Beginner Manual Flash at short distances.

So, guide number is calculated (but often lied about) at 100 ISO, so at 50 ISO you open up 1 stop, at 200 ISO you close down by 1 stop.
Guide numbers are caculated at the flashgun's maximum zoom setting, which is often too narrow to illuminate all of the subject. It's a pretty meaningless figure because of this, but I suppose it's a starting point of sorts. At wider zoom settings the guide number will be lower.
Hotshoe flashguns tend to produce very hard light, which may not be what you want. If you need to diffuse the light, this will spread it over a (much) larger area, which will substantially reduce the guide number.

That's the reality, but ignoring all of these highly relevant factors, if the guide number is 61(m) and the flash to subject distance is 1m, then the apeture will be f/61 at full power, let's call that f/64....
At half power it will be f/45
At quarter power it will f/32
At one eighth power it will be f/22
At 1/16th power it will be f/16
At 1/32nd power it will be f/11
etc

So, at 0.5m you will be 2 stops down, e.g. at 1/32nd power it will be f/22

Real world, because of the factors mentioned, f/22 will probably be about 1/8th power.
The shutter speed is of course irrelevant when the only light source is the flash.
 
Garry Edwards said:
So, guide number is calculated (but often lied about) at 100 ISO, so at 50 ISO you open up 1 stop, at 200 ISO you close down by 1 stop.
Guide numbers are caculated at the flashgun's maximum zoom setting, which is often too narrow to illuminate all of the subject. It's a pretty meaningless figure because of this, but I suppose it's a starting point of sorts. At wider zoom settings the guide number will be lower.
Hotshoe flashguns tend to produce very hard light, which may not be what you want. If you need to diffuse the light, this will spread it over a (much) larger area, which will substantially reduce the guide number.

That's the reality, but ignoring all of these highly relevant factors, if the guide number is 61(m) and the flash to subject distance is 1m, then the apeture will be f/61 at full power, let's call that f/64....
At half power it will be f/45
At quarter power it will f/32
At one eighth power it will be f/22
At 1/16th power it will be f/16
At 1/32nd power it will be f/11
etc

So, at 0.5m you will be 2 stops down, e.g. at 1/32nd power it will be f/22

Real world, because of the factors mentioned, f/22 will probably be about 1/8th power.
The shutter speed is of course irrelevant when the only light source is the flash.
Click to expand...
Thank you, that's really helpful. That gives me somewhere to start and depending on the diffuser's affect on the flash, I can work from there. At close distances I want to try and be using apertures between f/11 and f/22.
 
I created a spreadsheet and printed off a table of working dstance vs flash power for any given aperture and ISO combination for my YN560 when I first got it, but tbh it's most of the time faster to shoot, review, adjust and reshoot than try and calculate anything!
 
Nothing to add to the above, you can chart it till you're blue in the face, but the reality is...
  • You know what modifier you'll use (if at all) ergo what zoom setting.
  • You know the working distance
  • You know the aperture you'll need
  • The SS is fixed
So your only other variable is your ISO, just set it up, start at 200 ISO and quarter power, adjust from there :)

You only have to 'experiment' for one shot, after that you'll have an idea of your settings when you set up the shot. Job done.
 
Thank you all, this has been really helpful. I was only one stop off when I took the first shot this morning, and I'm pretty sure that's because I misjudged the diffusion I had added. Managed to change working distances and flash power for each shot as needed without much hassle.
 
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