How is flash used to freeze movement

ziggy©

ziggy©

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I have been adviced that a flash can be used to freeze movement. I have a 580EXII and would like to understand how the flash can be used specifically for macro photography.

I understand the basics but would appreciate some more information on the following

- According to tutorials i have read, when you press the shutter button the camera will only meter for the background whereas the flash will be used to meter the foreground. Is this correct or have i misunderstood it? If correct, how does the camera know at what distance the background starts and how does the flash know how far is the foreground? Does this also mean that the flash can only be used to illuminate the foreground?

- I also found that the shutter speed has no impact on the exposure if flash is used. i dont have my camera with me to test this but doesnt this mean that you can have a shot where the background is blurry and the foreground is not?

- How can i use the flash to freeze movement?

Thanks
 
ziggy©;3698944 said:
- According to tutorials i have read, when you press the shutter button the camera will only meter for the background whereas the flash will be used to meter the foreground. Is this correct or have i misunderstood it? If correct, how does the camera know at what distance the background starts and how does the flash know how far is the foreground? Does this also mean that the flash can only be used to illuminate the foreground?
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Using background/foreground is a bit confusing here, the camera will meter for the ambient light and and then pop the flash to that same level, you can tell the camera to underexpose the ambient but still pop the flash to make whatever it reaches a correct exposure, what the flash can reach will tend to be the foreground if you are using an on camera flash.

If you use off camera flashes you can use them to light up the background (bit ambitious if the background is a mountain range, less so if its a room)

ziggy©;3698944 said:
- I also found that the shutter speed has no impact on the exposure if flash is used. i dont have my camera with me to test this but doesnt this mean that you can have a shot where the background is blurry and the foreground is not?
Click to expand...

no impact to a point, faster than 1/200th or so you might begin to see the flash catching the shutter (or the flash enters high speed sync mode)

Yes, you are correct, you could have a 1 second handheld exposure where everything is all wavy and wobbly but just one flash pop off which only lasts 1/1000th of a second or less and that bit that was flashed should look very sharp.

ziggy©;3698944 said:
- How can i use the flash to freeze movement?
Click to expand...

As above the flash itself is very very quick, something like a 580ex on minimum power is 1/28000th of a second, but as you increase the power the flash duration increases. To do this and not get the blurry background effect you need to control the ambient light and get it darker then pop the flash as the correct exposure.

This all gets a little fiddly so might be a bit over the top, if its just normal stuff you are shooting then just firing the flash as normal will probably be enough, if you are wanting to freeze bullets then its quite a lot more faff!
 
nano said:
As above the flash itself is very very quick, something like a 580ex on minimum power is 1/28000th of a second, but as you increase the power the flash duration increases. To do this and not get the blurry background effect you need to control the ambient light and get it darker then pop the flash as the correct exposure.
Click to expand...

Excellent explanation!

I think i sort of understand the basics of it now. What i am still not sure of is how do i know how much power i need to use to get a correct exposure. Especially if when shooting during the day with lots of sunlight.

For example, if there is a lot of sunlight that is causing the subject to overexpose, how can the flash be used to reduce the light hitting the subject? I know with the camera metering you reduce the exposure settings but with the flash if you reduce the flash power the harsh light from the sun is still hitting the subject.

Thanks
 
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For macro try - body in full manual, spot metering with ISO200, f16 and a shutter of 1/200 set the flash to ETTL and play - the flash should automatically determine its own output to expose correctly but you may need some FEC
 
Dogfish_magnet said:
For macro try - body in full manual, spot metering with ISO200, f16 and a shutter of 1/200 set the flash to ETTL and play - the flash should automatically determine its own output to expose correctly but you may need some FEC
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thats generally what i end up doing but maybe bumping up the iso to get some ambient light in the scene to make it look as natural as possible rather than nuknig the scene as with macro there tends to be very little light with the close focusing distances.
 
You can go as low as 1/15sec with flash, this was shot of people dancing at a wedding i did with a friend last year, but 1/40sec would have been better at the time i think.

 
Dogfish_magnet said:
For macro try - body in full manual, spot metering with ISO200, f16 and a shutter of 1/200 set the flash to ETTL and play - the flash should automatically determine its own output to expose correctly but you may need some FEC
Click to expand...

How will the flash determine its output? Will it use the settings i choose for shutter and aperture or does it do its own metering?

And also what is the benefit of using Manual mode in this situation? The only reason i can think of is that the camera doesnt then choose a silly shutter speed that is not hand holdable.

Presumabely f16 and 1/200 will only be correct if they result in correct exposure. If not i then have to either decrease the aperture or increase ISO.
 
ziggy©;3703360 said:
How will the flash determine its output? Will it use the settings i choose for shutter and aperture or does it do its own metering?

And also what is the benefit of using Manual mode in this situation? The only reason i can think of is that the camera doesnt then choose a silly shutter speed that is not hand holdable.

Presumabely f16 and 1/200 will only be correct if they result in correct exposure. If not i then have to either decrease the aperture or increase ISO.
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Manual is is the best way to keep control of your DOF and shutter speed. Obviously you have a limited DOF with macro anyway, and if you use shutter priority you risk the camera choosing a ridiculously low f-number, and you get an OOF shot. Conversely, if you use aperture priority the camera might choose a ridiculously long shutter speed, and you get a blurry shot.

In manual you control both the DOF and the shutter speed, and all that's required of the flash is to look at the scene with the settings you've chosen, and decide how much power to use so the shot is exposed correctly.

That's my understanding of it. :)
 
GrittyShaker said:
In manual you control both the DOF and the shutter speed, and all that's required of the flash is to look at the scene with the settings you've chosen, and decide how much power to use so the shot is exposed correctly.
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How does the flash decide how much power will be required?

Lets assume i want to use f16 and 1/200 which will give me enough DOF and a fast enough shutter speed to handhold the camera. Lets also assume that these settings without the flash will not produce the correct exposure i.e. the image will be dark and i want the flash to produce the remaining amount of light that will be required for the shot to have correct exposure. How does the flash know what amount of light it will need? Does it have its own metering system?

I cant wait for this bank holiday this weekend to try out everything i learnt from this thread :lol:
 
ziggy©;3707281 said:
How does the flash decide how much power will be required?

Lets assume i want to use f16 and 1/200 which will give me enough DOF and a fast enough shutter speed to handhold the camera. Lets also assume that these settings without the flash will not produce the correct exposure i.e. the image will be dark and i want the flash to produce the remaining amount of light that will be required for the shot to have correct exposure. How does the flash know what amount of light it will need? Does it have its own metering system?

I cant wait for this bank holiday this weekend to try out everything i learnt from this thread :lol:
Click to expand...

Have a read of this
 
ziggy©;3707281 said:
How does the flash decide how much power will be required?
Lets assume i want to use f16 and 1/200 which will give me enough DOF and a fast enough shutter speed to handhold the camera. Lets also assume that these settings without the flash will not produce the correct exposure i.e. the image will be dark and i want the flash to produce the remaining amount of light that will be required for the shot to have correct exposure. How does the flash know what amount of light it will need? Does it have its own metering system?
Click to expand...

Your 580EXII uses Canon's E-TTLII system. There's an explanation on how it works on these two pages:

http://cpn.canon-europe.com/content/education/infobank/flash/flash_basics.do
http://cpn.canon-europe.com/content/education/infobank/flash/speedlite_compatibility.do
 
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