Flash and shutter speed

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I have tried reading a few things about this but its gets a bit too technical before I have even gotten a grasp of the basics.

I have a canon 450d and a canon 430exII flash. It’s set to ETTL; basically I put it on the top and did nothing else.

Now I understand that the light coming out of the flash can only travel at one speed and that it’s massively quicker than the fastest shutter speed of the camera.

I have recently voyaged into full manual. I typically tend to set the aperture I want and then adjust the exposure until it’s in the middle of the bar, after reading a recent book they suggest where possible to leave the ISO at 200. Now I understand that as the light disappears the shutter will be open longer and the longer its open then the more chance there is of a blurred image. For some reason I seem to have it in my head not to go below 1/60th of a second, unless you are photographing stationary things with a tripod. I also have another speed in my head as well, 1/200th but I don’t know where I got that from, something to do with synch speed.

So, if I have my flash on can I increase the shutter speed even though it’s indicating being under exposed? What sort of shutter speed do I use with a flash?

Thanks in advance
 
The sensor is covered by the shutter curtain, which is literally like a pair of curtains. These physically move across the sensor in order to expose it to light. First one curtain opens and starts sliding across the sensor. The second curtain follows behind at a short interval afterwards, leaving the sensor completely covered at the end of the exposure.

For fast shutter speeds, above the flash sync speed of your camera body, there is not enough time for the first curtain to be fully open before the second curtain begins to follow. Rather than have the entire sensor exposed during any part of the exposure what you get is a travelling slit between the curtains as they move across the sensor. If the flash was to fire at these higher shutter speeds you would only get part of the sensor exposed to the flash. The other part would be hidden behind the second curtain as it moves across. What you would see is a sort of shadow or dark band across the image where the flash was not included in the exposure.

The way around this is to use a feature of the flash called High Speed Sync. This fires the flash as a series of incredibly rapid pulses, spreading the delivery of flash light over an extended period of time rather than one single, quick pulse. This makes the flash light behave more like continuous light and allows you to use flash at any shutter speed you wish. The downside is that the effective power of your flash is reduced because some of the light is being wasted as it strikes the curtains moving across. The faster the shutter speed the narrower the slit between the curtains and the weaker the flash appears to be.

To shoot in this way you need to enable High Speed Sync on your flashgun. I don't know for sure, but I imagine the 430EX does have that feature.

Check this out - http://www.youtube.com/watch?v=Xnn5nzPvoIM&feature=related.
 
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I think on Canon units there's a button on the rear with a lightning symbol that, when pressed, puts it into the high-speed sync mode that Tim has mentioned. For close-up work within a few feet you'll hardly notice any change if you're letting the camera dictate flash power - it's only when you want to pump a shedload of light out that you'll notice the change, or when trying to light stuff far away, because the flash just won't be capable of doing what it does in a single flash below the sync speed.

Just relying on flash and totally killing ambient (to have a studio-type set-up) then i generally work at 1/125th or 1/20th because effectively, the shutter speed has no effect because the aperture is doing all the work.

The shutter comes into play when you want ambient to play a part in lighting the scene and the shutter speed you use is all dependent on the amount of ambient available, the amount of ambient you want in shot and the ISO you're using. Outdoor stuff I'm still looking for shutter speeds that allow me to handhold (say 1/100th) but I' generally adding fill light into the shot so I'm underexposing slightly. I just meter the scene and get the exposure I want (underexposed of course) and then add in flash until it looks like how I want it to.

This is a pretty typical example of how I'm using flash to fill in and work with the ambient instead of trying to overpower it:


Rob's Big Chub by Pat MacInnes, on Flickr

The flash matches up pretty nicely with the ambient, although I didn't gel the flash because I like the white light against the warmth of the sun. Lit with a softbox to give soft light:

This is a good example of me killing the ambient and relying more on hard light to create the definition and detail in the guy's clothes and hair. Again, this was just about getting the exposure right and then adding flash in until it looked right for what i wanted:


Setting up by Pat MacInnes, on Flickr
 
Blasted said:
I have tried reading a few things about this but its gets a bit too technical before I have even gotten a grasp of the basics.

I have a canon 450d and a canon 430exII flash. It’s set to ETTL; basically I put it on the top and did nothing else.

Now I understand that the light coming out of the flash can only travel at one speed and that it’s massively quicker than the fastest shutter speed of the camera.

I have recently voyaged into full manual. I typically tend to set the aperture I want and then adjust the exposure until it’s in the middle of the bar, after reading a recent book they suggest where possible to leave the ISO at 200. Now I understand that as the light disappears the shutter will be open longer and the longer its open then the more chance there is of a blurred image. For some reason I seem to have it in my head not to go below 1/60th of a second, unless you are photographing stationary things with a tripod. I also have another speed in my head as well, 1/200th but I don’t know where I got that from, something to do with synch speed.

So, if I have my flash on can I increase the shutter speed even though it’s indicating being under exposed? What sort of shutter speed do I use with a flash?

Thanks in advance
Click to expand...

There are a lot of questions hidden in there, but the answer to the one you've asked is 1/200sec or longer in normal flash mode. The camera won't actually allow you to set anything higher than that.

For faster speeds, that's when you engage HSS (high speed sync) mode - which the 430 gun has. That will work at any higher speed right up to 1/8000sec, but the power output is greatly reduced. It's fine at close-ish range, for couples and small groups, that kind of thing.

Get a handle on how a focal plane shutter works, as Tim described or google, and things will start to fall in to place.
 
Nice shots Pat.

I always shoot in M mode and yes, the meter will show that the image is going to be under exposed.
This is because it doesn't take in to account the flash. I find most of the shots i take come out well even though the meter is showing it will be well underexposed. I always bounce the flash and i think for indoor use this works well.
Like the OP i set the f stop to what's required, normally use ISO200-400 and a shutter speed of around 1/125 but it varies.
As already mentioned, 1/200 is the normal OB flash max shutter speed. Its also an option for setting this speed, or Auto, for flash in AV mode i believe.

Once you enable Hi-speed sync on the flash then the full speed of the flash will be available.

I took this one at 1/800 with bounced flash.
501860941_JeuVP-M.jpg
 
dave_bass5 said:
Nice shots Pat.

<snip>
Click to expand...

Yes :)

Once you enable Hi-speed sync on the flash then the full speed of the flash will be available.
Click to expand...

Not sure what you mean by this :thinking:

I took this one at 1/800 with bounced flash.
Click to expand...

Why did you use HSS for that shot? When the only light is the flash, you will get a much shorter effective exposure time (the flash duration) using normal sync flash, and use a fraction of the power.
 
HoppyUK said:
Yes :)



Not sure what you mean by this :thinking:
Click to expand...

I just meant that untill HHS is turned on you wont be able to set the shutter speed past 1/200 (or 1/250, cant remeber now) using an external flash.



HoppyUK said:
Why did you use HSS for that shot? When the only light is the flash, you will get a much shorter effective exposure time (the flash duration) using normal sync flash, and use a fraction of the power.
Click to expand...

The shot wasnt planned so the camera was still set from me shooitng in another room, i just walked in and caught my son in mid air.
I was experimenting with the camera and my brand new 580EXII.
 
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