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# Thread: need help calculating aperture with speedlights

1. ## need help calculating aperture with speedlights

Hi all.

I did some test shots with my Metz 44 on manual on my Bessa R4A with Polypanf 50 ISO.
I shot everything at 1/125, at approx 1,2 meters, flash bounced from white ceiling.

I did test shots of a rather dark scene at apertures 2.8, 4, 5.6 and 8 with flash power set at 1, 1/2, 1/8 and 1/16.
Guide number of flash should be 44 at 100 ISO, so that would be 44/1.4=31 at 50 ISO, correct?

If I remember correctly, the correct aperture is guide number divided by distance to subject.
The only good negatives are f/2.8 and 4 at 100% flash power an f/2.8 at 50%, the others are too thin.

But these apertures are much bigger than my simple calculation suggested (i.e. GN31/1.2m = f/25). Where did I go wrong? How do you take into account the bounce flash when calculating aperture?

Need input...

Rgds,
Gerd.

2. when attempting to calculate exposure using bounce lighting there are two major factors only one of which can be calculated.
1. total distance from flash to reflecting surface to subject
2. % of light reflected by the reflecting surface - this may be very low

3. The reflectance can be assume to around 80% with white ceiling but as in previous post you must consider distance between flash and bounce surface and then to subject and also consider now the light is no longer straight ahead. The amount of illumination fall off no longer follow the inverse square law as it's no longer a point light source but a fairly large light source. But your result is what I would expected in such a situation.

4. This is the procedure I usually follow when bouncing flashlight off the ceiling.

1. Focus at the bouncing point on the ceiling to measure the 'flash to bouncing point' distance. (i.e. 1.2m)
2. Stoop a little to be the same height as the photographing subject.
3. Focus at the bouncing point again to measure the 'bouncing point to subject' distance. (i.e. 1.6m)
4. Now I get the total length of the path between the flash and the subject (1+3) (i.e. 2.8m)
5. Estimate the reflective ratio of the ceiling. (i.e. 25% : 0.5stops blighter than the 18% reflective card)
6. Aperture calculating

(example)

film ISO100
flash GN 28m (ISO100)
total travel of light: 2.8m
-> 28/2.8 = F10 (provided that the bouncing surface is 100% reflective)
estimated reflevtive ratio: 25% (2 stops of light loss)
-> F5.0
safety margin (only for print films): half a stop
-> F4.0 (2.5 stops from F10 in total)

This procedure is only applicable when the bouncing point is located just between the flash and the subject (the incident and reflective angles should be the same), and the ceiling should not be too low. but it does work well for that use. You may feel somewhat apprehensive to estimate the reflective ratio, but most people usually have good experiece of estimating the ratio already. If the color of the bouncing surface is as blight as the 18% grey card, that means 18% of your flashlight will reach the subject, which is equivalent to 2.5stops of a light loss. What you have to do is compensating the aperture from there. And this is not so different from compensating your exposure setting when shooting under the available light depending on the in-camera reflective light meter.

5. The traditional GN calculations are only good for direct flash. If you're bouncing it, a good rule of thumb is to overexpose by roughly 2 stops (if direct flash calls for f/11, then use f/5.6 for bounced flash). However, there are many variables involved like the color or reflectance of the ceiling and its distance to the subject.

6. hi gerd.

i am kind of a luddite when it comes to these sorts of things but whenever i use a flash and bounce it ...
i need to add to the distance from the thing i am lighting when i figure out my fstop. for example ..
if the "thing" is 8 feet away, and i bounce to it, well it is xfeet from the ceiling and the ceiling is yfeet from me
s0 it will be x+y feet ...as others have mentioned the total amount of light doesn't make it to the subject, so you have to
take that into account too ...
sorry to suggest this, you might think of picking up a cheap flash meter ... years ago i picked up one and it makes it easy to
make sure my i am exposing things correctly ... some, even the cheap ones, allow for 3 readings, so you can do ambient readings
as well as flash readings at the same time ...

have fun!
john

7. I will second the flash meter. Luna Pro F or Luna Six F, same thing I think.

8. Guide Number arithmetic only applies to direct flash...a Guide Number is flash-to-subject distance ordinarily, but by bouncing that calculation is useless.
The ceiling-to-subject distance might seem to have a factor, but when a flash is bounced up to the ceiling, the ceiling is a large virtual source which is NOT governed by the Inverse Square Rule of small source falloff in intensity.

To illustrate, I just did this with my Metz 54MZ, which has GN106 with 'normal lens' coverage angle. With my subject exactly 7' from where I stand, GN arithmetic says f/15.
Most every speedlight is overrated by the manufacturer (virtually every brand is guilty of this!), and most will actually flash meter LOWER than rated by about -1EV! Direct flash from Metz 54MZ standing 7' away indicates f/11 +0.0EV with Minolta flash meter (GN77) vs. manufacturer rated aperture says f/15 (GN106)
Standing 7' away from subject, and using ceiling bounce which is 7' above the subject (subject to center of bounce) meters f/4 + 0.6EV, or GN35.

Measured direct flash = GN77, while ceiling bounce measured GN35...about -2 + 0.3 EV loss due to bounce from measured GN for direct flash.

9. Thanks all for the help!
So an new gadget it is!