Wireless Flash Control on the Canon 7D

[lostsoulal2]

I was experimenting using the wireless flash control function of the 7D to control several speedlites and came across the following anomaly:

That whatever setting I used, the built in flash on the camera always fires in conjunction with the other speedlites.

Searching for this on the Internet, I found the following:

"I only want the external flashes to fire, but my built-in flash still fires?

When you set the camera to only control the external Speedlites, the built-in flash will still fire it's pre-flash to communicate with the external Speedlites. After the preflash, the built-in flash will emit a very low power flash when the shutter opens to signal the external flashes to fire. This sync-flash will not contribute to the final exposure (or minimally so)."

I was trying to take a fairly close-up photo of a lens inside a light tent and was frustrated as to why I kept on seeing the built-in flash's reflection on the lens, even though I had selected speedlites only.

It seems the built-in flash is not capable of only emitting IR light - unlike the elderly ST-E2 controller, which uses IR LED's to communicate with the external flashes. The built-in flash emits the full spectrum of light which includes IR light that is received by the slave flashes. The ST-E2 pulses light in the same manner, you just can't see it.

So while the 7D is more versatile as you can control more groups with greater flexibility than the ST-E2, in situations where the camera is located near the subject, please be aware that the built-in flash can contribute to the final exposure.

 

[ruvor]

stick a piece of unexposed processed slide film over the built in flash.

 

[itsdavedotnet]

it's more like using a 580ex as a commander than an ST-E2.

ruvor's trick is meant to work well, or use a card to bounce it away or something, or gobo it out from the subject.

 

[specialman]

Nikon does a handy little thing called am SG-2 diffuser, that was designed to lessen the impact of the pop-up flash when used in conjunction with its SU-4 TTL slave units. It'll fit your hotshoe. You can get 3rd party ones off fleabay too that do the same job. Might be an option...

On the subject of the pop-up adding to the overall exposure, do you ahve to set the camera's flash in the menu system? With Nikon's CLS it's the same thing –*you basically set the pop-up flash to act as a trigger and not as a flash*–*but there's a notorious quirk that if you don't press the right button when in the menu system, the flash remain active.

 

[Spiritflier]

It's something I noticed too when I got my 7D... I've photographed my fiance a couple of times with it and she finds it a bit distracting compared to when I've used my ST-E2. The pop-up flash made no difference to the exposure 'cos I made sure by switching the strobes off and firing a shot... It was almost pitch black!

I think I'll give the 35mm slide a try-out though. Thanks for the tip.

Cheers,
Si

 

[grumpybadger]

I know the internal flash fires to communicate but I've seen no evidence of it during any of my exposures

 

[HoppyUK]

I know the internal flash fires to communicate but I've seen no evidence of it during any of my exposures

I've never seen any evidence of the triggering flash either in real photos. But I did test it once and if you shoot really close, like under two feet in dim light, you can see it in the image very clearly.

This infrared LEDs thing is interesting. I've not been able to get a true answer on this, despite my best googling, but I think both the Canon ST-E2 and Nikon equivalent are actually just low-power flash guns with a semi-IR filter over them, ie dark red.

I say semi-IR because I'm thinking that if they were full IR they would effectively emit nothing as there's very little IR light in electronic flash. I also have a so-called IR flash trigger (Elinchrom) and when that fires you can quite clearly see a dark red/purple light.

Easy enough to check - can you visually see a dark red light from the ST-E2 if you look straight at it when it fires?

 

[hollis_f]

Nikon does a handy little thing called am SG-2 diffuser, that was designed to lessen the impact of the pop-up flash when used in conjunction with its SU-4 TTL slave units. It'll fit your hotshoe. You can get 3rd party ones off fleabay too that do the same job. Might be an option...

Only problem is that there's a little switch on the Canon hotshoe that prevents the pop-up flash from working when there's something on the hot shoe. It is possible to shave a bit off the SG-2 to make it work. A google should find it.

 

[lostsoulal2]

Sorry for short delay:

stick a piece of unexposed processed slide film over the built in flash.

I did not have any slide film, but had a flash diffuser to use, but that gave me another issue as it initially disabled the hot shoe..........



