GN Rating for Speedlights

[TCampbell]

The guide number is the distance that a flash can adequate illuminate a subject assuming ISO 100 and f/1.0.

But if you already know the guide number of the flash and you aren't using modifiers, then you can use the original rule... which is that the guide number is the distance at which the light can adequately illuminate a subject for an exposure assuming ISO 100 and f/1.0 -- then you can calculate the f-stop for any other distance (or if you want to change the f-stop you can calculate the new subject distance... of course the math works both ways.)

What is this f/1.0 business?

f/1.0 means nothing to Guide Number definition, it is just another aperture.

If f/8 at 10 feet, or f/4 at 20 feet, or f/16 at 5 feet, is the proper exposure, then the GN is 80. No f/1.0 about it.

But yes, f/1.0 at 80 feet would also be GN 80. I suppose you are making up your own definition from this numerical concept involving the number 1, but f/1.0 seems mighty impractical, and a confusion. No one else does that. Absolutely no need of it.

guide number - Google Search

No link on that first search page mentions f/1.0 once.

I'm guessing you're new to photography... or certainly new to manual flash photography.

f/1.0 is used because it's a baseline that makes the math easy. Nobody expects you to own an f/1.0 lens, much less shoot at f/1.0 even if you had such a lens.

By using f/1.0 it means you can either:
a) divide the guide number by the f-stop you really plan to use to determine the distance, or
b) divide the guide number by the subject distance you plan to use to determine the f-stop

I wish I could make this stuff up. No... I've just been shooting for a lot of years and most of the years I shot with film and completely manual flash. We had to know this stuff because it's not like we could peek at the film to see if we got the exposure right before taking the next shot.

 

[WayneF]

I'm guessing you're new to photography... or certainly new to manual flash photography.

f/1.0 is used because it's a baseline that makes the math easy. Nobody expects you to own an f/1.0 lens, much less shoot at f/1.0 even if you had such a lens.

Sorry, you misjudge that too. I was using Guide Numbers fervently in the late 1950s. Doing much dark room work then too. Had a Nikon F in the 60s, I was an avid beginner back then, but actually, I have come a ways since, and seen a thing or two. Remember FP flashbulbs?

f/1.0 is not a base line for GN. What good is it if we have no f/1 lens? LOL
It is a baseline for EV (equivalent exposure setting groups), maybe you could be confused and mixing GN with EV (f/1 at 1 second is 0 EV), but EV has no relationship with Guide Number either.

No source explaining guide number ever mentions f/1. Because, it is not needed, it has no relationship, it plays no part, it is a distraction. You will not find any reference online mentioning f/1.0 in association with Guide Numbers. Well OK, I do remember seeing a Pentax mention of f/1 once - some nut there in Tech Pubs I suppose. I remember being puzzled about it then too, because there simply is no importance of f/1.

Guide Number = f/stop x distance.

The entire whole point of GN is to plug in any f/stop, and it works. Inverse square law is a square relationship, as is f/stop, and this equation makes the messy math cancel out. That equation does allow f/1.0 too, but it certainly is NOT about f/1.0, no more than it is about f/22. I suppose the only possible thing about f/1 is then that distance happens to match that GN number. Which may seem magic to a math beginner, but that is no miracle, that is how 1 works. But that f/1 number is totally useless if we are using f/8. Yes, I think this is pretty funny.

By using f/1.0 it means you can either:
a) divide the guide number by the f-stop you really plan to use to determine the distance, or
b) divide the guide number by the subject distance you plan to use to determine the f-stop

Then simply explain how is f/1 is used, or how does it help that, in any way? How is it different if we never think of f/1? Rhetorical, I know there is no possible answer, simply no way f/1 has any effect on GN whatsoever.
What we need to remember is the equation, not the distance where f/1 might work. LOL

Yes, it seems silly. It can't hurt me if you want to imagine it that way, but it seems a sin to mislead newcomers with crap. That is my objection.

 

[photoguy99]

To be fair, I've been taking photos for quite a while and never ran across the f/1.0 explanation before. Clearly it's a common explanation but not universal.

Unlike Wayne, I think it's slick and clever and useful.

At this point you guys are talking past one another. By which I mean Wayne isn't paying any attention to anything you say because he's itching for a fight.

 

[WayneF]

At this point you guys are talking past one another. By which I mean Wayne isn't paying any attention to anything you say because he's itching for a fight.

I think it must be the other way around? I am asking how does f/1 influence the definition of GN? No answer.

 

[photoguy99]

You're being argumentative and refusing to see the point.

The two definitions are identical in the results they produce.

One is simple: The GN is a certain distance.
The other is complex: The GN is the product of these two numbers neither of which is fixed but which vary in lock step.

Just because you've never heard of the simple one doesn't mean it's crap. It is in fact a much better definition.

 

[WayneF]

You're being argumentative and refusing to see the point.

The two definitions are identical in the results they produce.

One is simple: The GN is a certain distance.

This one is simply wrong. GN is certainly NOT any specific distance. It is a relationship of distance and aperture (the equation).

The other is complex: The GN is the product of these two numbers neither of which is fixed but which vary in lock step.

