I have a Canon 5D Mark II and normally shoot with natural light (I try to stay away from flash). However, I am taking on more situations where I need to use my hotshoe flash (Canon Speedlite 580 EX II).
I have not been able to figure out how to easily get the flash to get the right exposure without having to use the Automatic/ETTL mode. I normally shoot in AV mode, and would like to get the flash to sync with it. What would you recommend I try? Canon's flash manual isn't very clear.
Canon flash is a bit convoluted. Exactly how the flash behaves varies by camera mode, in some modes the flash will try to act as main light, in other modes it will try to act as fill flash.
Unless I misremember, in AV mode the flash will automatically, always, try to behave as a fill flash; the photo will be pretty correcly exposed per the light-meter without flash and then the flash will try to fill out the foreground only. If this means a three-second shutter speed then so be it.
I prefer to use M mode when flashing, in this mode the flash will be the main light and I have explicit control of the natural-light exposure.
Oh, and "right exposure" with a Canon flash means ETTL. By definition, pretty much. I can't rememeber if the 580 series have their own independent light-sensor on the front, as Nikons have had for a couple of decades now, but if so they are the first Canon ones to have it. Lacking this, a flash has no way of measuring the light emitted without resorting to TTL.
With manual flash, you have two separate exposures you'll have to set and balance - background exposure and subject exposure (illuminated by flash). Both are affected by aperture and ISO, while only background exposure is affected by shutter speed and only subject exposure is affected by flash power setting.
First, you'll set your aperture, ISO and shutter speed so that depth of field and background exposure is like you want them to be. For getting more accent on your subject, you might want to underexpose background by a stop or two. With digital, it's easy to confirm the exposure by taking a test shot (flash powered off) and studying the histogram. For most efficient flash power usage, your shutter speed would be equal to your camera's sync speed (for a 5D mark II, 1/200 s according to specs, but that often causes a dark band with flash, 1/160 would be safer). Using high-speed sync is rarely a good idea.
Next, you'll adjust flash exposure by setting your flash power to a guessed approximate value (don't worry, you'll get a feel for setting the initial power after you've done this a few times) and pop another test shot. Adjust flash power to get the exposure you like and voilà!
With multiple flashes, you'll adjust power for each flash one after another.
As you can see, operating a flash in manual mode is a bit time-consuming, not very well suited for "capturing the moment" or moving around a lot. That's what you have TTL for. Manual flash is for when you sculpt out an image the way you want it to be. Therefore, Av mode may feel awkward with manual flash (you don't have control over shutter speed to control background exposure) and M mode might be better suited.
Don't forget that you can also set the parameters for using flash in Av mode on Canon 5D Mk II cameras, with respect to shutter speed. In the 'Custom Function' Menu, > 'Exposure' (C.Fn I), > 'Flash Sync, In Av Mode. You have 3 options: 1. Auto 2. 1/60 to 1/200 3. 1/200 fixed
I think that Auto is the default setting, the only problem with this is your shutter speed can drop and cause motion blur. If you set the option to 1/200 fixed, then you will lose any ambient light in your exposure if shooting indoors. I use option 2. 1/60 to 1/200 and normally get fairly decent results when I'm shooting on the hoof. Hope this helps.
Essentially, you'll need to adjust aperture, ISO, and flash power until you get proper exposure. If your image is too bright, tighten your aperture, decrease ISO, or decrease flash power until it's properly exposed. If it's too dark, do the opposite.
If you like to do math in your head, you can use the guide number of the flash as an aide for proper exposure.
Keep in mind that your flash shines more light on objects that are near to it, so if your flash(camera)-subject distance is varying, you'll have to quickly adjust the flash power to compensate. In quickly-changing situations, you'll probably want to use TTL.
Adjusting shutter speed affects only the ambient light, up to your flash sync speed (usually 1/200th or thereabouts.)
Strobist's Lighting 101 and Lighting 102 series are fantastic intros to manual flash operation. I've only scratched the surface here.
It sounds like you're bumping into the issue that the "Canon EOS Flash Bible" calls "EOS Flash Photography Confusion".
Whenever you take a flash photo, you have two main sources of illumination: the ambient (i.e., all the existing light in the scene that isn't flash), and the light coming from the flash. The two sources combine in any flash image you're taking, and can be balanced against each other to taste however you want. You can have an image be mostly flash with no ambient (brightly lit subject, black background) up to mostly ambient with a small bit of flash (aka "fill flash"--because the flash "fills in" the shadows).
Ambient is controlled by iso, aperture, and shutter speed (this you know). Flash, however, is controlled by iso, aperture, flash-to-subject distance, and the flash power setting.
The camera doesn't know how you want to balance the flash against the ambient, but makes a "best guess". So the behavior changes with the shooting mode.
In M mode, you're in full control. How you've set your iso and aperture and shutter speed control your flash/ambient balance if your flash is in eTTL. If your flash is in M mode as well, you can control the flash power output directly, too.
In Av and Tv mode, the assumption the auto-exposure system makes is that you want flash for fill. So, the camera settings will be made based on the ambient metering, and will be pretty close to what you would have had without the flash. So, in low-light situations (which is where most people begin using flash), Av can yield very slow shutter speeds (unsafe for handholding) and low levels of illumination.
In P and Auto modes, this behavior changes to something more like how P&S cameras behave. The flash/ambient balance will be set to fill in good light levels, but in lower light levels, the balance will shift to mostly flash so that the background can be black, but the subject will be well-lit, and handholding shutter speeds are always used.
My suggestion to you, assuming by "correct exposure" you mean P&S flash behavior in Av, is to look at your camera's custom settings, and change the shutter speed used for flash in Av mode. Shutter speed is your one independent control for flash/ambient balance, because it will affect your ambient, but not your flash. The faster the shutter speed (up to your maximum sync speed), the more illumination in the image comes from the flash. Most Canon bodies have the ability to force the shutter speed in Av to be your sync speed, or from 1/30s-x-sync (i.e., handholding safe speeds), which will then make it behave more like a P&S flash.
However, if you really want to learn flash and balancing ambient against the flash, putting your camera in M mode and your flash in M mode, and reading the following two articles on flash vs. ambient, may be the fastest way to learn.
dont agree - 1/200 shutter speed and -2 or -3 EV on Speedlight (compensation - TTL) and you dont loose the ambient light if you have got propoer exposure - fill flash
There are two different situations.
Bright ambient light: Turn on high speed sync on the flash. Lower the ISO
Low ambient light: Select higher ISO until the shutter time is acceptable
