Well here is the low down on off camera flash. There is indeed a standard output measure that is a lot more reliable than the watts per second of the studio strobes. The power output is given in guide number (either in meters or feet). I say it's more reliable because not all studio strobes effectively convert the w/s into light energy, so you will find that some better (and more expensive) strobes will put out more light for the same w/s rating as a cheaper strobe. With guide number, unless they are using misleading info, that's the amount of light being put out.
Cheaper flashguns usually have less power, less features, less f-stops range and longer recycle times and of course less durable build.
For example my Metz 48-AF cost me about 3 times what I paid for the Vivitar 285HV. I've had the Metz for twice as long and the Vivitar already has a nasty crack on the plastic mounting shoe while the Metz is as good as new.
Although the type of battery you use will a lot of times change the recycle time for the flash, it will not be that way for all flashes. I had for example a small Metz with a guide number of 20 (m) and it took a full 10 sec to recycle no matter what battery you put in it.
My Vivitar goes from 10 sec to about 5 or 6 sec if I use NiCad batteries instead of alkalines, and my Metz 48 is usually in the 2 to 3 sec range. So it can be more than just an extra second or 2, waiting 10 seconds for your next frame can feel excrutiatingly slow when on a shoot.
I can change the power on my Metz from full to 1/128 power while the Vivitar goes from full to 1/16 power and some cheaper flashes have only full power and auto (thyristor controled) settings. So you have to adjust power by putting the light closer or further away. This of course will also affect contrast and might create uneven lighting due to the inverse square law. It's much better to have a wider power control range, allowing you to fine tune the light output without changing flash to subject distance.
The full power output of my Metz flash is 48 m ISO 100 @105mm, the Vivitar I think is 41 m ISO 100 @105mm (for more info on guide numbers read
this article by David Hobby aka The Strobist)
I can mount the Metz on my camera and enjoy features such as tilt and swivel head, high speed sync, second curtain sync, P-TTL metering, focus assist beam (not much of an issue with the K-5, but it's there if you need it) and can even fire the Metz wirelessly (optical P-TTL) with the wireless triggering mode on Pentax bodies and flashes. Not that my flash spends much time mounted onto the camera (I'm a radio firing, off camera, manual flash power loving kinda shooter =P ) but again it's nice to have for those ocasional moments where you can't be placing lightstands and your best bet is to bounce that light of a ceiling or wall and pray for the best.
Some of the pricier models also give you the option of using an external battery pack to power the flash, giving you even faster recycling times and more flashes per charge.
Now that I've given you a run down of how a "cheap" Vivitar flash compares to a more expensive flash unit, it is important to note is that the Vivitar is a classic and probably one of the best "cheap" non dedicated off camera flashes you can buy (it runs for about 85$ from Adorama).
Buying a cheap flash with a guide number of 20 m is just wasted money, it won't serve for much more than a hair light and shooting through or bouncing off an umbrella always makes you loose a few stops of light, so by the time the light has reached your subject, it's probably weaker than the light put out by your built-in flash. Granted it will be nice soft directional light, but very underpowered forcing you to shoot open wide with fast glass or with higher ISO. You can certainly forget balancing strong sunlight with those flashes. I speak from my experience with the Metz Mecablitz 20 (and it cost me 60 bucks, a mere 20$ less than the much better Vivitar).
If you have the money, buy a couple of Metz 48-AF (I think the current version is the 50-AF) or equivalents, if you're on a very tight budget, consider something like the Vivitar 285HV (you will at some point have to replace the plastic hotshoe; they are prone to breaking). I have also heard some good things about those Yongnuo flashes (quite cheap for their feature set), but since I have never used one (nor held one in my hand) I can neither recommend nor discard them as an option.
Originally posted by wildweasel 
Post #15 by joe.penn
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