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Old Aug 17, 2004, 10:29 AM   #1
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Gentlemen -

Here's the setup:

300D in Av mode, ISO=400, 28/135 IS lens @ approx 90-110mm. fstop = 6.3-7.1

White balance set to 'flash'

Canon 420 EX Speedlight (on camera) function switch to the right, H 'Flash' symbol.

Scene: Indoor horse arena, ambient lighting = decent eye levels.

horse and rider 40 to 50ft distant. Tracking them at a 'walk'.

Problem: I am or was under the impression that the flash would increase the light level thereby shortening the shutter speed in order to capture (freeze) motion. Instead I am getting 'ghosting' on the image which appears to actually be the tiny movement within the 1/30 of a second when the shutter is open.

I will try to get an example up here tomorrow
1. What am I doing wrong?
2. Am I expecting to much?
3. Both of the above?

Serious responses please.
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Old Aug 17, 2004, 12:47 PM   #2
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The 'H' is, I believe, for high-speed shutter sync. That is, if you set the shutter speed above 1/200 on your camera in Tv mode, the flash will still be able to light your scene correctly. However setting it to 'H' will reduce the output power of the flash.

Your ghosting is most likely because there is significant ambient light, and the camera sensor picks that up. You mentioned a shutter speed of 1/30 which can result in ghosting in this case. Note that the camera, in Av mode, picks the shutterspeed based on the ambient light it measures, not based on the flash! Try to switch to manual mode and pick a shutterspeed of say 1/125 with the 6.3 f-stop.

Barthold
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Old Aug 17, 2004, 12:54 PM   #3
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Starting to make sense - thanks

Anybody else ??
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Old Aug 17, 2004, 1:33 PM   #4
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Setiprime wrote:
Quote:
Starting to make sense - thanks

Anybody else ??
Yeap... In the High-speed sync mode, the flash is actually outputing multiple strobes (ie the ghosting which you're seing) similar to a stroboscope light on the dance floor!

This mode is not designed to freeze an action, but rather for the lower intensity flashes pulses to follow the narrow slit between the 1st and the 2nd curtain in your dSLR! (If it did put out a stronger burst initially then the flash has no charge left to follow up with multiple lower bursts). It's used primarily for flashfill up close with wide aperture (hence faster shutter) to blur out the background...

Make sense?
http://www.photozone.de/3Technology/flashtec5.htm
http://eosseries.ifrance.com/eosseri...k_synchfp.html

You're better off not to use FP or High-Speed sync, and go for the distance with the normal mode; However,with the 420EX there's not enough oomph to shut out the ambient light and Barthold is right... ie you need a more powerful flash to be able to increase the aperture/shutter to cancel out the effect of the natural light @ 40 to 50ft.

As an example: The Guide Number is 138 (ISO-100, feet) at 105mm flash/zoom setting for the 420EX. At 40 ft the maximum aperture possible is only f/3.5 (and less with f/2.8 @ 50 ft) -> You'll need a more powerful flash or up the ISO. You can also increase the shutter speed since most light will only be coming from the flash now :?
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Old Aug 18, 2004, 6:42 AM   #5
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NHL-

After shooting for 40 or so years, I got to thinking I knew a bit about shooting.

Obviously this is an area that my technical knowledge is inadequate. It is comforting to know that I can call upon those with experience and receive adult, intelligent, explanations. Thank God at 65 I can still learn !!

Thank both of you (Barthold/NHL) for getting me on the right path to understanding.

Historically your answers have been so complete on many subjects that all I can do is to concur.

Once again thanks !!
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Old Aug 18, 2004, 8:18 AM   #6
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FYI

Here's more info on why it's better to shoot at regular X-sync to freeze an action rather than High-Speed (FP) sync: http://webs.lanset.com/rcochran/flash/hss.html

"When you change shutter speed from the ordinary sync speed to the "high speed sync" range, it's the only photographic situation I know of where speeding up the shutter causes the exposure time to slow down."
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Old Aug 18, 2004, 10:22 AM   #7
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NHL-

That makes me feel a little better, at least I'm not the first and only one that misunderstood. It really had me thinking my mind was gone. Sometimes Logic isn't logic.
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Old Aug 19, 2004, 1:58 PM   #8
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Setiprime,

You're welcome. Flash exposure issues are confusing!

As NHL points out, the 420EX isn't the most powerful flash. But from my experience a bit of underexposure is OK, in Photoshop you can get several stops of underexposure back by playing with the levels. I like doing that better than going over ISO 400. Above that the noise starts to become noticeable.

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