Why Does Dragging the Shutter Work?

I know to do it and how to control it, but I don't understand the concept behind it. I like to expose the scene so that ambient is about two stops dark. What does it matter if I achieve that through shutter speed, aperture, ISO, or anything else? If the scene is metering two stops dark (and I'm using ETTL, btw) does it really impact the quality of the image if I did that through a wider aperture vs. a faster shutter speed?

It depends on how much of the ambient you want to be part of the photo. Some people, don't want their background to be underexposed by 2 stops. Some like to still retain detail in the background.

Nothing to do with the quality of the image, just what the overall image looks like.

I've dragged the shutter in a few of my shots, just to bring out some other lighting that was around, etc.

Honestly I don't understand the question.

Peacefield wrote in post #11742032
I know to do it and how to control it, but I don't understand the concept behind it.

You are lowering your ss to get more ambient in. You can do same by using wider aperture or higher ISO. Usually your aperture will be based on the DOF that you need. For shooting single person shot, I may use wider aperture, higher ISO or lower ss or combination of all these to get more ambinet in. Most times you lower ss as you are in low light and probably at the higher ISO limit of your camera or what you think will provide you nice quality prints.

Hope this is clear.

Now here I am confused. Here you saying you want to lower ambient (opposite of dragging the shutter where you want to bring the ambient up). You mention using wider aperture which will bring up the ambient (like dragging shutter explanation that I gave up) vs faster shutter speed which will lower ambient like when trying to knock down ambient.

All the attributes (ISO/A/T) can be adjusted to give the desired level of brightness in the frame, when dealing with ambient light only.

You have to creatively control the Aperture for depth of field. With the desired depth of field set, the ISO/Shutter will be adjusted to capture the desired brightness in the scene.

Add strobe power into the mix. Aperture is still your depth of field control, but becomes tied to the strobe power to ensure correct subject exposure. The size of the aperture determines how much light gets to the sensor/film plane over the duration of the strobe burst.

The shutter timing is set to capture ambient light.

The strobe duration may last only 1/1000th of a second to give correct subject exposure.
Ambient may need 1/125th to correctly expose.
The difference between the 1/1000th strobe exposure and the 1/125th ambient exposure is termed "dragging". Effectively 2 exposures in 1 shot.

ISO is sensor/film sensitivity so it has an effect on both Aperture and Shutter when changed.

Shutter speed is the only variable that does not affect flash exposure; ISO and aperture do. That's the reason it's used to control the ratio of flash light to continuous (=natural) light.

Tobi

Here is an extreme example of dragging shutter.
I think the shutter was about 1.5 or 2 seconds.
Rear Curtain sync on the speedlite.

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Tobi. wrote in post #11742167
Shutter speed is the only variable that does not affect flash exposure; ISO and aperture do. That's the reason it's used to control the ratio of flash light to continuous (=natural) light.

Tobi

There is max sync speed to consider when exposing in bright ambient situations. Getting good exposure at max sync can mean using filters or HSS which does affect the flash exposure as a result.

Negativ3 wrote in post #11742240
There is max sync speed to consider when exposing in bright ambient situations.

This doesn't affect exposure as such though, but only cuts off the light below a certain line where the shutter curtain casts a shadow on the sensor/film.

HSS is a totally different story and has nothing to do with 'dragging the shutter' (A terrible name IMHO). Deliberately chosing a long shutter speed that is still beyond max sync so it requires the use of HSS is either nonsense or the application is so weird that it won't be a problem for the person using it.

Of course filters do affect overall exposure. What's left to control ambient light but not flash light is the shutter speed. There's simply no other way to balance a short impulse light source and a continuous light source other than to do it via exposure time.

Tobi

Tobi. wrote in post #11742342
This doesn't affect exposure as such though, but only cuts off the light below a certain line where the shutter curtain casts a shadow on the sensor/film.

HSS is a totally different story and has nothing to do with 'dragging the shutter' (A terrible name IMHO). Deliberately chosing a long shutter speed that is still beyond max sync so it requires the use of HSS is either nonsense or the application is so weird that it won't be a problem for the person using it.

Of course filters do affect overall exposure. What's left to control ambient light but not flash light is the shutter speed. There's simply no other way to balance a short impulse light source and a continuous light source other than to do it via exposure time.

Tobi

Agreed, but when trying to figure out how these things work, it is necessary to understand how to maintain good, full frame exposure above max sync.

bobbyz wrote in post #11742142
You are lowering your ss to get more ambient in. You can do same by using wider aperture or higher ISO.

