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Flash trigging (from built in flash)


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I have a Canon Rebel T4i camera which triggers an external flash using the built-in flash…


1) Will this cause confusion for capturing highlight images (even though the strength of the built-in flash is so much lower)?

2) If yes, would it cause confusion if I redirected the built-in flash so it does not directly hit the subject?



Thank you,



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hi Jason,


I will attempt to answer your question about your flash situation with the Rebel T4i.


The short answer is: the only flash that should fire during a capture sequence is the Flash Assembly - this is the "light" that creates your "LPs" (or Light Positions). The Georgia Okeeffe Museum has a useful video for illustrative purposes that you can watch here in re to hardware:


"Camera Flash Assembly"



Since the T4i has a flash hot shoe, I'd recommend using "Pocket Wizards" to trigger your flash. The older models work (PW Plus II receiver/transmitter) as well as the newer "Mini TT1" and the "Flex TT5".


If "wireless" triggers are cost prohibitive, you could replace that technology with the use a long (wired) "sync cable" to trigger your flash— meaning could could go "wired" to your light source. If you use a long sync cable — make sure to take precautions to prevent against any movement in your setup. IF your camera(s) do not have a "flash sync port" under that little rubber flap on the side of the camera you are going to need a "Hot Shoe to PC Cord Adapter" to get the sync cable from your camera to the flash unit.


I hope that this helps you along the path to success.



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  • 4 weeks later...



Thanks for the reply. Despite knowing the right way to do it, last month I was travelling through the highlands of Guatemala and was not able to find a radio trigger or sync cable before my capture session so here is the work around that I used:


1) I set the external flash to be 8x the power of the built in flash so almost all of the illumination was from the right direction

2) I used the built-in flash to wirelessly trigger the external flash for my highlight image capture sequence


This resulted in 2 reflections (one large from the external flash and a small one from the built in flash) on the reflective spheres. Since the built-in flash did not move, the reflection from it was always in the same place on the spheres which made it easy to paint out. This ensured that the right light position was calculated for the RTI.


3) After capture, I recorded a Photoshop action to automate the painting out the reflection of the built-in flash for the entire sequence. Every image received identical treatment.

4) Then I processed the RTI as per instructions after that.


I don't know if the added light from the built-in flash may have adversely affected the quality of the RTI, but we were very pleased with the outcome. Thanks for developing such a robust and useful technique.



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  • 2 weeks later...

Dear Jason,


thrilled that you had success in the highlands of Guatemala! Thanks for the feedback and for sharing your experience with this community.


What you did, would certainly work — obviously it did. As this technique gets better, the software also gets better too. In the most recent version of the RTI builder, the software "should" (it most always does) have the ability to track a moving highlight to get the LP (light position). It has "brain" enough to know that it needs to look for the moving target, and treat the stationary light source as something to avoid.


We have seen examples of this with situations like windows, skylights and lights that we can not turn off --- anyhow, this is good for you to know when you come up against this again.


Your post processing mind was and is on the right track!


Nice job at turning up the flash to full power. I'm curious, can you please tell me what flash you were using?


good work.


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  • 7 months later...

I know its an old post but...


The Metz 15 MS 1 ring flash is a small lovely external slave only flash unit that I use a lot.


Is trigged by the built in flash (or another one of course). Its a ring flash but because its slave nature you can just use it where you want and not only attached to the lens filter ring. That's why I use it so often.


In order to avoid intruder lights from the built in unit, this flash comes with a plastic clamp that you use to cover it but leaves some kind of light to go "outside", enough to trigger the Metz unit.


Even if this clamp works in 80% of times, there's some chance of getting a red reflect in the image. I avoid it by putting some black paper in front of the clamp, wich almost never avoided the flash to get trigged.


But wireless and sync cord based triggers are found at ebay by a couple of bucks. They worth a try.

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Dear MJM,


I wanted to suggest that you stay away from any Ring Flash.


It does sound like you are able to trigger your Ring reliably, but the Ring Flash will produce a 'ring' on the surface of your shinny black sphere. This is not the desired result.


A specular 'dot' or crisp small round highlight light will produce a much more accurate light position and surface normal for your final product.


If your goal is to get accurate scientific data, you might want to consider this approach.


I hope that this helps.



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Re:  secondary reflection from the built-in flash, is there a way to remove not only the secondary reflection from the sphere, but also subtract the effect of this light on the subject?  For example, could you take an ambient light image with the built-in flash only, then subtract this image from the entire RTI image capture sequence?  I seem to recall some discussion of this in the RTI class--I should review my notes. 


Using neutral density filters and increasing the power of the moving flash is a better alternative, but I could imagine situations when one doesn't have the right ND filter or there's a need to remove ambient light from the image sequence, such as a room with a lot of light coming through a window or overhead lights that can't be turned off, or outdoor RTIs. 


As long as the moving flash unit is much brighter than the secondary or ambient light source, the adverse effect should be small or negligible, but in some situations it might be significant.  As Jason mentioned, the secondary light source could have an effect on the quality of the finished RTI because it effectively adds a constant "ambient" light to the entire image sequence.  If there's a way to subtract the ambient light component, it should improve the final result.

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Speaking of ring flash, indeed, as any macro use it can have their issues. In this case anular reflections. But that only occurs with a macro use. Distances longuer than X doesn't give that effect. The Metz unit as even the possibilitty of redirect the lamps angle to "close"or "open"the ring depending on the distance to the object.


In any case your observation is right and should it be mentionned and is something not so evident as red lights and led signals reflects for some flash and cameras will.

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  • 3 weeks later...

Our team had the same problem.  We were using the T3i Rebel and found the "primer flash", as customer service calls it, does not affect the overall quality considerably.  Granted, you may have to review your images individually and hand pick the flash since their can be some hiccups when highlighting light detection on the spheres.  We ended up doing the same thing you did Jason by changing the external flash power, which can also be coupled with changing the shutter speed and ISO on the flash and camera to negate the effects of the primer flash.  

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