If you are poor like me and can't afford the newest (newer) flashes ($200+ for a 430EX) you have to make due with less expensive or older flashes. In my case, I went with the Sunpak Auto 383 Super (review) and multiple Nikon SB-20s (review). While I was able to save 50-70% of the price of a 430EX on each, I had to give up some features.
These older flashes do not support the newest flash balancing algorithms like E-TTL II (for Canon -- Nikon has an equivalent). E-TTL II is pretty neat because it meters the flash through the lens, exposing based on what the camera sees, and also takes into consideration the focal distance of the lens (full description here at Bob Atkins' site). Of course, neither my SB-20 nor my 383 supports TTL (through the lens metering) of any sort with my 20D. Instead, I must either balance the flashes using manual controls (awfully slow and fiddly) or use the auto mode.
The auto mode is an automatic metering mode based entirely on the flash. First, the user sets the flash to auto and the camera aperture to the value specified for the given auto mode (often, flashes have a few options for auto mode for different apertures). As the flash fires a photocell on the flash records how much light is reflected back from the subject, and when a predetermined amount of light is recorded, the flash is quenched (shut off). Essentially, this lets the user get automatic exposure without any communication between the flash and the camera.
Both the 383 and SB20 support auto mode. The 383 has a photocell just above the hot shoe (the green circle on the left of the image):
And the SB-20 has a photocell in the lower right of the large red window on the front of the flash:
Auto mode is very easy to implement because it is directly on the flash but has a few limitations because there is no camera-flash communication during the exposure. First, since the photocell can't see exactly what the camera sees through the lens, the results will be inaccurate for very wide or very telephoto lenses. Also, since the photocell is at a different level than the lens, it could be occluded by objects or not aimed at the same place when in close. Finally, since the photocell measures all the light coming in, you'll get a bad exposure if something bright is closer to the camera than your subject.
That said, if you just want to get some snapshots and don't want to worry about balancing the flash in manual mode, auto mode is very useful. Put another way, auto mode lets the flash automatically adjust for distance -- in manual mode, every time you move closer or farther away from the subject, you'd need to adjust the exposure. For kids, that kind of constant adjustment isn't possible!
When I got my Sunpak 383, I quickly realized the auto mode wasn't very good. I just couldn't seem to get a good exposure! After that, I kind of abandoned auto mode until the other day when I thought I'd try an SB-20 in auto mode and did a little comparison. Turns out my 383 just has a crappy auto mode (not sure if I can generalize it to all 383s though -- does anyone else have a 383 that works well?).
To start out, I tried my SB-20 in auto mode by switching the main switch to A, set the ISO 100 with the slider at left, and set the desired f-stop using the slider at right (F/8 in this case -- note the yellow indicator). This means I should be able to set my camera to F/8 and get good exposure no matter what my distance to the subject is. The back of the SB-20 looked like this:
So, I fired off the following shot of the stuff sitting on top of my television (all of these shots are pretty much straight from the camera):
Chimping the result, I thought it looked a little hot, so I dialed it down one stop, and got:
Then, stepping back and zooming in to test if the auto mode was working, I got:
So, by the looks of it, the auto mode on the SB-20 works very well. And, in hindsight, I really didn't need to stop down below the SB-20's settings -- it was very close to good exposure and the stopped down version is a little too dark. Since I usually underexpose flash shots a little bit (you can always increase exposure from the RAW file, but if you blow whites out, you can't get that detail back), I'll probably stop down 1/3 of a stop compared to the flash setting, but it really seems like the SB-20's auto mode is right on.
Now, on to the Sunpak Auto 383 Super. Auto settings on the Sunpak are a bit more confusing. First, I set the left-hand slider switch to the red A setting and the top slider to ISO 100. In the F/stop window, f/5.6 shows up (actually, more like f/6.3) which is what I should set my camera to. Also, I set the bottom slider to full (A) -- my understanding is that the auto only works when the slider is set to full (disclosure: I faked the f/stop value because it was in shadow, but that's how it looks!).
Note that the 383 has three auto settings, but as far as I can tell, it is only a mechanical calculator. Going from red to yellow doubles the distance and opens up two stops (consistent with the inverse square law: doubling the distance should require four times as much light).
So, given these settings, I set my camera to f/6.3 and took a shot under the same conditions as above, and got:
Obviously, way too bright and I completely lost all the highlights on the fan. So, after some playing, I settled on f/11 as a similar exposure to the SB-20 (yes, that's 1 2/3 stops down!):
So, obviously, the SB-383 works, but it needs to be stopped way down. Consistency (in this tiny test) seems to be good across images, but it disturbs me that the Sunpak runs about 2 stops too hot.
Another thing that worries me about the Sunpak's auto mode is the photocell is very, very narrow (less than a millimeter in diameter, about three millimeters deep). I wonder if it gets very good coverage of a wide angle lens. Even more problematic is the height of the photocell on the 383 -- with a hood on a lens (or even a relatively large lens) the cell will be occluded for at least part of the range. I feel like the SB-20 has a much better design.
My conclusions? I like the SB-20 auto mode a lot better, and it just reinforces that if you are on a really tight budget and need a flash, start with an SB-20! For just $30 you get a flash with zoom, bounce, manual mode, and auto modes. The only thing the Sunpak has that SB-20 doesn't is a rectangular head (useful for putting modifiers on it), a little more power (GN120 vs GN100), and a swivel head (so you can bounce of the ceiling in a portrait orientation. While each of those is significant, paying three times as much tempers the advantages. There's a reason I have three SB-20s and may buy up one or two more...
Anyway, if you'd like to get an SB-20, check eBay. Prices seem to be pretty stable at $30 (including shipping) but there are often even better deals if you look hard enough. Here's the current listings if you are curious: