[Roy C]

Being a complete idiot where flash is concerned I am still trying to understand flash sync speed. Messing around with the pop-up flash yesterday I set an aperture of f11 in AV mode and popped up the built in flash, it returned a shutter speed of something like 1/8 sec (it was a darkish room). I then fixed the flash sync speed to 1/250 via C.Fn 1-7 and shoot again with the aperture and ISO the same but now I am shooting at 1/250, How come?. am I right in thinking that with flash sync in auto the whole scene is exposed correctly but when you enable flash sync to 1/250 only the main subject is correctly exposed (with the background being dark).

[LHB1]

Roy,
For 35mm cameras, you are mixing two different things: flash sync and proper exposure (shutter speed and aperture combination). The flash must fire during the period when shutter is open to admit light to film. The fastest speed at which the flash will coincide with open shutter is the max flash sync speed for that camera/flash combo. Proper exposure is the setting of proper shutter speed and aperture to get the correct amount of light on subject. The shutter speed must also be at or less than the flash sync speed of that camera/flash combo or the light will not be passed thru the shutter to film. Hope this helps.

LB

[Hanno]

I am just as baffled with a couple of things. Here is the scenario:

I am in program mode and get 60th exposure and f4 aperture.

I then switch to aperture priority and still am at f4 but exposure goes to 2 seconds. Why?

[Overread]

*from what I understand of flashes and metering in canon cameras*

When you use the camera to meter a scene it will read through the lens the availible lighting and, based on the settings and mode you are in, povided the info or change settings so that you will get a "correct" exposure back, correct exposure will be based on your metering mode as normal as well as any possible exposure compensation that you decide to use.

Now when you start using flash for support the camera still can only meter the scene with the current amibent lighting - it does not therefore take into account the fact that the flash is being used*. When you press the shuttter button there is a preflash before the flash fires (this is increadibly quick and hardly noticable) which lets the camera meter the scene with the flash and it sets the flash power output based on that reading - the aperture, shutter speed and ISO all remain the same.

When you used the custom function you forced the camera to shoot at its max sync speed always when the flash is active, which is why the shutter speed jumped up even though the scene before you was still dark. In that case when the flash preflashes it will return a result which will tell the flash to put out more power to the scene to light it better.


The above is why a lot of flash photography where flash is the dominant light source and not simply being used for fill, is done in manual mode - since you the shooter have to take into account the effect of the flash on the scene. You can use an external flash meter and also make use of the histogram in order to take test shots to balance your settings and the flash power output to adjust the effect you get - experience also comes into play here as well.

As for the effect Roy got with the flash of a lit subject and back background (lowkey shooting) that is the effect of the flash light being strong enough to light the main subject, but not having enough power to push light into the further off background - the light diffuses more the further it gets away from the flash and by the background areas the light just does not have the power to return a proper exposure (without blowing out the main subject of course). Using a small aperture and fast shutter speed aids in this process as it increases the amount of light needed at an interval in order to get a correct exposure - the background areas are effectivly exposing without the additional light - hence why you get a black result


* this is excusing full auto modes where the popup flash activates on its own - in those cases the flash is taken into account when used. This I suspect is why Hanno is getting the difference in shutter speeds - in full auto the flash is taken ito account as being active, whilst in aperture priority it is not. This is important as it allows the photographer to use the flash in the semi auto modes as a fill light source as default, rather than the dominant light source - which is what you will get using it in the auto mode every time.

[Roy C]

Originally Posted by Overread

*from what I understand of flashes and metering in canon cameras*

When you use the camera to meter a scene it will read through the lens the availible lighting and, based on the settings and mode you are in, povided the info or change settings so that you will get a "correct" exposure back, correct exposure will be based on your metering mode as normal as well as any possible exposure compensation that you decide to use.

Now when you start using flash for support the camera still can only meter the scene with the current amibent lighting - it does not therefore take into account the fact that the flash is being used*. When you press the shuttter button there is a preflash before the flash fires (this is increadibly quick and hardly noticable) which lets the camera meter the scene with the flash and it sets the flash power output based on that reading - the aperture, shutter speed and ISO all remain the same.

When you used the custom function you forced the camera to shoot at its max sync speed always when the flash is active, which is why the shutter speed jumped up even though the scene before you was still dark. In that case when the flash preflashes it will return a result which will tell the flash to put out more power to the scene to light it better.


The above is why a lot of flash photography where flash is the dominant light source and not simply being used for fill, is done in manual mode - since you the shooter have to take into account the effect of the flash on the scene. You can use an external flash meter and also make use of the histogram in order to take test shots to balance your settings and the flash power output to adjust the effect you get - experience also comes into play here as well.

As for the effect Roy got with the flash of a lit subject and back background (lowkey shooting) that is the effect of the flash light being strong enough to light the main subject, but not having enough power to push light into the further off background - the light diffuses more the further it gets away from the flash and by the background areas the light just does not have the power to return a proper exposure (without blowing out the main subject of course). Using a small aperture and fast shutter speed aids in this process as it increases the amount of light needed at an interval in order to get a correct exposure - the background areas are effectivly exposing without the additional light - hence why you get a black result


* this is excusing full auto modes where the popup flash activates on its own - in those cases the flash is taken into account when used. This I suspect is why Hanno is getting the difference in shutter speeds - in full auto the flash is taken ito account as being active, whilst in aperture priority it is not. This is important as it allows the photographer to use the flash in the semi auto modes as a fill light source as default, rather than the dominant light source - which is what you will get using it in the auto mode every time.

Thanks for that 'overread' I think it has finally sunk in.
What I could not understand was when I set the flash sync to 1/250 the shutter speed remains at 1/250 no matter what aperture or ISO I select - but what you are saying is the the power output of the flash is is variable.

[Overread]

yes 0 though the popup on the camera has a limited range of power options compared to say a 430 speedlite, plus you lack the abilty to dial in the manual power output of the flash.

Also the 1/250sec custom function is basically forcing the shutter speed up to 1/250sec when ever the flash is used - bascially its giving you a good fast shutter speed when your using the flash outside of manual mode - thus in cases where your in a dark spot you can use the autoflash and it won't still give you a slow shutter speed in the dim lighting (remembering that the camera meters through amibent lighting and since the flash is not contributing to that at that time its metering in dark conditions)