspective, the number followed by "t" is the amount of energy left in the capacitor banks. "0.5" means half). What this means is that the flash still has 50% of it's power left to discharge. That is quite a bit of light left that can affect your exposure. The "t 0.5" value is also known as the effective flash duration.

There is another number used in the lighting industry - "t 0.1" - which is the amount of time it takes for the unit to discharge 90% of it's full power (leaving 10% of the energy in the capacitor banks). The "t 0.1" value is also known as the total flash duration. You usually won't find this number specified by a manufacturer because it isn't a sexy. The t 0.1 duration is about 3 times longer than the t 0.5 value.

So what does this all really mean? It means that if you want consistent exposure you need to know your strobe's total flash duration and use a shutter speed slower than that value. Let's say, for example, that you own a monolight that advertises a flash duration of 1/500th. of a second. Unless the manufacturer specifically says that this is the "t 0.1" value, your total flash duration is in the neighborhood of 1/160th. of a second at full power. If you use a camera sync speed of 1/250th., your film or sensor won't receive the full amount of light emitted by the strobe. You will, in effect, under-expose the shot because your shutter curtain closes before the strobe has discharged all of its energy.

When you dial down your strobe, say to 1/2 power (reducing exposure by 1 stop), the flash duration decreases because the strobe interrupts the discharge. Now your total flash duration might be 1/300th. of a second. But your resulting exposure won't be 1 stop darker, it might only be 2/3 stops darker. This is about the time you start cursing your strobe or meter as being inaccurate.

Another side effect of using too high a sync speed is exposure falloff in the direction of shutter travel. As the second shutter curtain travels across the film (or sensor) plane, it prevents exposure to part of the sensor. Let's say that the shutter travels from top to bottom. The bottom of the sensor receives more light than the top of the sensor, causing light falloff.

In locations with low ambient lighting, you are almost always safe using a shutter speed of 1/60th. of a second with any brand of strobe equipment. That is, just about every strobe out there has a (t 0.5) value of at least 1/300th. of a second. But how much head room you have above 1/60th. depends on your strobes. Here are a few monolight examples of manufacturer and advertised flash duration. I've picked models where the number is the advertised w/s power rating.

Interfit Colorflash 500 -- 1/500
Bowens Esprit 500DX -- 1/700
Profoto CPPS 600 -- 1/850
Elinchrom Style 400BX -- 1/900
Hensel Integra Pro 500 -- 1/1600
Elinchrom Style 600RX -- 1/2050

I recently had the opportunity to play with the new Canon 1D MkIII. The Canon rep made the point of noting that the camera is being advertised as having a sync speed of 1/60th. of a second with studio strobes and 1/250th. of a second when using a speedlight. When pressed as to the reason why he said it's because of the longer flash durations exhibited by studio equipment. I also suspect they'll use it as a reason to recommend purchasing speedlights to the less well informed.

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Re: Flash and Shutter Speeds
Old 04-22-2007, 11:38 AM   #6 (permalink)
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All your answers are gold. This stuff just isn't in any book (at least that I have found).

This really clarifies the sync-- and speed concept to me. I have been using different speeds, but not with any reason. I have noticed the exposure problems, and you can bet I have cursed my meter to no end

Thanks again!
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Re: Flash and Shutter Speeds
Old 04-22-2007, 03:46 PM   #7 (permalink)
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Just for the record since lots of us use Alien Bees, the t0.5 numbers are:

B400 - 1/6400th second
B800 - 1/3200th second
B1600 - 1/1600th second

Here is what Alien Bee's websie says about T values

"Flash manufacturers do not rate flash duration as the entire time the flash tube is emitting light however, because the end of the slowly dimming 'tail' does not contribute much to the overall exposure (or to blur). t.1 is defined as the time during which the flash lamp output is above 0.1 (or 1/10) it's peak intensity. t.3 you may guess is the time the output is greater than 0.3 (about 1/3) peak, and t.5 is how long it's above 0.5 (half) the peak value."

I've shot with many different strobe systems over the years. I have never had any problems with consistency of exposure when shooting at sync speed or slower. I usually try to shoot at 1 stop below the maximum because I'm comfortable with that. It has never failed to work.

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