Comparing flashes Math problem

Looking at two different flashes you have:

1. GN 58, Focal Length 105, ISO 100
2. GN 38, Focal Length 50, ISO 100

Now I know that GN/mm(focal length)=aperature. But if you just plug those numbers in you dont come out with ISO 100. So how do you actually solve for this? Im wondering if I need to learn my f stops

The beauty of modern digital cameras is that you don't have to know anything: You simply take a picture and look to see if it's OK. If not, you delete it, change something and take another picture. Repeat.

If you take enough pictures you will begin to have a "feel" for what is a good starting point for your camera and flash settings. Often the camera has an automated mode which can get you most of the way there.

To get all the way there, just do some empirical tests which is easy to do.

I guess that folks with experience just have to make fewer repititions.

P.S.: The 580EXII manual has a GN table. At 50mm, ISO 100 the GN is 42 for full power (1/1 setting).

In essence then these two cameras would be very similiar then.
I realize that the flash would figure it out for you but believe it or not where I live it is very hard to walk into a shop and compare because they probably only have a select few. And the fun thing is "ALL" the shops carry these exact same select few. So my shopping has turned to the internet. haha

40Dude thanks for that ps.

Did you read the tutorial on guide numbers I recommended in your last thread?
https://photography-on-the.net/forum/showthre​ad.php?t=208520

Hard to directly compare unless you can get GN for each, using the same FL coverage area.
In comparing two GN58 units (Canon 580EX and Metz 58AF) they have same GN for 105mm, and then the same GN at 50mm...GN42.
So one could assume that the GN38 of the second flash in the OP would be less power than both GN58 flashes (comparing GN38 vs. GN42)

There's more to a flash than just the GN.

40Dude6aedyk wrote in post #6902719
The beauty of modern digital cameras is that you don't have to know anything: You simply take a picture and look to see if it's OK. If not, you delete it, change something and take another picture. Repeat.

If you take enough pictures you will begin to have a "feel" for what is a good starting point for your camera and flash settings. Often the camera has an automated mode which can get you most of the way there.

To get all the way there, just do some empirical tests which is easy to do.

I guess that folks with experience just have to make fewer repititions.

P.S.: The 580EXII manual has a GN table. At 50mm, ISO 100 the GN is 42 for full power (1/1 setting).

The PROBLEM with modern digital cameras and flash units is that you don't need to know anything. It limits growth and general uderstanding of photography and more importantly, light. Seems to be the sad trend of the future.

On the plus side, those who learn light and flash and photography will out shine those who are solely dependent on the automation available.

Titus213 wrote in post #6904335
The PROBLEM with modern digital cameras and flash units is that you don't need to know anything. ....

Hey, I got new Canon 1DMkIII. How do I take a picture?
(BTW, I already threw away the manual because I wasn't going to read it anyways.)

Curtis N wrote in post #6902811
Did you read the tutorial on guide numbers I recommended in your last thread?
https://photography-on-the.net/forum/showthre​ad.php?t=208520

Ya I did. And It did finally dawn on me that distance was not focal length and that was where I was getting confused. I was putting focal length in the equation.
Now it does seem to me that unless you had the flash in your hands then you wouldnt be able to measure what the GN number is for a certain focal length.

Now I want to get back to your flash thread and see what else you wrote. I do appreciate the cliff note version you wrote out.

elitejp wrote in post #6907388
Now it does seem to me that unless you had the flash in your hands then you wouldnt be able to measure what the GN number is for a certain focal length.

Think of GN sort of like engine horsepower. You don't measure it, the manufacturer does. It's a product specification that you can use as a guide, if you understand it.

If my flash has a GN of 120 feet at ISO 100, that means that if the flash is 12 feet from the subject at full power, I'll need f/10. That's because 120 divided by 12 equals 10.

Conversely, if I want to shoot at f/16, I know I'll need the flash to be 7.5 feet from the subject. 120 / 16 = 7.5

Curtis: since your around heres one more question a bit off topic
what does this mean flash speed 1/8500～1/600? is this good or bad?

(im translating from chinese so take that into account if their is a different word we like to use in english)

elitejp wrote in post #6907443
Curtis: since your around heres one more question a bit off topic
what does this mean flash speed 1/8500～1/600? is this good or bad?

(im translating from chinese so take that into account if their is a different word we like to use in english)

I would translate "flash speed" as "flash duration". The numbers you quote are standard for any speedlite type flash unit and nothing special. This makes sense since the physics of producing a flash are pretty much identical in all flash units. See this link that PacAce has posted previously:

http://www.chem.helsin​ki.fi …sh-discharge/regular.html

And also this link posted by Dr. Zivnuska:
http://www.photosbykev​.com …non-580ex-flash-duration/

big thumbs up for all the help

elitejp wrote in post #6902644
Now I know that GN/mm(focal length)=aperature.

Substitute mm(focal length) with "distance" and you're right...