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09-29-2011, 06:27 AM   #1
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Camera exposure compensation vs flash compensation

Hi,

If the subject is wearing a white T-shirt, i would set the EV to +0,7 or +1 to make sure that the white shirt come out as white. And if i am shooting the flash as fill light, do i just set the flash compensation to -1 or so? Is this the right procedure? I am shooting in manual mode by the way.

What is the relationship between the flash compensation vs the body's EV compensation in this case?

I am rather confused by these 2 terms.

Thanks
raider

09-29-2011, 08:08 AM   #2
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Flash Ev is just going to boost or cut flash power. Just like going from 1/4 to 1/2 power when using manual flash to boost flash power. Ev comp works just the same with or without flash to meter the scene. Need to get to work, don't have time for more.
09-29-2011, 08:22 AM   #3
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I just tested this out shooting my refrigerator. M mode with built in flash, DA35macro (normal) at about 3m as my kitchen is only 3m wide.
I am still not 100% sure of the best way to balance them but i'm pretty usre the flash comp is independent of EV comp.

Side note: either Nikon or Canon is like this, but the other one Canon or Nikon the EV is "tied to" flash comp. I cant remember which is which though.



Just note on how Exp comp works in various modes
In Manual mode if you dial in EV+1 this just moves the bar graph down to -1 so you manually adjust it up to the middle point this gives you effectively EV+1
In other modes Av,TV or P the adjument is made automatically by slowing the shutter in Av mode, or opening the apeture in Tv mode for example.

I shot M mode no flash EV0 f2.8 1/60 - fridge appears underexposed grey (or a bit yellow in my downlights)

with M mode no flash EV+1 f2.8 1/30 - the fridge appears whiter

with M mode with flash FEC 0 and EV+1 f2.8 1/30 - the fridge appears bright white with some blown highlights

with M mode with flash FEC -1 and EV+1 f2.8 1/30 - the fridge appears bright white with almost no blown highlights.
with M mode with flash FEC -2 and EV+1 f2.8 1/30 - very little difference to FEC -1 interestingly? only very slightly darker

with M mode with flash FEC 0 and EV 0 f2.8 1/60 - the fridge is not the same as FEC -1 EV+1 the histogram is noticably darker.

In my test in M mode it would appear that EV adjustment is "stronger" than FEC adjustment.

I was using ISO 800 and the in built flash is P-TTL so who knows whats going on inside its brain?.
I would like to try this later on with a flash unit in manual.
In my indoor test the availble light is tungsten and flash is cooler daylight so the colour temperatue varies. Would like to try this again in real daylight.


In the Auto modes the apeture and shutter speed changes as soon as the flash is used.

same scene in P mode
EV 0 no flash f2.8 1/60 - same result as M mode above
EV +1.3 no flash f2.8 1/25 - predictably brighter exposure of white object looks better

the flash sets the speed to 1/80 in P mode
EV +1.3 flash FEC 0 f4.0 1/80 - blown highlights
EV +1.3 flash FEC -1.3 f4.0 1/80 - no blown highlights but similar brightness to the no flash picture but without shadow areas. Cooler temperature due to daylight flash.


Subjectively the most desirable flash result is with some FEC -ve to "kind of" offset the EV +ve


I've no idea if that was helpful at all.
Sorry, my gallery allowance is on the limit and and flickr is not set up yet so I'm a bit stuck to post photos at the moment.
09-29-2011, 04:14 PM   #4
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Thanks for that Steve.

I am still a little confused though.

I am pretty sure the exposure compensation should be set to match whatever thing we are shooting...so if we are shooting white object, EV+1 and if we are shooting dark object, EV-1. It is the relationship between FEC and EC that confuses me.

In what circumstances should we adjust the FEC to 0, and when should we use -1 and -2?

09-29-2011, 05:02 PM   #5
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I'm more confused than before I tested it.

Basically if you are going to use some EV+ to get the "whites" corrected and you are using some flash then not dialling in some FEC- appeared to blow out whites in my test.

But EV +1 wasn't balanced out evenly by FEC of -1 in my test.

EV comp is pretty predictable in shooting but the effect of light from flash and FEC appears to be infuenced by ISO, distance to subject etc.

Id like to know if with a dedicated flash in M mode that dialling in FEC of -1 is the same as 1/2 the power output. ie is it analogous to the Exposure comp adjustment in normal shooting.

I can only recommend that you experiment except use daylight instead of tungsten light like I did last night as that made it hard to assess what is "equivalent" to what due to the different colour temps.

Try M mode with flash and P-TTL and then just pick the one that looks the best I suppose.

Sorry I don't have the knowledge when it comes to mixing ambient and flash other than the basics. I was interested to test what works best.
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