You can see the small microswitch which is located on each side of the hot shoe guide (present from the 450D / 40D onwards) -but as long as the tab which secured the diffuser was only inserted halfway, the built in flash worked........

I still got a small amount of unwanted flash reflection in my subject matter, but I was shooting at about 18" focal length.

I tried it with another subject about 5-6 foot away and could not see any interference - so its only in short/macro shots that the 7d's in built in flash can intrude.

I will have to see if I can beg or borrow some slide film as you suggested to test it out.

I've never seen any evidence of the triggering flash either in real photos. But I did test it once and if you shoot really close, like under two feet in dim light, you can see it in the image very clearly.

This infrared LEDs thing is interesting. I've not been able to get a true answer on this, despite my best googling, but I think both the Canon ST-E2 and Nikon equivalent are actually just low-power flash guns with a semi-IR filter over them, ie dark red.

I say semi-IR because I'm thinking that if they were full IR they would effectively emit nothing as there's very little IR light in electronic flash. I also have a so-called IR flash trigger (Elinchrom) and when that fires you can quite clearly see a dark red/purple light.

Easy enough to check - can you visually see a dark red light from the ST-E2 if you look straight at it when it fires?

The ST-E2 has 2 separate sections with plastic covers on them - the top is the Transmitter (with a Black coloured plastic cover) the one below is the AF assist beam (with a dark Red plastic cover).

As I depress the shutter button half way - 2 Red lamps illuminate from the lower AF assist section and projects a red coloured horizontal/vertical grid on the subject until focus is achieved.

When I fully press the shutter button I can then see a single Red lamp illuminate from the top section, but could see anything on the subject and presume that the black plastic cover is the semi-IR filter?

Thanks.

 

[HoppyUK]

<snip>

The ST-E2 has 2 separate sections with plastic covers on them - the top is the Transmitter (with a Black coloured plastic cover) the one below is the AF assist beam (with a dark Red plastic cover).

As I depress the shutter button half way - 2 Red lamps illuminate from the lower AF assist section and projects a red coloured horizontal/vertical grid on the subject until focus is achieved.

When I fully press the shutter button I can then see a single Red lamp illuminate from the top section, but could see anything on the subject and presume that the black plastic cover is the semi-IR filter?

Thanks.

Thanks for checking that Lostsoul.

It looks like the ST-E2 works just like the 580EX in master mode, but with the visible light dimmed by a dark red filter, rather than being blocked by a full IR jobbie. Makes sense.

It also means that you could get the same effect by putting a dark red filter over the 7D's flash. I've done something similar using the cellophane wrappers from a box of Quality Street. Seriously, I think it would work nicely.

The yellow/orange one also does a passable job of gelling a flash to tungsten for matching interior lighting

 

[lostsoulal2]

Hi Richard,

I am allergic to "Quality Street", (plus loads of other foods) so will not be able to try this out, but presume that I can use a Flash Gel from Lee filters or equivalent? Perhaps a ND filter gel?

Thanks Roy

 

[HoppyUK]

Hi Richard,

I am allergic to "Quality Street", (plus loads of other foods) so will not be able to try this out, but presume that I can use a Flash Gel from Lee filters or equivalent? Perhaps a ND filter gel?

Thanks Roy

You don't have to eat them You could just get a box to try, and secure with a rubber band - maybe two or three together. See how it works. An ND filter will just dim the light, possibly reducing range. The idea of the coloured filter is that it will let through at least some light unhindered, sufficient to signal the slave, but kill enough of the visible light so it doesn't intrude in the image.

This filter is what you're trying to replicate, from Nikon http://www.warehouseexpress.com/buy-nikon-sg-3ir-ir-replacement-panel-for-built-in-flash/p1030321

I have also seen people use a card deflector, which completely blocks the pop-up flash from the subject, but sends light in the right direction for the slave to pick up. That would be a complete solution, if your slave is in a suitable position and you can rig up something neat.

Also, if you shoot at a higher f/number and lower ISO, that will reduce the effect as the master signal flash fires at a constant level regardless.