Exposure with flash

[jessytom]

I have a 430 exII flash and rebel xti body. How do I sync them so the camera recognizes the exposure. I've just learned about metering and it currently isn't registering it properly on my camera when the flash is used. Thanks!

[Skirball]

Make sure the flash says ETTL in the upper left corner.  Use the mode button to flip through the different modes:

http://learn.usa.canon.com/app/pdfs/quickguides/CDLC_Speedlite430EXII_QuickGuide.pdf

Also, you should be aware of how your camera shooting mode meters based on having a flash present.  Here's an article:

http://www.scantips.com/lights/flashbasics4.html

It's just the first decent one I found, I didn't look into it too much, but it covers the different shooting modes and how they react to flash.   it's written for Nikon, but should translate the basics fine (just note that A = Av, S = Tv, etc).  I highly recommend a solid understanding of the different shooting modes to understand flash photography.

[RobertTheFat]

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@Skirball wrote:

Make sure the flash says ETTL in the upper left corner.  Use the mode button to flip through the different modes:

http://learn.usa.canon.com/app/pdfs/quickguides/CDLC_Speedlite430EXII_QuickGuide.pdf

Also, you should be aware of how your camera shooting mode meters based on having a flash present.  Here's an article:

http://www.scantips.com/lights/flashbasics4.html

It's just the first decent one I found, I didn't look into it too much, but it covers the different shooting modes and how they react to flash.   it's written for Nikon, but should translate the basics fine (just note that A = Av, S = Tv, etc).  I highly recommend a solid understanding of the different shooting modes to understand flash photography.

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Gisle Hannemyr wrote (in English) a good article a few years ago on the eccentricities of Canon's ETTL and how to use it properly. A quick Google search didn't turn the article up, and I don't have time just now for a more exhaustive search, but it should be out there somewhere.

[Skirball]

Sure, and if you really get into flash then it's probably worth looking into the eccentricities - although you'll probably discover them sooner or later just through use anyway.

But I just meant to reinforce that the camera/flash behaves very different in green box or P mode than it does in Av or Tv mode.  And that it's worth understand how, and in what conditions.  Mostly, it's worth knowing that Canon flashes will always default to fill flash when in Av, Tv, or even Manual with eTTL.  I believe that Nikon systems don't work the same way.  It's worth understanding these basics or you're going to run into trouble - as people usually do when using Av in a really dark situation.

[jrhoffman75]

Is this it?

http://dpanswers.com/content/canon_flash.php

[RobertTheFat]

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@jrhoffman75 wrote:
Is this [the Gisle Hannemyr article that Bob referred to earlier]?

http://dpanswers.com/content/canon_flash.php

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Probably. I'm not sure it was organized just that way before, but the content seems to be at least as detailed. It's probably a little more up-to-date, but still not recent enough to mention the 600EX-RT. If anybody runs across a later version, please let us know.

[jessytom]

Thank you everyone! This has been incredibly helpful! I'm just learning about my camera and trying to take better pictures of my kids. Your responses and the links have cleared this up dramatically. Much appreciated!

[ScottyP]

Hi

In Manual using ETTL the camera's meter will still show an under exposure but the flash makes up the difference you need (if it can) when you take the shot.

Keep in mind that the meter is telling you the ambient light, and flash power drops off insanely fast over even short distances (inverse square law) so if your camera meters for something/someone in the foreground, your background will be way underexposed if you don't manipulate the base exposure to be within a stop or closer of what the meter says is good exposure.

[lniles]

Yes I have the same flash.  On my 7D or 6D (also the EOS family) I just need to put the flash in ETTL mode, like the others have said.

[TCampbell]

The camera should METER for the exposure as if there is no flash... but then provide flash when you take the shot. This is the preferred behavior.

Light has a problem sometimes called "fall off" but the technical explanation is called the "inverse square law". This says that as the distance from the light source (and it works with many things other than light) increases based on the square root of 2, the power will decrease by half.

In other words if you have a subject 5' away... then anythng 7' away (5 x 1.4...and 1.4 is the a rounded value for the square root of 2) will get half the light. At 10' it's half again (or 1/4) and at 14' it's half again (1/8th) and at 20' it's half again (or 1/16th). This creates wonky flash exposures where your intended subject is properly lit... anyting closer is blown out.. and anything much farther drops to darkness very quickly. The image doesn't look very natural.

It turns out if you "meter" for the ambient light and let the shutter stay open long enough for ambient light... but burst the flash anyway... the intended subject gets correct light ... and the background fills in with ambient light causing an overall image actually looks pretty good.