Flash basics

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rob13

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Rob
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I need some basic assistance in using flash. I feel quite competent with adjusting aperture, shutter and ISO in natural lit areas, however dont know where to start with flash. I get very mixed results so keep running it on program mode with ETTL.

What mode should I be running with? TV/AV or manual? Its metering for a scene I cant understand as when you point the camera at something, unless the flash fires then its metering for a naturally lit one.

I understand not to run a shutter higher than 1/250 as it can't sync fast enough, so should I keep my shutter on 1/200 at all times unless I want to drag the scene? Should I stick with ETTL or should I be exploring the manual settings?

I'm using a Canon 40d and a 430EXII.
 
Depends entirely on what you are looking to do, if you want to use the flash as a fill-in flash use Av or Tv. If you want the flash to be the main source of lighting then i'd suggest full manual.

When using flashes you have to realise that you are trying to expose two different things. You need to expose your ambient light (by the use of shutter speed) and then expsure the flash light (by the use of the aperture)

Changing your shutter speed has no affect on the flash light (unless over 1/200) this is why you set the ambient with shutter speed to the desired effect. Then you change your aperture to the flash output you are looking for.

Hope that helps :)
 
Well I tend to use ISO when the above two are met first.

Say for example i'm in moderate lighting at dusk, and I want a narrowish aperture of say F8 to keep focus on the whole face on a portrait, then I want a shutter speed of around 1/125 to keep ambient down but not eliminated. But then I find that the ambient is WAY too dark, so I can boost my ISO up to suit :)
 
Also if you can't get your flashguns powerful enough, then boosting the ISO from ISO100 to ISO200 means you'll need around half the power you did before :)
 
As Jamie says, with flash there are two exposures - one for the ambient light, and one for the flash.

With normal flash, flash exposure is controlled by the power output - and also the f/number and ISO, but not the shutter speed. (High Speed Sync mode is different, leaving that aside for now.)

The ambient exposure is controlled by all three factors - f/number, ISO and also the shutter speed. But not the flash power output, as this only affects the foreground (flash illumination falls off very quickly over distance).

In this way it is possible to regulate the balance between the flash and ambient light, using the power output to adjust the flash (without affecting the ambient background) and the shutter speed to regulate the ambient light (without affecting the flash).

With Canon, using any of the camera's auto modes on E-TTL, the camera will generally attempt to balance both the flash and ambient exposures, although there are differences. On Tv, the camera will increase/reduce the f/number as far as it can, but it soon runs out of adjustment. On Av, the camera will let the shutter speed run as long as it needs to balance the flash, possibly up to several seconds. Using P, the camera will only let the shutter speed drop to 1/60sec to balance the ambient light, working on the assumption that it's better to let the background go dark rather than risk subject movement bluring the background.

Av therefore gives you most scope, and you can adjust the balance between the flash and ambient light ratios with +/- compensation on the camera to lighten/darken the background (by altering the shutter speed) and +/- compensation on the gun to lighten/darken the flash (by altering the power).

Edit: going back to High Speed Sync, when you're in Av and the camera is adjusting the shutter speed to balance the ambient light, if you're shooting in bright light, eg using fill-in flash, the shutter speed will quite often want to run faster than 1/250sec. That's when you use HSS, which increases the duration of the flash so it is long enough to cover the full cycle of the focal plane shutter, so you can use any shutter speed, right up to 1/8000. Just switch it on, that's all there is to it. The drawback is that HSS uses a great deal of flash power, even though most of it is inevitably wasted by the way the shutter has to operate, but it's fine when you're close - like maybe ten feet away or less, it varies of course. It's a fantastic feature, and daylight fill-in makes for great pictures. I seem to use it all the time :)
 
rob13 said:
Excellent explanations guys, its just what I was looking for thanks.
Click to expand...

You're welcome Rob. I've added a bit above about High Speed Sync which your 430EX gun has :thumbs:

Edit: Dredd, that's a very good book :)
 
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