Originally posted by tabl10s From prior experience, I know if the flash output id too strong, the background looks like it's night. I would then open the lens to compensate. Can't this be done with the K-5?
If the flash is too strong (ie it overexposes the subject) then opening the apeture wider will overexpose it even more!
Usually with P-TTL flashes including the built in flash it will reduce the power so the subject isnt overexposed (thats the theory anyway).
If you are very close, too close to the subject then that flash will blast them and overexpose anyway.
Obviously, if you are too far away then the flash power (particularly the built-in) wont reach the subject.
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With backlighting or really any flash photography outdoors when there is a fair bit of ambient light you have to treat it like you are doing 2 exposures at the same time.
There is the background - which is all ambient light because the flash does reach to the distant background generally
and the foreground subject - which is mostly lit by the flash.
So for the background you can set whatever you want to get a correct exposure but you should try to keep the shutterspeed slower than 1/180s. This is because when you pop up the flash or use a normal external flash, the camera will limit the maximum speed to 1/180 if you had it higher. This will casue the background to overexpose. "Twitch" was explaining this above to answer your original question.
If the flash is P-TTL then using that mode will regulate the power to just put out enough light to light the subject properly (in theory anyway). It might still look a bit under or over when you check it. You can apply a bit of + or - flash exposure comp to alter to taste.
The idea is to balance the exposure from the flash and the exposure from the ambient light.
a bit of basic theory
If you were doing everything manual, with a manual flash at fixed power
The flash fires off all its power in about a millisecond or so - doesnt matter exactly. If you are only lighting with flash then if you have a 1/180s shot and a 1/30s shot they will both look the same as the amount of time the shutter is open doesnt matter all the flash light is deliverd to the sensor in a tiny fraction of that.
So..... if you want to alter the amount of light from a flash getting to your sensor you can adjust the apeture or you can adjust the senstivity(iso). Ajusting the shutter speed does nothing.
Now, of course ambient light for the background exposure can be altered by varying the apeture, iso and the shutter speed.
To balance the ambient lighting whilst keeping the flash exposure the same you can adjust only the shutter speed. Shutter speed has no effect on flash.
You just have to keep the shutter below 1/180 because of the camera's limitation.
In reality with a manual flash you can also adjust the power output on most of them and of course for P-TTL its all done for you.
If you're interested have a search around on youtube for "flash tutotrial" or "strobist tutorial" and theres quite a few videos showing many tecquires to use flash.
also theres this
Strobist: Lighting 101
and I found this one helpful
http://neilvn.com/tangents/flash-photography-techniques/
....and if you dont have an external flash get one. It doesnt have to be an expensive full P-TTL, capable of high speed sync but being able to go wireless is a definte plus.
then get out there and practise what you read about
happy flashing