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it's not as small as the tiny 220ex (which is pocket size) but compared to the 580ex it is a lot smaller, and still fits in a jacket pocket. The 430ex has an impressive-for-it's-size guide number of 43 (I explained guide numbers on my 580ex review) but all you need to know is that it's only slightly less powerful than it's bigger brother and much more powerful than the 220ex- and it comes in a small package and is fairly affordable compared to the 580ex.
The extra power of it's bigger brother comes in handy when shooting bounce off a high ceiling, or when trying to overpower the sun, but for casual use or for when you can raise the iso the flash is powerful enough for most situations.
it is a great flash to start off with, and can be used as a wireless slave (not a wireless master though, for that you'll need a top-of-the-line 580 or older 550ex). My recommendation would be to use this flash in conjuction with a 550ex, both are affordable and that would give you a 2 light setup with wireless capability for less than the cost of a 580ex.
Build quality is great, nothing feels cheap and everything is logical and easy to adjust (not that it matters if you just use it on automatic mode).
If you're on a budget and can live with the reduced power get this (or if you just want a smaller flash), get a 550ex if you want more power/wireless, and avoid the off-brand flashes from sigma etc, they're junk.
I used this flash professionally for a couple of years shooting weddings and portraits - quick cycling so it's ready to shoot again shortly after firing (especially with a Quantum battery pack), works great with a Canon 5D and the 5D mk 2, eTTL II was mostly ok and it's got plenty of range for most situations.
I'm not sure how many jobs I used this for and it only started showing it's limitations when I got into using off camera flash - the lack of a PC socket means a remote radio trigger like a Pocket Wizard can only be used with an auxillary hotshoe adapter which while possible, can be a bit fiddly, especially if you're working in a hurry.
In the end I upgraded to the 580 EX II precisely because it had a PC socket (and the extra features and weather proofing are obvious bonuses too), but I keep the 430 and use an extension cable to hand-hold it off camera. A great flash, and the limitations won't matter to most people.
I have only just become a user of decent flash, preferring in the past to use natural light, or just not take the photo, firstly as I didn't own a decent flash unit and secondly because I preferred the light given off naturally. However as my skill as a photographer improved so did my need to use flash grew, finding more situations where although the light was good it wasn't right, or taking more formal portraits etc.
Not having money to burn on something I wouldn't be able to use, I plumped to start with Canons small pro-flash the Canon Speedlite 430 EX II, this would give me most of the control of the bigger 580 EX (without the ability to use it as a master) and should I eventually decide I want the bigger version I could still with use the 430 as a slave etc.
So far I have used the flash mainly off camera to take static portraits, and find that the ability to use manual settings invaluable, varying the power of the strobe in 1/3 stop increments or changing the angle of the flash, rotates 180 degrees and from straight up to a 90 degrees giving a good area coverable. When I have used the flash on camera the ETTL system seems to work perfectly, only occasionally blowing highlights if small areas of white are included.
My only gripe is that you cannot use this flash as a master unit to control other flashes you might have, it can however be used as a slave if you have the right master unit (580 EX)
Full technical specs can be found on canons website, so are not included here.
This is my workhorse portably flash. I used it in conjunction with the Canon 5D body, and it's not let me down yet.
It's equally at home being used directly on the camera, or being taken off and triggered with a cable or wireless trigger (you'll have to buy these though). I have used it for weddings, portraits sessions, pet photography and studio photography all with great sucess.
The auto function is really excellent and in combination with the camera judges the light levels well, although it can sometimes overexpose and blow the highlights, so I tend to dial down the exposure compensation and then bump it up a little in post processing.
There are a good range of third party accessories available that fit this flash, from ring flashes to diffusers and just about everything in between.
Be warned, this flash does eat batteries if you use it on full power. However, when used on quarter power, I can get a whole wedding reception out of one set of rechargables!
For the money, this is an excellent value flash. A good introductory flash compared to the 580, or a good slave model for wireless photography if you already have the 580 model.