Flash sync speed.... what's the point?

[T0MAT01]

I was using my nifty fifty the other day in AV mode fixed to f/1.8 and decided I needed some fill flash. As the shutter speed was 1/2000th @ ISO 100 and the flash sync speed is 1/200th on my 350D it was pretty obvious that it would overexpose the scene, so I hit the high speed sync button on my 430EX and hey presto the shutter speed was back up to 1/2000th and the shot turned out just as I had hoped.

That left me wondering why the sync speed is set at 1/200th of a second when the flash is obviously capable of performing at 1/2000th, and indeed, why there is a sync speed at all.

Is it an accuracy thing? Or am I missing something?!!

[stevegreen]

I will probably get shot down in flames here but I'll qualify this comment by saying that my knowledge of flash is very limited, however, I am sure that using Av mode will not give you 'fill flash' as the camera meters for the flash to expose the background correctly. To get the flash to fill in I think you need to select either M or Tv.

I am probably completely wrong but I felt like replying as flash is something I need to learn and getting shot down in your beliefs is a great way to do that

[Liquid101]

Originally Posted by T0MAT01

That left me wondering why the sync speed is set at 1/200th of a second when the flash is obviously capable of performing at 1/2000th, and indeed, why there is a sync speed at all.

Is it an accuracy thing? Or am I missing something?!!

Your camera assumes you haven't got a flash capable of shooting high speed sync, however your flash unit is capbable, and needs to 'let the camera know'

Originally Posted by stevegreen

I am probably completely wrong but I felt like replying as flash is something I need to learn and getting shot down in your beliefs is a great way to do that

There is nothign to stop you making fill in flash shots while in A/Av mode. You simply set your flash up to fire 1-2 stops below the metered reading.

[stevegreen]

Originally Posted by Liquid101

There is nothign to stop you making fill in flash shots while in A/Av mode. You simply set your flash up to fire 1-2 stops below the metered reading.

As a rule though is my understanding correct that it automatically meters to light the background in Av/A mode or am I completely mistaken?

[Peakoverload]

There is a difference between the x-sync speed and FP (Focal Plane) flash (which is what you are using at high shutter speeds.

The x-sync speed denotes the highest speed that the flash can sync with the two curtains of your shutter. If you put your flash in manual and set your camera to 1/500th and took a photo you would see either the first or second curtain closing across the frame i.e. you'd get a big black blob on one side of the photo.

How do you get around this? You use FP (if you flash supports it). What's the difference? Well, 'normal' flash fires once (ignoring any anti red eye and AF assist) and that's it. In FP the flash fires multiple times in very quick succession which removes the problem of the curtains closing. What it also does is drastically reduces the power and therefore the throw of the flash. If you are close to your subject this may not be a problem but if you are further away or trying to light a large group the flash simply may not be powerful enough and you have either an underexposed or black background and possibly under exposed edges creating a kinda vignette effect.

[Ian_S]

Everything you ever wanted to know about EOS flash photography and then some....

[dood]

Originally Posted by stevegreen

I am sure that using Av mode will not give you 'fill flash' as the camera meters for the flash to expose the background correctly. To get the flash to fill in I think you need to select either M or Tv.

My understanding is that in Av mode, the camera metres for the scene as though there was no flash being used. This will result in long exposures in dimly lit scenes. If a flash is used, it will act as a fill in. This can be used to good effect if you want the flash to freeze a nearby subject whilst allowing the ambient lit scene to blur out. However, in most cases, you would need to set the shutter speed manually to prevent the motion blur from long exposures. Of course this applies to dimly lit scenes, and is not relevant to a bright scene where the flash is only used as a fill.

[T0MAT01]

Originally Posted by stevegreen

As a rule though is my understanding correct that it automatically meters to light the background in Av/A mode or am I completely mistaken?

As I understand it Steve, you have it back to front!

In AV mode the camera will expose for the background and then meter the flash for the foreground subject.

Originally Posted by Ian_S

Everything you ever wanted to know about EOS flash photography and then some....

Thanks for that, I'll have a look.


Peakoverload: Thanks for that, it makes a lot of sense. I knew there must be a technical reason for it!