Manual flash power...how do you know?

G'morning, Curtis -

The Auto was pretty inconsistent for me - I'm going to try to repeat the experiment tonight, but in full manual mode and was hoping for some sort of rule of thumb or SOMEthing to help me get a ballpark idea for the manual flash.

As much as I love my nephews, they're NOT going to sit still for 10 minutes while I shoot 'n' chimp...so I've gotta be pretty close in the first couple shots before they turn into fidget monsters.

Though I use a Vivitar 285HV your Sunpak should have a similar calculator on the back of the unit.

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markhyo wrote in post #5237157
Though I use a Vivitar 285HV your Sunpak should have a similar calculator dial on the back of the unit.

It doesn't. It has sliding scales.

cdifoto wrote in post #5237162
It doesn't. It has sliding scales.

Right. But they work the same way, once you figure out what the numbers mean.

Exactly what I was getting ready to post Curtis. It's a different calculator but the principle works the same way.

Curtis N wrote in post #5237171
Right. But they work the same way, once you figure out what the numbers mean.

Of course. Just didn't want people to panic when they looked at their 383 and saw there is no knob.

More on how distance affects the amount of light:
Fill light at sunset

My 1-cent flash "meter" in images 5S & 6S.
Simple 2 Light Portrait Set-up
A basement, "2" lights, & 45 minutes later...
In Post #15, there's a link to: Low cost off camera flash tutorial for beginners

mnealtx wrote in post #5235871
I've read through all the stickies and everything else I can find - no luck with the information I'm trying to find.

Given a manual flash (Sunpak 383) - how do you know what power to set it at to get the right exposure?

I've seen the posts about using the guide number, (GN 120 @ f/4 = 30 feet between flash and subject), but how do you know that's the right amount of light, without tons of fiddling and chimping?

GUIDE NUMBER makes it all simple!!!

In this case at ISO 100, the guide number at Full power is 120. So if you focus an the distance is 30', the guide number result in f/stop of f/4 (120/30 = 4). If you set it to 1/2 power, the guide number is (120/1.4 =) 86, so for same distance, 86/30 = f/2.8. If you set it to 1/4 power, the guide number is 60 (120/2 = 60) and the f/stop to use at 30' is f/2

If you upped the ISO to 200, multiply all of the guide numbers in the preceding paragraph by 1.4 and use that in the computations (ISO 200 GN=168). If you upped the ISO to 400, multiply all of the guide numbers in the preceding paragraph by 2 (rather than by 1.4) and use that in the computations (ISO 400 GN=240)

Then you have to understand also that the GN gives you a normal exposure, but you might want to deliberately underexpose by some amount, in order for the lighting to not look abnormally bright. For example, fill flash on a subject whose face is turned away from the sun should probably be exposed at -1EV rather than the exposure indicated by the guide number. So using ISO 100 examples, you might shoot at f/2.8 and use 1/4 power rather than 1/2 power.

Aha... so the distance calculations *should* yeild a proper exposure, then....THAT'S what I was looking for!!

So (example given for the 383 on Auto [out of the manual pdf]) ISO 100 @ f/4 shows a distance of 30 feet (full power). So, to get the proper exposure, the flash needs to be 30 feet away from the subject (assuming straight-on flash)?

Now, for Auto flash (again, using examples from the manual since I'm at work), for full power, f/4 (yellow zone) it shows a distance from 4 feet to 30 feet. In that instance, I would need to be in the middle of the range shown, or around 16 feet, to have the best chance of a proper exposure, even with the thyristor controlling the flash duration, correct?

Then, any bouncing or modifiers would decrease the distances accordingly, and I'll have to figure *that* out from experience.

Thanks for the explanation, everyone - that makes a whole lot more sense, now!

Wilt - so for each stop of change in ISO, or "notch" of power output change, there's a corresponding 1 stop change in light output?

Wilt wrote in post #5237340
GUIDE NUMBER makes it all simple!

Sure it does, if you only have one guide number.

Today's cameras have about fourteen different ISO settings. How quickly can you figure the GN of the Sunpak 383 at 1250 ISO?

Take it one step further with a modern flash that has eight different guide numbers, depending on the zoom setting. Combine these two factors, and you have over 100 different guide numbers.

That's why they have distance scales.

More about guide numbers here:
https://photography-on-the.net/forum/showthre​ad.php?t=208520

Curtis N wrote in post #5237655
Sure it does, if you only have one guide number.

Today's cameras have about fourteen different ISO settings. How quickly can you figure the GN of the Sunpak 383 at 1250 ISO?

Take it one step further with a modern flash that has eight different guide numbers, depending on the zoom setting. Combine these two factors, and you have over 100 different guide numbers.

That's why they have distance scales.

More about guide numbers here:
https://photography-on-the.net/forum/showthre​ad.php?t=208520

Changing ISO is not bad...ISO 100 to ISO 1250 is 3-2/3 EV difference...call it 4EV difference for convenience (after all well know the GN is a 'guide' after all! ) So simply multiply the ISO 100 GN by 4x!

What makes GN complex is only the changing Angle of Coverage for the Focal Length zoom heads and the fact that user manuals often neglect to provide a usable guide number for the FL settings that are available.

As Curtis points out, the display on most flash units makes the variable easy...it tells you for the current f/stop and ISO what the max distance you can get proper exposure...so simply multiply the f/stop * max distance = Guide Number.

mnealtx, if you have the flash on Auto, the flash regulates itself down in power to make the shot with your selected aperture. It aways has Full power available if needed, but it also can reduce itself down very low in power. The display tells you the maximum distance achievable for your selected aperture and ISO assuming max power.

I think (for right now, anyway, while I'm still getting my toes wet) that I'm glad the 383 is a non-zooming flash!!!

The control panel gives a near and far distance in the various Auto modes. It seems logical (which means I'm probably wrong) that exposure would be the most accurate if I'm in the center of the range, would it not?

I'd swear I read something talking about that in some thread, a while back...but can't find it now.

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Great stuff here guys, thanks a lot! It will help me too

mnealtx wrote in post #5237875
The control panel gives a near and far distance in the various Auto modes. It seems logical (which means I'm probably wrong) that exposure would be the most accurate if I'm in the center of the range, would it not?

It should be accurate throughout the range indicated.

With traditional auto flash (or E-TTL for that matter), by far the biggest variable is the luminance of the subject (white dress, black tux). Within the indicated distance range, all other factors that could affect the accuracy are insignificant by comparison.