I've just been given a Sigma EF-500 DG ST flash unit. I've not really had much experience using flash. How can I get started? Are there any good learning resources?
The best place to start is the Strobist website. I could spend hours and hours typing up stuff and giving you my experience, but the Strobist is the place to get started on. It's all free, with explanations, and "assignments" designed to help you learn. It's the best, free, resource I know of.
There are some good learning resources. I'm sure others will point them out. :) I want, though, to give you some background on the flash you have.
Sigma makes flashes that are "system dedicated" — there's different versions made for Pentax, Canon, Nikon, Sigma, or Sony. You can use it on the "wrong" brand (Sony excepted — they're weird) but will only get the most basic functionality — it will fire at full power, or at ¹⁄₁₆th power, depending on how you've got it set. And, without specific-brand communication with the camera, it won't zoom (to deliver more powerful light at narrow focal lengths). Or, depending on the combination, it might not fire at all -- a weird quirk of the Sigma "ST" models.
A more advanced model would give more manual control, but this is the entry-level model. On the plus side, Sigma's entry-level models are quite powerful (usually no less powerful than their higher-end models).
Assuming you do have it matched to your camera, the camera does all of the intelligent work — tells it where to zoom, tells it how bright to be, and etc. If you want to leave auto modes, you're limited to that very simple all-or-¹⁄₁₆th manual choice (although the zoom should still work). And if you want to tell the flash to provide more or less light, you're limited to whatever flash exposure compensation settings your camera body provides.
There's no fancy features like wireless or high-speed sync or stroboscopic effects, or even moderately-common wishes like second-curtain sync. And because of some design decisions, it won't work with many (most? all?) third-party radio trigger systems (or even simple remote cable solutions).
There's nothing wrong with this — it's really a good entry-level flash — but you'll definitely benefit from understanding the limitations of what you've got. (Revision: okay, actually, there may be something wrong with this, because it's not very flexible. It is very much an auto-mode flash, and is particularly poor as a manual-control flash, which means a lot of the standard basic flash advice doesn't apply. Particularly, it isn't very good for Strobist use.)
Oh, one more thing: if you have a recent camera model, you may need to send the flash back to Sigma for a firmware upgrade (free, but you pay to ship it there).
There are probably better guides, but here's the very basics:
- Get the flash off the camera, if at all possible. There are several ways to do this, that range from remote triggering systems to extender cables and poles, but get it off if possible.
- If you can't get it off, try a technique called bouncing the flash. Basically point the flash straight up (Or at any other white surface), and it will give some interesting effects.
- If you are shooting in a situation where the flash is the main source of light, I suggest you shoot in manual mode, assuming your flash has ETTL or similar system (See mattdm's post). If you are shooting outdoors and want a fill flash effect, shoot in Av mode. Just be careful about the shutter speed;-)
Hope these tips help!
@John Cavan pointed out the Strobist. I'd add Syl Arena's Speedliting. You'll get great advise there as for flash techniques and accessories.