Originally posted by Ash Hi Will.
Cool stuff - should be lots of fun experimenting.
Yep, it has been already.
Quote: The calculation to work out the flash intensity you need is quite simple:....
I hate when somebody says that. ;-)
Quote: ...flash exposure at ISO 100 is GN = aperture × distance or distance = GN / aperture (working it out for full flash intensity), but it gets a little more complicated when you have 2 or more flashes working together. Though if just using one, then just remember for the aperture you've worked it out at, just reduce the flash intensity by one stop if you want to open the aperture by one stop, or alternatively, distance the flash source 'one stop' away from the subject (on a logarithmic scale) if you find that the flash intensity is one stop too strong for the exposure at full power, and so on...
I also hate it when someone uses the phrase "logarithmic scale..." :-)
But I think I've got the idea. The guide number for the Metz 58 AF-1 is 191 (ft) @ ISO 100. Let's round that GN up to 200 because it makes it possible for me to do the math without having to borrow my daughter's school calculator.
So if I'm 10 ft from my subject, shooting at ISO 100,
and with the flash at 1/1 full power, here's how I would do the calculation:
GN/distance = aperture
200/10 = 20
And then remembering that my guide number isn't really 200 but only 190, I'd reckon then that I should set my aperture to f/18 or f/19. Right?
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Now, if I wanted to increase the ISO to, say, 400, I could then DOUBLE the guide number (that's where the logarithmic scale comes in, I think), so shooting at ISO 400, my calculation would be:
400/10 = 40
But that's with the flash at full power (1/1). Since I don't have an f/40 on my camera/lens, I'll need to make some adjustments. And this is where it gets a little sketchy for me.
I don't have any trouble doing these calcs if I'm simply shooting without flash and trying to adjust my manual exposure settings. F/22 is two stops faster than f/44, so if my shutter speed at f/44 was 1/100th sec, I'd need to bump that up to 1/400th sec to shoot at f/22, or 1/1600th sec to shoot at f/11.
But working with the flash, I'm not sure it works that way. If the calculated right aperture with flash at full power is f/40, and I want the aperture somewhere around f/10 or f/11 (about four stops faster), what adjustment to I make to the flash? Would I set it at 1/4 or 1/16 or something else? If I'm opening the aperture four stops, I think I'd need to move the flash output to 1/16, right? Or have I got it all completely messed up?
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I will note that the Metz 58 AF-1 seems to be trying to help me out here, if I can only learn to understand what it's saying better. With the flash mounted on the K20D's hot shoe and flash in M mode, the flash seems to be able to tell what the camera's ISO is, and it shows me a distance on the back display that seems to indicate how far Metz's engineers think I should be standing from the subject. (I know it's flash-to-subject distance that matters, not photographer-to-subject or camera-to-subject. But remember, at the moment, the photographer, flash, and camera are in the same place.) Right now I have the DA 40 on the camera and the closest zoom setting on the Metz is 35, so I've got it set there. As I change the ISO on the camera, the distance displayed on the Metz's led changes; likewise, if I change the power of the flash on the Metz without changing the ISO, the distance number changes. So perhaps I don't need to do math at all?
Will
Thanks,
Will