Flash power, iso, shutter speed and f stop...

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trencheel303

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In really basic terms, the way I found easiest to think about it is you have 3 variables which you adjust to make a balanced exposure.

ISO
- higher = more sensitive to light
- Lower = Less sensitive

Aperture
- higher = less light enters camera (more dof)
- lower = more light enters camera (shallower dof)

Shutterspeed
- higher = less light
- lower = more light

So you are balancing all 3 to get correct exposure. With the flash you are adding a 4th variable.

So if you want to brighten a scene with your flash at 1/32 you can do any combination of the following:
-Increase ISO (results in more noise)
-Lower F-stop (lets more light in but shallower DOF)
-Slower shutter speed (more light in)

What you choose to do is down to you and the subject of the photograph and the amount of ambient light you already have.

If the subject is too dark in the example you gave however the bg of scene is exposing correctly but not your subject in order to balance the two you either need to move the flash closer or increase the power on it IMO.
 
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riiiight

ambient exposure is normal exposure so controlled by light level (uncontrolled factor) iso, aperture and shutter speed

flash exposure is instant so unaffected by shutter speed, the light level here is flash power and controllable, the other factors are iso and aperture

altering iso and aperture affect both exposures, altering the shutter speed affects only ambient but is limited by sync speed

so if you are at sync speed (1/320th afaik for a 1 series) and want to bring more ambient into the picture lower your shutter speed. If you want to reduce the ambient then you need to reduce aperture or iso, this affects both exposures so you would need to increase the flash power to compensate

if your ambient light was too bright and your flash too dark you would first need to push the shutter to your sync speed to control the ambient, then if you have made the ambient darker than you need open the aperture, or if the ambient is about right increase flash power, or if the ambient is too strong reduce iso/aperture until its right then increase flash power until the flash exposure is correct (in doing this you can run out of flash power)

as an aside the light level produced by the flash is affected by the modifiers used and the subject flash distance so you can make power by moving the flash closer or ditching a mod
 
Aperture controls the subject's exposure, shutter speed controls the ambient. As above, the iso changes the light sensitivity and thus affects your flash power primarily. This is true for manual though. With ttl, you have to mess with the flash exposure compensation a bit.
 
What it boils down to is:

f/number affects everything.
ISO affects everything.
Shutter speed only affect ambient.
Flash power only affects flash.

Which is why Av is handy for working with flash. You set the ISO and f/number and leave them, then use the flash power and shutter speed to control the flash/ambient balance.

High Speed Sync is not really 'flash' at all. It is continuous light, same as ambient, but only for the duration of the full shutter cycle. That is not to be confused with shutter speed - at higher shutter speeds the full shutter cycle duration hardly changes at all between say 1/250sec and 1/8000sec.

If you think of HSS as flicking on the room light just before you press the release, and then switching it off again immediately afterwards, maybe that makes more sense? HSS reacts to shutter speed just like ambient, although of course you can control its power.
 
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