Help - Strange results with Remote Flash Triggeres
 

I was taking some shots the other night to get an image of the person dreaming to comple my Dream Car image for this months competition.

I had my Canon 20D tripod mounted and a single flash fireing into a brolly off to one side. The flash was being triggered at this stage by a cable linked to the PC socket on the camera. I then decided that I needed a bit of fill in flash from the pop up on the camera. I took this opertunity to get rid of the wire trailing across the room to the brolly flash and fitted a remote light sensing trigger that would sense the pop up firing and fire the brolly flash.

Took the shot, the pop up fired, the brolly flash fired and guess what - the image was DARKER :confused: Hadnt really got time to mess around then so refitted the cable from camera PC socket to brolly and this time I got the fill in the I wanted.

This morning I've had the chance to experiment and see what happening. I set everything up and took some pictures of a white wall (which weren't that exciting :D ) but did enable me to see the amount of ilumination I was getting by looking at the resultant histograms - see attached). I had the camera set on manual 125 sec f8.0 ISO 100, flash sync set to 1st curtain sync (although I tried 2nd curtain and the results are the same)

First I took just the wall to guage the amount of ambient illumination. Then just the pop up flash to see how much light that was giving and then added the remote flash triggered by the slave. This is where thing get really odd and the illumination was LESS. How can that be :confused:

Then I tried the remote flash only triggered by the cable, then flipped the pop up as well. This gives the result I expected but didnt get using the slave trigger ie more light than either the remote or the pop up alone. I wondered if the remote slave was not acting fast enough to trigger the remote flash before the shutter closed. So I set up a second remote flash unit. Triggered the first one by the cable and the second one by a slave unit. White out - which proved that the slaves are acting fast enough. Flipping the pop up again resulted is a drop in ilumination - again slightly lower than the pop up + wire triggered remote on their own

Can anyone explain whats goin on and how to set things up for using multiple flashes triggered by slaves AND using the pop up of in fill?

Clive when I had this problem I found that the pop up was giving off to much forward light. What I did to get the lighting right was to put a cover which is masked on one side over the pop up this masked the light going forward and sent the light backwards, it was the only way to get it to trigger the slave. I would think that is the same problem you are having when using the pop up for triggering, its giving you false lighting.

Christine

Perhaps I should have said - the remote flashes are definitely triggering but somehow the camera is recording less light even though there is more light being generated. If it was just that the remotes were not triggering, then the histogram should be exactly the same as for just the pop up - but in fact the exposure is less :confused:

The 20D pop up will be sending out a pre-flash for colour balance setting. This pre-flash will be triggering your slaves. You need a newer type of slave trigger that copes with preflash.

I've not seen them sold in this country but I did see them list on a USA website. They are quiet expensive compared to the cheap old style ones.

Rob - You may be right, although there is no discernable pre flash. Is there anyway to turn this off or override it? I've got red eye and focus assist both set to off. Following your post I've tried setting the white balance to manual which you would have thought would eliminate the need, but it seems to makes no difference :(

Clive,

You can verify if you are getting a pre-flash by looking through the viewfinder when you take the picture. If you have a pre-flash, this can be seen through the viewfinder immediately before the mirror goes up.

My Pentax(es) can have the pre-flash disabled by adjusting the flash settings combined with using manual exposure, I'd imagine there is a way to do this with the Canon.

Duncan

So info on the Canon ETTL system

http://photonotes.org/articles/eos-flash/

I don't believe you can disable the pre-flash. Difficult to see as it happens milli-seconds before the main flash.

I bought a cheap slave unit with the view of modifying it. It was a project for the winter months but there you go.

In the end I bought an off camera cord and extended the cable to 5 metres.

Hi Clive

I think it's a bit more complex than that. My understanding is that, assuming that you are not somehow overriding the ETTL, the camera will attempt to perfectly expose each picture. It does this by measuring the amount of light on the subject and switching the flash(es) off when it calculates that there is enough to generate a good exposure based upon the ISO, aperture and shutter speed.

In other words, all of your histograms should look exactly the same in an ideal world no matter how many flashes you use unless the camera is unable to quench the flashes quickly enough or if, say, one flash at full power was insufficient to light the scene properly.

In the real world however, it is possible that some light from your slave is getting back to the camera directly which the camera assumes has been reflected from the subject and the camera is fooled into thinking that more light has reached the subject than actually has done. It therefore quenches the strobes a fraction earlier than it otherwise would giving a slightly darker image.

The way to compensate for this is to adjust the flash exposure compensation function on your camera.

Does that make sense?

Oh, if you were to take a picture whilst pointing all relevant strobes at a bathroom mirror you should be able to see which strobe(s) are affecting your exposure from the resulting picture.

Sean

Another link giving info on problems

http://photography-on-the.net/forum/...ad.php?t=44351