flash exposure correction + or -?

[ramig]

got the sf20, but I am a little confused about one thing. some of you, in previous posts, suggested to reduce the flash exposure to -2 or so. but when I set the flash to 'minus' something, it increases the meter mesure, indicating larger space cover. when I set it to 'plus', it decreeses the space covered. so I guess to reduce the flash power I have to set it to '+', right?

[jackflesher]

No. You set the exposure compensation to "-". First off, make sure

the flash is on your TTL camera, the flash is turned on, AND the

camera is activated by slightly pressing the shutter release. I don't

have my set-up in front of me now, but if the SF20 is still denoting

additional range, it is because the flash is telling you that you now

have the same relative amount of light available at a longer

distance, because you have reduced the amount of light you expect out

of it.

 

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:-),

[ramig]

Jack, thanks for the answer.

it makes some sense, which is much more than it did before. but

still, for example, with -1, it changes the meters mesure from 9.9 to

14. as you put it, I expect less flash light, but I reduced it for

the 9.9 meters, there is where I expect less light (I knew from the

begging that I would get less light on 14 meters, I didn't have to

change anything for that?) so do I get it all wrong? I am not just

trying to argue, I spend 4 hours last night trying to figure out what

could it mean. thanks again.

[jackflesher]

Rami:

 

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Let's try this explanation:

 

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1)Keep in mind that light falls off according to the inverse square

law just like f-stops -- hence we lose 1/2 the effective output with

our flash every time we increase the distance by a factor of the

square root of 2, or 1.4.

 

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2)Furthermore, keep in mind that the flash has a maximum output, and

that with TTL we will get the exact amount of light we need if it is

equal to or less than the maximum output we asked for.

 

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So, if we tell the flash we want to reduce its EFFECTIVE output by

1/2 or 1 stop (by dialing in -1 on the exposure compensation) we now

can gain extra distance by the factor of 1.4. BUT, anything exposed

at the longer distance will be underexposed by 1 stop since that is

what we asked for. Hence, your 9.9M going to 14M makes perfect sense:

At full power with normal exposure, it can get out to 10M; but since

you are asking for it to give you the EFFECT of half-power (1 full

stop less) it can produce that same effect by using its full power at

14M (10M X 1.4).

 

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:-),

[ramig]

Jack,

exellent explanation. I sware I already guessed it has to do with

squares... but to tell you the truth, I would never have been able to

clarify to myself exactly how. so now it does make full sense.

thanks.

[jackflesher]

Rami:

 

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Always happy to be of assistance to a fellow Leica shooter!

 

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;-),