Guide Numbers and Manual Flash.

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.

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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.

Ok I see what I did there...

25 •/• 3 = 8.333
25 •/• 6 = 4.166

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.

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.

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.

Quote:

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Originally Posted by Terry Christian

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

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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.

Quote:

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Originally Posted by mopar_guy

. 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.

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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.

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.

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.