Need help with changing BG brightness when using fill flash!


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Nov 4, 2006
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#1
Hi guys. Sorry I need some help here.

I've read under the section " Changing the brightness of the BG" in this article "Fill-flash use with EOS camera and Speedlite" at http://www.usa.canon.com/dlc/contro...t&articleID=106&productID=229&articleTypeID=5 that I can change the brightness of my BG without affecting the brightness of my subject by raising the ISO setting, shoot at a wider aperture or shoot at lower shutter speed.

However, when I tried shooting a scene at home (with a standing fan as my subject and dark hdb flats outside my window as my BG) using the suggested method, I was unable to attain what the article claims. When I either decreased ISO or used a smaller aperture, the subject brightness remained the same but my BG still remained the same brightness (not darker).

I used an external flash mounted on top of a 350D on AV Mode. Settings are 1/15s f/5.6 ISO 1600 for the 1st shot and 1/10s f/8 ISO 1600 for the second shot. I used evaluative metering and had my AF pt on my subject. External flash was on ETTL mode.

Any one got an idea? :dunno: Thanks!
 

raptor84

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Dec 6, 2005
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#2
In Av mode camera meters for the overall light and flash acts as a fill. Normally used when your backgound is overly bright and you dont want your subject underexposed or you need to fill in some shadown caused by strong overhead lighting.

Try going into manual mode as then the flash will beocme the mian light. How underexposed your background becomes will then depend on how you st your iso/aperatuer/shutter combo. Try setting like ISO 800 f/2.8 1/30 and f/4 1/125 to see the difference. Your initial 2 settings stated are only 1/2 stop apart so wont show much diff :)
 

Nov 4, 2006
555
1
18
#3
In Av mode camera meters for the overall light and flash acts as a fill. Normally used when your backgound is overly bright and you dont want your subject underexposed or you need to fill in some shadown caused by strong overhead lighting.

Try going into manual mode as then the flash will beocme the mian light. How underexposed your background becomes will then depend on how you st your iso/aperatuer/shutter combo. Try setting like ISO 800 f/2.8 1/30 and f/4 1/125 to see the difference. Your initial 2 settings stated are only 1/2 stop apart so wont show much diff :)

Hi. Thanks for the enlightenment. Just to confirm...by going manual mode it means the camera (and not the flash) manual mode right? And in this case the exposure on my subject won't change, am I right? :think:
 

Apr 15, 2008
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#4
Hi. Thanks for the enlightenment. Just to confirm...by going manual mode it means the camera (and not the flash) manual mode right? And in this case the exposure on my subject won't change, am I right? :think:
correct, if you're shooting in a very dark place :)

But if you're not, say...in a brightly-lit hall, ambient light will definitely affect exposure :think:
 

Nov 4, 2006
555
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#5
correct, if you're shooting in a very dark place :)
Hmm my scenario is: Im sitting in a well-lit (by florescent lamp) room and I shoot a standing fan as my subject with dark HDB flats outside my window as the BG. So the standing lamp receives the same amt of ambient light as me (but not the BG). Does this fits your description of "shooting in a very dark place"?
 

Apr 15, 2008
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#6
Hmm my scenario is: Im sitting in a well-lit (by florescent lamp) room and I shoot a standing fan as my subject with dark HDB flats outside my window as the BG. So the standing lamp receives the same amt of ambient light as me (but not the BG). Does this fits your description of "shooting in a very dark place"?
yea, guess so. Since the BG is waay too far to be affected by either of the 2 light sources :think:
 

Nov 4, 2006
555
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#7
Hmm...thanks guys. Ok now I understand. What if I use EC? Would it acheive the same effect as M mode? :think:
 

catchlights

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Sep 27, 2004
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#8
the lighting ratio are too great, if outside your house is a cloudy day, it should be f8, ISO 1600 at 1/1600 sec..

compare to your camera setting shooting in the room, the b/g is just simply white wash.

btw,
1/15s f/5.6 ISO 1600 for the 1st shot and 1/10s f/8 ISO 1600
is less than half stop different.


any also, when you using your flash as fill flash, which mean the ambient light is still the main light, if you darken the main light, you will underexpose your subject.
you want to darken your ambient light, you flash has to be the main light, so your subject will not be affected by ambient light.
 

Nov 4, 2006
555
1
18
#9
Try going into manual mode as then the flash will beocme the mian light.
This manual mode means flash M or camera M mode? I tried with flash M mode and it works. But not camera M mode. Does this means that my flash has become the "main light" and not a fill flash anymore? Are there any other ways I can change the BG brightness while keeping the subject brightness the same w/o using flash M mode coz it's very hard to estimate the amount of flash light given out? :dunno: Thanks!
 