Yes, that is the full relationship, and the extreme beauty of Guide Number. It has nothing to do with f/1. I explained that.

The distance that some f/1 user would realize is of zero importance to me using f/8.

It would seem you are not grasping that.

 

[photoguy99]

And now you're doing it to me. Dogmatically insisting that your definition, despite being completely equivalent to another, simpler, one, is RIGHT and others are WRONG.

Well, stick this in your pipe and smoke it: The correct definition is:

DIstance to subject multiplied by the distance from the rearmost optical surface of the lens to the plane of focus when the lens is focused at infinity, divided by the diameter of the iris opening.

Now that's a handy definition! And, incidently, exactly equivalent to the other two we've seen.

 

[WayneF]

And now you're doing it to me. Dogmatically insisting that your definition, despite being completely equivalent to another, simpler, one, is RIGHT and others are WRONG.

That is simply because right is right, and crap is crap.

Well, stick this in your pipe and smoke it: The correct definition is:

DIstance to subject multiplied by the distance from the rearmost optical surface of the lens to the plane of focus when the lens is focused at infinity, divided by the diameter of the iris opening.

Now that's a handy definition! And, incidently, exactly equivalent to the other two we've seen.

Your quote, wherever it came from, is trying to say (subject distance) * f/d, which is like Guide Number.

f/d is fstop, except f is certainly NOT defined to the rearmost surface. Extreme wide angle lens, the point is way behind rear element (so the SLR mirror can be raised). Telephoto lens, it is in front of front element (by definition). Most lenses, at some interior point, hard to define, designer needs to specify.

And d is not the physical aperture diameter, but is the apparent effective diameter seen through the magnification of the front elements.

And subject distance is not about the lens distance, but is about the flash distance. If flash is on the camera, they can be the same.

If you replace yours with flash distance x fstop number, it would be right. (I am just trying to help you).


So, I am asking either of you, please explain how f/1 is in any way important or useful to the definition of guide number?
How is it used, what does it do? Be specific. It is your claim, you claim some advantage, so how is it seen useful?

I have GN 80, and want to shoot at 10 feet. How do I use f/1 to determine this?

Frankly, I would divide 80/10 and get f/8.

ROFL

 

[photoguy99]

Sure, no problem.

GN 80 means that, at f/1.0 I need to have the flash 80 feet (meters?) away. Let's assume feet.

Since I want to shoot at 10 feet, obviously I can't use f/1.0 which my lens doesn't do anyways. So I have to stop down. Since I need to divide the distance by 8, I will need to, correspondingly, multiply the f-number by 8. So, f/8.0. That was SUPER HARD OMG.

 

[WayneF]

Sure, no problem.

GN 80 means that, at f/1.0 I need to have the flash 80 feet (meters?) away. Let's assume feet.

Since I want to shoot at 10 feet, obviously I can't use f/1.0 which my lens doesn't do anyways.

Right, so f/1 is simply not important, not in any way. 80 feet is not important, not in any way.

I can contrive some way to say 154.6 feet too (multiply by square root of shoe size maybe), but it is simply not a factor.

They simply are not factors, not in any way.

What I asked was, how is f/1 important in any way? I do understand there is no way you can answer.. It's not.

GN 80 / 10 feet = f/8

LOL

 

[photoguy99]

One more time, because I am dumb:

The use of f/1.0 makes the definition easier to grasp.

Rather than explaining to the noob that it's the product of this thing and that thing, you give them a simple definition. THEN you walk them through the process. A simpler definition has value. And you're either an idiot or a troll and I don't care which. The fact that you sign off with LOL or ROFL or some similarly imbecilic acronym makes me suspect 'troll' though.

 

[WayneF]

One more time, because I am dumb:

The use of f/1.0 makes the definition easier to grasp.

Rather than explaining to the noob that it's the product of this thing and that thing, you give them a simple definition. THEN you walk them through the process. A simpler definition has value. And you're either an idiot or a troll and I don't care which. The fact that you sign off with LOL or ROFL or some similarly imbecilic acronym makes me suspect 'troll' though.

No, I am just a seeker of truth and justice. I like to chase and debunk false crap (because, right is right). Surely you know how it is when someone on the internet is wrong?

So specifically HOW does f/1.0 make the definition (definition of distance x fstop) easier to grasp? At best, it seems only a superfluous example, but not pertinent to any case at hand. An example is one thing, but calling it a definition is a load of crap. If you believe it does help, surely you can say HOW does it help? What does f/1.0 do? Where do I plug it in? I may be doing it wrong, because I never do, yet it seems to always work out OK.

 

[photoguy99]

Nope. Done explaining it. You've proved your trollness by circling back around and trying to make me bite again. It's been explained already, go re-read the thread if you're really interested.

You're not, you won't, that's ok with me.

 

[WayneF]

You have explained nothing. I've asked a few times, and nothing. Of course, I'm just having fun, because I know you can't, it makes no sense.

 

[photoguy99]

Trolls never cease to amaze me. "You've explained nothing" when there's 100s of words of explanation in the thread, right there in front of you.

Do they actually think anyone will believe it? Do they believe it themselves? It's like standing in the rain declaring what a sunny day it is. It doesn't make you look smart, it makes you look insane. Mysteries abound!