Shivatron
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prowlerregal wrote:
Shivatron,
Thanks for the reply. That does help. I have another question. Based on your explanation, in manual mode, does the camera never recognize that you are using flash? How do I set the correct exposure when I want to use flash in manual?
The camera does recognize your flash if it's an EX-compatible speedlight. You'll notice this by the lightning bolt icon in your viewfinder (I think, long time since I used a 20D  ). However, in manual mode, there will be no change in the reading: the camera knows the flash is there but does nothing about it.
So what can you do? If you're using E-TTL, you have it easy. First you need to decide if you want to see the ambient exposure or not. In other words, do you want the scene to be lit only by the flash or do you want to see the ambient light in the exposure as well?
If you want the scene to be lit only by the flash, that's easy. Set a smallish aperture (not too small -- your flash will have to work harder -- say f/8), and then set a fast shutter speed (1/100 or 1/200 of a second). In other words, in manual mode, intentionally underexpose by 4 or 5 stops. You will notice that the ambient (non-flash) light doesn't appear much in the exposure.
I'm going to dig up an example image, and then finish this post. Stay tuned...
EDIT: All right, I'm back. Here's an example of what I'm talking about above. It was taken in a lit hallway, but I cranked the shutter speed (1/500s) and dropped the ISO (ISO 200) so that the ambient light and the flash is the only light that shows up in the picture.
http://www.shivamayer.com/Portraits/20051113-004244.jpg
If you want to see the ambient light, you have it a bit harder. You still want to underexpose (otherwise your picture will be blurry), but you need to expose enough to register the background light. This is often called "balancing the ambient". You will almost certainly want to use your lens wide open (biggest f-number) for this.
Say your scene meters 1/5 second at f/2.8. Well, you think, that's too damn slow, my picture will be blurry! But you still want the ambient light. So you pick 1/30s at f/2.8. Still a litte slow, but hopefully the exposure will be "frozen" by the flash so it doesn't look blurry. (At least that's the idea, sometimes it doesn't pan out.) And then the flash provides the correct exposure to make up the difference (you don't need to worry about setting that since E-TTL does it for you).
Here's an example. It is really dark here, ambient meters over 1/2s f/4 at ISO 800. I'm brave and want a lot of ambient light so I go for a really slow shutter, 1/5s at f/4 ISO 800. The flash does the rest.
http://www.shivamayer.com/Life_at_the_AMS/20050903-235747.jpg
Now, the last thing to remember is that this is an oversimplification. There are a LOT of other things to consider. For example, Flash Exposure Compensation (FEC) is often required to get a correct flash exposure. Also, E-TTL metering sometimes does wacky things so I often use my flash in manual mode.
Here are some basics to remember:
1. Aperture controls ambient exposure and flash exposure, but when using E-TTL, flash exposure will be held constant by the flash.
2. Shutter speed only controls ambient exposure.
3. ISO controls ambient exposure and flash exposure, but when using E-TTL, flash exposure will be held constant by the flash.
4. FEC can be used to change metered flash exposure.
Good luck!
Edited by Shivatron on Dec 31, 2005 at 05:19 PM GMT
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