Last edited:
Jul 31, 2006
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#10
The background brightness is theoretically determined by shutter speed, aperture and ISO combination and independ from the flash illumination. Therefore, use Av or Tv mode may not change the background brightness. One example is your own setting, when you closed the aperture from f5.6 to f8, the shutter speed automatically changed from 1/15 to 1/10 to compensate, the result turn out that background brightness is kept the same.
Solution is using M mode, in which you can close the aperture with out changing the speed. Or alternatively, you can use Av mode, but use exposure compensation function to control the background brightness.
 

raptor84

Senior Member
Dec 6, 2005
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#11
This manual mode means flash M or camera M mode? I tried with flash M mode and it works. But not camera M mode. Does this means that my flash has become the "main light" and not a fill flash anymore? Are there any other ways I can change the BG brightness while keeping the subject brightness the same w/o using flash M mode coz it's very hard to estimate the amount of flash light given out? :dunno: Thanks!
Manual mode on body. This makes your flash the main light. How it meters depends on weather you set ettl or average (custom function on body or flash). Your ISO/aperature/shutter settings will then determine how the overall scene is exposed.
The longer the shutter,wider aperture or higher iso will not only affect the amount of ambient light mixed in with the flash but your ISO/aperture setting will also affect the effective power of your flash with might cause it to over expose(even at min flash output theres too much light) or under expose(flash cannot properly illuminate scene ).
 

Nov 4, 2006
555
1
18
#12
The background brightness is theoretically determined by shutter speed, aperture and ISO combination and independ from the flash illumination. Therefore, use Av or Tv mode may not change the background brightness. One example is your own setting, when you closed the aperture from f5.6 to f8, the shutter speed automatically changed from 1/15 to 1/10 to compensate, the result turn out that background brightness is kept the same.
Solution is using M mode, in which you can close the aperture with out changing the speed. Or alternatively, you can use Av mode, but use exposure compensation function to control the background brightness.
Manual mode on body. This makes your flash the main light. How it meters depends on weather you set ettl or average (custom function on body or flash). Your ISO/aperature/shutter settings will then determine how the overall scene is exposed.
The longer the shutter,wider aperture or higher iso will not only affect the amount of ambient light mixed in with the flash but your ISO/aperture setting will also affect the effective power of your flash with might cause it to over expose(even at min flash output theres too much light) or under expose(flash cannot properly illuminate scene ).
Hmm..thanks for the reply! I understand it. Does this mean that I can't keep the exposure on my subject constant if I use M mode on camera and ettl mode on flash? Since if i "close the aperture with out changing the speed", I would be reducing the amt of light reaching my cam in the entire scene, including the subject itself. Thx
 

Nov 4, 2006
555
1
18
#13
the lighting ratio are too great, if outside your house is a cloudy day, it should be f8, ISO 1600 at 1/1600 sec..

compare to your camera setting shooting in the room, the b/g is just simply white wash.

btw, is less than half stop different.


any also, when you using your flash as fill flash, which mean the ambient light is still the main light, if you darken the main light, you will underexpose your subject.
you want to darken your ambient light, you flash has to be the main light, so your subject will not be affected by ambient light.
Hi thanks for ur reply too, catchlights! Sorry I forgotten to thank you. Haha coz I couldn't find a quote frm ur post. ;p
 

raptor84

Senior Member
Dec 6, 2005
4,726
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Singapore
www.furry-photos.com
#14
Hmm..thanks for the reply! I understand it. Does this mean that I can't keep the exposure on my subject constant if I use M mode on camera and ettl mode on flash? Since if i "close the aperture with out changing the speed", I would be reducing the amt of light reaching my cam in the entire scene, including the subject itself. Thx
Your background would be more under exposed but your flash wil increase its output power to try and achieve the same exposure. Depending on how faar away your b/g is the flash might not have enough power to properly illuminate it and so it might be underexposed but if your intended subject is near enough it should still be properly exposed. Again it will differ between ettl and average flash metering.
 

Nov 4, 2006
555
1
18
#15
Your background would be more under exposed but your flash wil increase its output power to try and achieve the same exposure. Depending on how faar away your b/g is the flash might not have enough power to properly illuminate it and so it might be underexposed but if your intended subject is near enough it should still be properly exposed. Again it will differ between ettl and average flash metering.
Hmm oh yes. That's what I'm referring to "your flash wil increase its output power to try and achieve the same exposure.". My flash doesn't seem to have this effect. Hmm...It produces the same amount of light :dunno: There's only ETTL/Multi/M on my flash. What's average flash metering? Nikon? Thx
 

raptor84

Senior Member
Dec 6, 2005
4,726
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Singapore
www.furry-photos.com
#16
When on ettl mode you can set ttl or avg flash metereing via a custom function on the body. Check the manual. You mean that your subject gets dimmer as you decrese your ev? If you are on ettl it could mean that your flash might not be able to output enough light.
 

Nov 4, 2006
555
1
18
#17
When on ettl mode you can set ttl or avg flash metereing via a custom function on the body. Check the manual. You mean that your subject gets dimmer as you decrese your ev? If you are on ettl it could mean that your flash might not be able to output enough light.
Hmm..I found a mode under custom functions...under ETTL II there's evaluative and average. I suppose that's what you're referring to? I shall try again at night. Hahaha much thanks guys! ;) Smhow I just don't get the efx i want.
 

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