camera exposure compensation (XTi) and 430EX problem

[izabela_mroczko]

Hi everybody,

I have a problem with exposure compensation when using external flash, 430EX. I

have a Canon Rebel XTi. When I shoot in P mode, I take a test shot and check my

histogram. When image is underexposed I turn the knob to open up the exposure

depending how many stops my image is underexposed. When I take a shot, the

image still looks the same and histogram doesn't move, no matter if I open up

or close down the exposure. It only works when I use exposure compensation in

the flash itself. Then the exposure and histogram change.

Can anybody help me with that? Thanks!

izabela

[roger pfister]

I bet you are setting the NON FLASH exposure comp using the Av button. ON the 400D (XTi) set it (Flash exp com) via the menus.

[cliff_henry]

Your basic problem is using P mode. I suggest you try M mode indoors and Av mode outdoors. Shooting with a flash is two different exposures - one for camera using ambient light and a separate exposure for the flash. Each exposure can be compensated differently.

 

The following sites should help. Also, you can search PN for flash compensation. There is a lot of info to study.

 

http://photonotes.org/articles/eos-flash/

 

http://planetneil.com/tangents/flash-photography-techniques/

 

http://strobist.blogspot.com/2006/02/welcome-to-strobist.html

[eyepasha]

Izabela, in general you should use Flash Exposure Compensation (FEC) to adjust the exposure when using ETTL flash based on what you are describing. But it may help you to realize that exposure compensation is basically adjusting the exposure for your ambient light. And FEC is adjusting your exposure for the flash light. In practice that means that exposure compensation will allow you to adjust the background exposure and FEC will allow you to adjust the exposure for that subjects in focus of the flash.

 

If you look back at your pictures and examine the EXIF data you may be able to see (unless the background is well beyond 3 stops light ratio) that exposure compensation is in fact allowing you to make adjustments. The combination of EC and FEC can provide you very subtle control over the image - as long as the balancing is within the range of the camera's sensors.

[eyepasha]

Cliff is correct and pointing you to articles that explain the difference between the two. Where I *might* disagree, Cliff, is that P mode with the Xti (basically ETTL) works fine with EC and FEC together.

 

But... I am never going to actually disagree with anyone arguing for using M mode (indoors or outdoors) ;-)

[bob_osullivan]

P mode put's limitations on shutters speeds available and selects ISO up to only 400. So that could also be contributing.

 

Learn to use M, A and Tv modes.

[christophe_waterplas]

Do you "turn the knob" while pressing the exposure compensation button (marked Av/+-)? If not then turning the wheel in P mode will select another shutter speed/aperture without changing the exposure (program shift).

[ajb2222]

Bob O'Sullivan, Jan 22, 2008; 02:51 p.m.

 

<i>P mode put's limitations on shutters speeds available and selects ISO up to only 400. So that could also be contributing.</i>

<br>

<BR>

P alows me to set ISO all the way up to 1600

[izabela_mroczko]

Thank you all for your help. I am using P mode because of my job requirements. I have to shoot in that mode. Personally I prefer Av and M modes and I agree with you all on that. Although all the answers where very helpfull to my problem :-)

Yes Christopher I do turn the know while pressing compensation button. :)

Izabela.

[roger pfister]

AV button does NOT set the flash exposue comp on the XTi.

 

As I said before ;) the Fash exposure comp is set via the menus - TAB 2 to be exact see Page 75 of the manual.