Is it possible to delay the flash on a T5i to fire at the end of a long exposure shot

Question

I'm wanting to take long exposure shots of dancers and would like the flash to fire at the end of the exposure instead of the begining to increse the depth of field right as the shutter closes. Is that possible using the on camera flash of a T5i and if so what are the settings?
Thanks,
John

Answer 1

When you use a TTL flash (Canon's E-TTL or E-TTL II) the camera uses a multi-zone metering system (think of an imaginary grid system overlaying your image) and (1) meters with no flash at all, then (2) fires a pre-flash at very low power while simultaneously metering again.  It then compares the difference in each zone between #1 and #2 to determine how much of a difference the flash made.  By doing this it can identify highly reflective surfaces (e.g. mirrors, glass, reflective metals, etc.) and eliminate those from the flash evaluation.  It can also find zones where the flash seemingly made no difference at all (e.g. if there was a light source in that zone... such as a lamp) and elminate those.   Light sources and reflections would ordinarily fool a flash metering system into thinking it had produced enough light ... even if it hadn't.  With those zones safely eliminated it can then compare the difference for each zone between #1 and #2 to determine how much addtional power should be necessary to correctly light them.
It does all of this during the pre-flash.   The shutter is closed when it does this (the light will not affect your image.)
Then the shutter opens and at some point it fires the flash again... only this time using the power level calculated during the pre-flash phase.  Only this second firing of the flash will show up in your image.
If you switch the flash to manual mode, you wont get a pre-flash.
The on-board flash is useful as a "fill" flash (decreasing the severity of shadows) and it can be used as a primary flash IF the subject is reasonably close (e.g. around 10' is fine... 14' feet is a stretch and beyond that... not so great.)
An externally attached speedlite flash can provide significantly more power.  It can also be located off-camera to provide better lighting (on camera lighting is "flat" because there's no highlight/shadow side to any 3D surface... textures look 2D as a result.)
As for "depth of field"... this is a different problem and the flash can help, but there are some caveats.
If you're shooting indoors with only the available light in the room, then you're likely in challenging circumstances and using lenses with wide-open apertures.  Hence the depth of field is fairly narrow.
If you could increase the lighting then you could reduce the aperture opening and increase the depth of field.
But there is a catch.... light exhibits a property sometimes called "fall off".  This is because it follows a rule in physics called the "inverse square law" whereby the intensity of light decreases as the distance from the light source increases.  Each time the distance from the light source increases by the square root of 2, the light intensity is cut in half (exactly half... that's not an approximation.)  E.g. the amount of light illuminating a subject which is 14.1 feet away from the flash will be exactly half as compared to a subject which was only 10' away.  The square root of 2 is approximately 1.41.  So 10 x 1.41 = 14.1.  
This means if you are photographing two subjects... one is 10' away and the other is about 14' away then the more distant subject will only receive half as much light.  Suppose we go to a crazy distance... and we have a flash 100' away.  NOW the second person has to be 141' away before they get half as much light.  This means two subjects within just a few feet of each will not receive "noticeably" different levels of light.
While placing the flash farther from the two subjects will reduce the difference in illumination on your near vs. far subject, built-in flashes cannot handle these larger distances... they don't have enough power.  But an external flash brings more power for you to do this.  
Canon's speedlite model numbers actually provide an indication of their power level.  If you delete the trailing zero from the model number (e.g. in a 430EX II you'd omit the "0" and get "43"... in a 600EX-RT you'd omit one "0" and get "60', etc.) the number you get is the flash "guide number" in meters.  The "guide number" is an indicator of how far the flash can adequately illuminate a subject... but the f-stop and ISO settings you use on the camera will change this.  So the "guide number" uses a baseline ISO of 100 and a baseline f-stop of f/1.  Of course you don't have an f/1 lens but the baseline is used because it makes the math easy. You divide the guide number by the f-stop you are using.  e.g. if you are shooting at ISO 100 and f/8 then you'd divide your guide number (suppose you have a Canon Speedlite 600EX-RT.. that's a guide number of 60 meters) and you'd get 60 ÷ 8 = 7.5 meters.  Convert meters to feet and that works out to 24.6 (about 25').  Of course that's at ISO 100. Bump up to ISO 200 and you can multiply that distance by 1.41... or bump up to ISO stops to ISO 400 and you can double the distance (about 50'... and that's at f/8).
I should caveat that this assumes you are pointing the flash directly at the subject and there are no light modifiers in use (no soft-diffusers, you aren't "bouncing" the flash off a ceiling or wall, etc.)
Here's a Youtube video that might help you understand the light fall-off problem.
Tim Campbell
5D II, 5D III, 60Da