Raising the camera's ISO won't allow for lighter backgrounds as both the subject (which is effected by the flash) and the background will remain the same..

Flash exposure is controlled by aperture, ambient by shutterspeed not ISO..

You can accurately control the brightness of the background by selecting a shutterspeed which is low enough to register the ambient light of the background.. It's really quite easy to do.. With your camera on manual. meter the background.. If you have TTL flash metering use those settings.. If you set up you background too brightly, you may have to underexpose the flash output..

Tobi. wrote in post #11742167
Shutter speed is the only variable that does not affect flash exposure; ISO and aperture do. That's the reason it's used to control the ratio of flash light to continuous (=natural) light.

Tobi

yogestee wrote in post #11742380
Raising the camera's ISO won't allow for lighter backgrounds as both the subject (which is effected by the flash) and the background will remain the same..

Flash exposure is controlled by aperture, ambient by shutterspeed not ISO..

You can accurately control the brightness of the background by selecting a shutterspeed which is low enough to register the ambient light of the background.. It's really quite easy to do.. With your camera on manual. meter the background.. If you have TTL flash metering use those settings.. If you set up you background too brightly, you may have to underexpose the flash output..

People seem to be forgetting that the level of ambient lighting is usually fixed but that of the flash is not. Flash to ambient ratio can also be controlled by changing the power level of the flash. Or by moving the flash closer or farther from the subject if the flash output is fixed.

Changing the aperture also affects the amount of ambient light captured, also. It just doesn't control the amount of flash captured. And changing the ISO also affects both.

yogestee wrote in post #11742380
Flash exposure is controlled by aperture, ambient by shutterspeed not ISO..

No.

ISO and aperture affect both flash exposure AND ambient.

The sentence I quoted from you is one of the most common misconceptions about flash photography.

Tobi

Tobi. wrote in post #11742508
No.

ISO and aperture affect both flash exposure AND ambient.

The sentence I quoted from you is one of the most common misconceptions about flash photography.

Tobi

I agree with Tobi. If you change aperture or ISO, flash exposure will be effected. Issue is ambient will also be effected by the same amount. Now if one wants to change the ratio between ambient and flash then ss will let you do that. Leo also mentioned control of flash power (through distance of the flash to the subject or adjusting the flash power itself on the flash unit).

Yogestee is saying it wrong. People should just do simple experiment rather than reading something on the web (some from very well known blogs) and quoting that.

There's no agreeing / disagreeing with Tobi.. It's a fact, regardless of if you agree or not... ISO, aperture, shutter speed all have an impact on flash exposure, just like ambient exposures.The impact of shutter speed on a flash exposure can have minimal impact from ~1/100 to 1/250, etc.. but it's still present.

Peacefield wrote in post #11742032
I know to do it and how to control it, but I don't understand the concept behind it. I like to expose the scene so that ambient is about two stops dark. What does it matter if I achieve that through shutter speed, aperture, ISO, or anything else? If the scene is metering two stops dark (and I'm using ETTL, btw) does it really impact the quality of the image if I did that through a wider aperture vs. a faster shutter speed?

Let's assume there is a certain level of ambient light, and the use of ISO200 and 1/100 f/4 captures 2 unit of background light on film. We have a flash, and it outputs light so that at ISO200 with f/4 results in the sensor seeing 4 units of light, and that four units of light constitues 'proper exposure'. So our background is underexposed by 2 units of light while the main subject is properly exposed. Stated in terms of EV (light values) the subject is 0EV and the background is -1EV. Now let's analyze how we might try to alter the balance of subject:background brightness (not an exhaustive list of possible ways to handle, but just a few!)...

1. We can speed up the shutter to 1/200. This causes the background to be further underexposed by -1EV. Since the flash (which is faster than 1/200) outputs the same amount of light on the subject, relative to the subject (which is at 0EV) the background is now at -2EV
   
   
2. We can change aperture to f/5.6. This causes the background to be further underexposed by another -1EV. The ETTL flash metering sees the less light, too, so it asks the flash to output more light (+1EV) which offsets the slower aperture, so the subject is still exposed to 0EV. But now the background is at -2EV, relative to the main subject.
   
   
3. We can change the ISO to ISO100. This causes the background to be further underexposed by another -1EV. The ETTL flash metering sees the less light, too, so it asks the flash to output more light (+1EV) which offsets the slower ISO, so the subject is still exposed to 0EV. But now the background is at -2EV, relative to the main subject.