I am going to purchase a portable flash unit in the next few days and have been torn between my final two options. I’ll get either the Canon 580EX II or the 430EX II Speedlite. I won’t list the specs and features of each unit. They are easily found at any retail site. So, why do I think I need a portable flash?

Ambient light is not always adequate. It can also be harsh, from the wrong direction, or have a strong color cast. When I was at a festival recently on a really bright sunny day, the harsh shadows cast on a lot of the faces of the festival goers would have benefited from a fill flash. Unfortunately, the festival was not one that lent itself to posing folks without losing out on capturing people in the moment. I think a number of photos would have been better with the fill flash of a powerful portable flash.

Portable flashes are far more powerful and versatile than the built in flash. The lowest end Canon Speedlite has an ISO 100 guide number of 72 in feet (22 in meters). That’s enough to give you a maximum shooting distance of 26 feet with a f/2.8 lens, at 100 ISO. The built-in flash units in Digital Rebels and xxD-series SLRs have a guide number closer to 43, which is almost two stops less. The 580EX II has a maximum guide number of 190/feet coverage at 105mm. More light output means a greater maximum range and the ability to use smaller f-stops for greater depth-of-field. It also provides the ability to bounce the light off reflective surfaces for a larger and therefore softer light source, something you can’t do with the built in flash.

Portable flashes are a consistent, daylight-balanced light source. They deliver the same color temperature as the midday sun allowing you to add light to daylight lit scenes without drawing attention to the secondary light source. A consistent light source allows for proper adjustments of white balance in camera or in batch post processing of Raw files.

Portable flash units freeze subject motion in low light. The short duration of the portable flash, when used as the primary illumination source, by default becomes your effective shutter speed. Portable flash durations are typically 1/800 second or shorter which is enough to freeze subject motion and eliminate camera shake even when using non-IS lenses. Given that your focus is accurate, you’ll get more consistently sharp photos in low light situations with a portable flash unit.

Portable flash units reduce your reliance on high ISOs for low-light photography. Even high end expensive DSLRs have their limits when it comes to low noise at high ISOs. Having a portable flash unit to provide an extra stop or two of light can mean the difference between shooting at ISO 800 vs. 3200 or shooting at f/5.6 instead of f/2.8.

Canon Speedlites are fully integrated with Canon EOS cameras allowing them to operate as one unit. This means that the flash and camera will automatically adjust themselves for correct synchronization, no matter what exposure mode you’re in. The Speedlite can also provide auto focus assist if there’s not enough light for the lens to focus properly. It will automatically emit a focus-assist beam onto the subject to help provide sufficient light for correct focus. If the flash exposure is too bright or too dark, you can adjust it from the camera, the same way you would any other type of exposure. The same applies to the ambient light exposure and the flash and ambient exposures can be adjusted separately from each other to achieve the precise balance you want.

So getting back to my original thoughts on my need for a portable flash, the EOS D-SLR/Speedlite combo can tell when you’re using flash in bright light to lighten shadows rather than as the primary light source and automatically reduce the flash intensity so it won’t overpower the ambient light. In addition, Flash Exposure Lock, Exposure Compensation, and Exposure Bracketing can also be set either on the flash or on the camera.

As for whether to get the 430EX II or the 580EX II, for me the major benefit of choosing the 580EX II would be power. The 580EX II can provide nearly twice the light output of the 430EX II. Yes the 580EX II has a longer feature list than the 430EX II, but most are beyond what I need it for now. But the only other way to get twice the 430EX II’s output at the same distance would be to add a second 430EX II flash unit. When you look at it that way, one 580EX II is considerably smaller and more economical than two 430EX IIs.

For more info on Canon Speedlites, the use of portable flash, and the inspiration for this blog post check out this article at The Canon Digital Learning Center.