Guide Numbers and Manual Flash.

ChristopherCoy · Feb 12, 2013

For rolls 5 and 6 of my 52 roll project I decided to try manual flash work with my Hasselblad. I've never really done it to the point that I'm fluent in its formula, so this is a perfect time to learn.

I understand that F = GN •/• Distance. So if I set my flash to manual ISO400, with a guide number of 25, then my aperture on my camera should be about f8 right? (f4 for ISO100, f5.6 for ISO200, f11 for ISO800 etc etc)

And if I am thinking correctly, this would only be for stop motion lighting right. Shutter speed would increase or decrease the ambient background lighting indoors right?

In addition, in manual mode on this flash GN 25 is the lowest setting, if I wanted to shoot at f4 instead of f8 according to the formula, my image would be two stops over exposed right? Is there a way to decrease the power further or is this where a ND filter comes into play?

This is what my flash looks like... I plan on bouncing the main head and using the sub flash for fill.

Chan Tran · Feb 12, 2013

The f/8.0 is correct only if your flash is about 3 feet from the subject. When you bounce it's difficult to estimate the light loss due to bounce.

ChristopherCoy · Feb 12, 2013

Ok I see what I did there...

25 •/• 3 = 8.333
25 •/• 6 = 4.166

Terry Christian · Feb 12, 2013

Be careful adjusting your shutter speed. Opening up the aperture to allow more ambient light to creep in is fine; but if you make the shutter speed shorter than your camera's sync speed, your flash will be underexposed, if it works at all. An ND filter would work, yes.

Or, if you are exposing your subject with primarily ambient light, using your flash for fill only and want to tone down your flash output for a more natural look, you can put an ND gel over the flash head.

mopar_guy · Feb 12, 2013

If I do a calculation like this, I always keep the distance units as part of the math to help avoid confusion.

f= GN/ distance(feet)

f=25 / 12 (feet) = 2 or
f=25 / 6 (feet) = 4

So at 12 feet (flash to subject distance) use f2.0 and at 6 feet use f 4.0.

Also make sure that the guide number for the flash is given in the same distance units that you use to calculate. The flash may have a standard Guide Number stated such as Guide Number=25 meters at ISO 100. If the flash guide number is given in meters, then you need to use meters to do the calculation. If the guide number is given for ISO 100 and you are using 400 film, then you will need to close down two additional stops to compensate for the faster film.

mopar_guy · Feb 12, 2013

Here's how I do this:

1) set up the shot and the you will estimate the Flash to subject distance (let us assume 15 feet).

2) Calculate the aperture: f=GN/Distance(feet). Assuming I am using my Olympus T-32 Flash that has a Guide Number of 104 (feet) or 32 (meters) at ISO 100 the math is as follows:

f= 104 (the guide number for full power in feet)/15 feet (the distance from my flash to the subject) or; f=104/15=7

The aperture calculates to f7 at ISO 100 for 15 feet. If I am using 400 speed film, I stop down 2 additional stops. In this case I will use f16 or put a 2 stop ND filter on the flash.

ChristopherCoy · Feb 12, 2013

Terry Christian said:
...but if you make the shutter speed shorter than your camera's sync speed, your flash will be underexposed, if it works at all.

It shouldn't be an issue should it? My Hasselblad has the leaf shutter in the lens and not a focal plane... And it only has a max of 1/500, and if I were shooting indoors I probably wouldn't be that high anyway.

ChristopherCoy · Feb 12, 2013

mopar_guy said:
. If the guide number is given for ISO 100 and you are using 400 film, then you will need to close down two additional stops to compensate for the faster film.

If I change the ISO on this flash to 100 then the min GN is 12.5, 200 is 18, and 400 is 25... Which makes sense now mathematically.

MattKing · Feb 12, 2013

If you are bouncing your flash, all your guide number calculations will need to be changed.

You will need to include both of the following in your calculations:
1) the total distance of the entire light path (flash to bounce surface to subject); and
2) the light loss caused by incomplete reflection by the bounce surface.

Guide numbers don't work well with bounced flash, unless you use a standard bounce reflector in a standard configuration, with a re-calibrated guide number.

mopar_guy · Feb 12, 2013

Since the flash has a GN of 25 @ ISO 400, there is no need to correct for the different ISO. You want to use ambient light for your base exposure and the flash for fill. You can estimate the balance of ambient to fill as a ratio: say 3:1 you would not want the flash to be more powerful so you can reduce the amount of light from the flash. The flash burst from electronic flash is quite short and Theoretically you will capture all of the flash's light on film if the shutter syncs properly.

Terry Christian · Feb 13, 2013

ChristopherCoy said:
It shouldn't be an issue should it? My Hasselblad has the leaf shutter in the lens and not a focal plane... And it only has a max of 1/500, and if I were shooting indoors I probably wouldn't be that high anyway.

Okay, you didn't say you we're using a Hassy before. I was just concerned because some cameras can sync as low as 1/60, like some focal-plane shutter SLRs. Shutters in d*g*tals seem to run roughly 1/200, and high sync speeds are useful for blocking out the ambient and enabling sufficiently open apertures to get shallow depth of field for portraits.

M Carter · Feb 14, 2013

A really quick way to bake this kind of info into your brain... is stick a polaroid back on and grab a notepad. There's something about taking a shot, examining it, coming up with what seems the correct next step, and trying it out instantly... vs. shooting a roll and processing and trying to suss out notes for every frame. In the pre-digital days it was a dynamite way to learn some more advanced stuff very quickly or test your crazier ideas.

There's still 2 decent flavors of pack film available after all... and peeling those things always brings back memories (for me anyway).

EDIT: you can likely get a hassy polaroid back for next to nothing thee days, if you don't have one...

John Koehrer · Feb 18, 2013

You can reduce the output by using a layer or two of lens tissue over the sub flash. Main flash too if you wish. Since you're dealing with guide numbers, there's a bit of testing to be done. The gn can get you close, but it' a guide not actual.

M Carter · Feb 20, 2013

John's tissue idea is really useful. When I'm at a bar with the mrs., I'm always tearing off little strips of bar napkin and licking the edge to cover the flash on her phone - if you curve 'em enough you get instant soft-box (OK, she's a always posting pics of dinner on facebook, it's not as effective for group shots but makes closeups really pretty...)

ic-racer · Feb 20, 2013

Chan Tran said:
The f/8.0 is correct only if your flash is about 3 feet from the subject. When you bounce it's difficult to estimate the light loss due to bounce.

What he said. I'd use auto.

mopar_guy · Feb 20, 2013

ic-racer said:
What he said. I'd use auto.

If you bother to read the Original Post in this thread, you would remember that this is a project to use manual flash. On purpose. Maybe the OP will end up learning something, which is not really a bad thing.

Guide Numbers and Manual Flash.

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