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Old Saturday 3rd July 2010, 08:00   #1
Coati
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Fill flash

Hi

Can someone advise what kit might be best for Canon 40D and 100-400 Combo

for use in the field (tropical ) IF I was to consider it.

A lot of birds in CR are troublesome; tending to be black lol, and always perching against a white sky; I have been told fill in might help.
However being dumb how could this work at 30 metres + at 400mm?

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Old Saturday 3rd July 2010, 13:18   #2
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You can use a Flash Extender (Better Beamer) to focus the light from a flash and increase the range. It is said to be good for adding at least a couple of stops, or doubling the reach.

http://www.birdsasart.com/accs.html#BEAMER

So, combined with a 580EX and used at f/5.6 and 100 ISO you could expect full illumination at (58 / 5.6) x 2 = 20.7m, if you could shoot at or below the sync speed. Since you only want fill, and not main light from the flash you would be down almost exactly 1 stop at 30m, which is a good starting point for fill.

So, yes, it could/should reach to 30m. However, if you need to shoot faster than 1/250 then you will run into problems as HSS loses power fast as soon as you start pushing shutter speeds beyond the sync speed of 1/250. If you go up just a small amount, to 1/320 the flash power will drop by about 1 stop and it continues to deteriorate the higher you push the shutter speeds.

That's the maths as I understand it. While I do have a Better Beamer, and have used it in the field with my 580EX, I have not undertaken any controlled experiments. Usually, when I use it, I dial in -2 FEC and pretty much let the thing do whatever it can to help the image. Typically I will use it to illuminate the shadowy undersides of BIF against a bright/backlit sky, so it ought to at least help out with your scenario.

I've just found a recent example, where I was having to shoot into white, overcast skies. According to the EXIF the distance is exactly 30m and I used the Better Beamer at -2 FEC for this shot. At 1/1000 I'm not sure how much help the Better Beamer was, but every little helps, I think. This was shot from the deck of a pitching boat, so framing was tricky and I was unable to fill the frame, but here is the unedited original and a version with a crop and minor tweaks.
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Old Saturday 3rd July 2010, 18:02   #3
Coati
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Excellent info Tim, thx very much!
Rich
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Old Saturday 3rd July 2010, 18:43   #4
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p.s. Of course, those calculations above were based on shooting at 100 ISO. If you bump the ISO then you gain roughly double the distance for every 2 stops you add. I say roughly because the inverse square law applies to point sources of light rather than flash heads with a combination of fresnel lenses to focus the light. I imagine that the rule will be close enough to get you in the ballpark.
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