Understanding Flash Unit Specs

Ive tried searching for this answer but havent come across anything that has actually helped. So I apologize in advance if it truly is right under my nose. But can someone give a very easy to understand lesson on what the different specs for a flash actually means?

for example: maximum Guide No. 58 ( meter; ISO100; 105mm)
Im already lost at the very first spec

ok maybe i got one down in tryin to compare flashes.

Guide number: The higher the guide number, the more powerful the flash

Yes, but that is not the whole story.

In the "very old days", the first electronic flashes had no automation and no manual controls. They would just fire a burst of light like an old flash bulb.

You would have to make a calculation dividing the GN (guide number) by the distance and that would give you the required f/stop.

The Canon 580EX has a GN of 58 at ISO 100, and with the flash head zoomed out to the max of 105mm. If you happen be using a lens with a focal length of 105mm or longer, you would divide the GN by your shooting distance and get the f/stop.

Example: GN 58 divided by a shooting distance of 7 meters you would get approx f/8.2

You would then set you your lens to f8 and take the shot.

The GN can also be expressed in feet instead of meters. The 580EX has a GN of 190 feet at ISO 100.

If you shoot at a different ISO, then you must use a different GN. For the 580 EX the GN at ISO 400 increases to 116 meters.

Also the GN changes if you set the flash head to a different setting. At a 50mm setting the GN is only 42 meters. Setting the head this way allows the flash to have a wider coverage but uses up some of the power. At a 24mm setting the GN is only 28 meters.

Here is a handy chart that shows different GN's for various Canon flashes at different settings: http://www.kjsl.com/~d​ave/speedlites.html

If you use "bounce flash" then all these calculations go out the window, because you don't know how well the light will be reflected or the actual distance the light will travel.

In the "OLD DAYS" an electronic flash usually had less power, and didn't have zoom heads, so it made the calculations a little easier, but I think you can now appreciate why they invented Automatic flash (like auto thyristor, TTL and ETTL)

Today ,Guide Number is usually just used to express the maximum power of the flash for comparison sake.

elitejp wrote in post #6894665
The higher the guide number, the more powerful the flash

Unfortunately, it's not quite that simple.

As Mike pointed out above, the coverage angle (often expressed as focal length equivalent) will make a huge difference.

Take a look at my previous attempt to explain guide numbers:
https://photography-on-the.net/forum/showthre​ad.php?t=208520

I appreciate the explanation for guide numbers. But really what I am after is trying to learn why this flash would be better than that one. I just used GN as a starting point. I would guess all flashes have a gn rating, and learning how to compare them is a good start. Like reading a motherboard its nice to know why this one is preferred over that one how the pieces work together.

Guide number, ETTL capability, master/slave capable, head swivel direction, high speed sync, stroboscopic, recycle speed, manual capable, are just some of the items to compare. Generally the more the flash will do the more it costs.