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things change, new features make a product better, innovations provide new opportunities, and ultimately the latest and greatest provides the best online bragging rights- but with that comes a high price, so sometimes it's a better option to look backwards and see what was the top of the line a few years ago- sometimes that can give you better bang for your buck than buying the midrange current generation product.
The canon 550ex fits into that 'better bang for your buck' category. There are some improvements in the newer product line, but you really do pay for it, if you're just looking for light then the light produced by this flash gun is as good as anything produced today.
The name 550ex means it has a guide number of 55, the 430ex has a guide number of 43, and the newer 580ex's have a guide number of 58. So what does that mean? Well it means
"The guide number represents an exposure constant for a flash unit. For example, a guide number of 80 feet at ISO 100 means that a target 20 feet away will be correctly illuminated with an aperture of f/4 (80 = 20 × 4) using a sensitivity of ISO 100. For the same guide number and an aperture of f/8, the light source should be 10 feet from the subject (80 = 10 × 8)"
so basically the higher the number the more powerful the flash.
A more powerful flash will be quicker to fire, because the flash needs time to 'charge up' when it fires at full capacity, so with a weaker flash you will be firing at close to full power quite a lot, so you'll get slower flashing, and also more power is just better for when you need to bounce off something or throw a lot of light into a scene- something like a softbox can eat your light quite easily so the 430ex doesn't cut it sometimes. So if you're in the market for a new flash then this is well worth looking at. Read my 430ex and 580ex reviews for more differences between the 3 flash guns.
Okay... instead of waffling on about how great this flash is, and how much it's increased my enjoyment of taking photos (indoors people, indoors!), i'm just going to go through some of the things that made me choose it over the 420EX... 1) Guide Number: it's a bigger, brighter flash, that means that you're more likely to get a photo that doesn't look like it was taken hanging out the side of a speeding train. 2) Slave units: You can use this as a master flash in a big set-up, should you choose to do this later on. From my point of view, i'm a "developing amateur" (apparently), and so wanted something that I could potentially be quite creative with later on. Having the option to be able to go away and do basic studio lighting using a couple of flashes was really good fun. 3) Tilt and swivel: if you're not familiar with flashes, you may not realise that they can fire in different directions, other than just straight into the subject's face. By bouncing the light of walls and ceilings, you can create a much more natural effect. This thing swings about in every direction you could want (note this in other flashes, because they don't all do this...) 4) Eye highlights: There's this clever reflector device that comes out of the top of the flash head (it's really difficult to describe, so i'll give up now.) which is a fairly unique feature. It creates a spotlight in the eyes of the subject, without bleaching the face - i.e. you get that sparkle in the eyes that really makes an exciting portrait. 5) Clever things: If you have a more advanced EOS camera, it shares and enchances the metering system and autofocus. So it can determine all the settings for you, meaning you get perfect pictures every time, just pointing and clicking, without going through all the maths (although all the manual features are there should you like doing all the maths...). It also does strobes and l
ots of other things that i'd understand if I was a better photographer. A proper flash really does add a different dimension to the way you can take photos, and can be a lot of fun. If used correctly, you can easily come out with brilliant portraits on a regular basis. So yes, a conclusion. Basically, the reason I chose this flash over the 420EX and any manual flashes, was the ease of use and the potential versatility it allowed me. If you just want to point and shoot, and use your SLR like a compact, then go for the 420EX, because the custom and clever bits will be wasted (as will your extra £100). This flash gun does anything that any other will do.