This is a set-up to use a manual flash unit on a microscope and fire the flash while the camera is in LiveView.
It would be useful to be able to use flash on a microscope directly from LiveView (EFSC) for very fastidious subjects like ciliates which may be disturbed even by first curtain shutter movements of the camera.
The Canon EOS 500D disables EFSC whenever a flash is physically connected to the camera hotshoe. Whenever a flash is connected, the camera goes through a full shutter + mirror movement BEFORE the exposure. More expensive EOS models allow the mirror to be disabled but the shutter movement still occurs.
The solution would be to release the camera and fire the flash independently of each other. However, the delay between pressing the shutter and the start of the exposure varies greatly; is too irregular to synchronise this with firing the flash reliably. Rik described the problem in detail in on older post: http://www.photomacrography.net/forum/v ... hp?t=13726 In his measurements, the trigger delay was between 70 and 500 ms.
I looked into this myself and encountered the same problem. I have now found a partial work-around.
This allows:
- exposure times down to 1/200 s, short enough to allow simultaneous continuous LED illumination that is bright enough for observer use on a 80:20 trinocular head
- firing the flash with EFSC enabled
- countinuous LiveView through EOS Utility Remote shooting
- intermittent shooting without flash use is possible through EOS Utility
- works at least 80 % of the time
The crucial observation was that the 500D, after a manual trigger, generates a voltage signal at the electrical shutter cable socket (2.5 mm jack) reliably around 70 ms before the exposure begins.
I have no idea why the camera generates the voltage signal but I found that the Cactus V6 transceiver interpretes this as a trigger signal and can in turn be used to fire a flash remotely.
You need two Cactus V6 for this; one is connected through a cable to the EOS 500D. It receives the voltage signal from the camera. It in turn send a wireless signal to a second Cactus V6 which has a DELAY timer function. The correct delay time has to be found for each unit (I have a unit with a delay time at 58 ms (more recent V6) and another with 68 ms (older version of V6)).
The camera flash is mounted to the hotshoe of the second Cactus V6 unit. I use a Yongnuo YN-560 III speedlite manually set to 1/128 power. The power setting remains largely unchanged; exposure is controlled though ND filters, through adjusting the camera ISO and though digital image processing.
Parts needed:
- Mechanical shutter release cable (seriously!) - for now this works only if the camera is triggered mechanically, it doesn't work though EOS Utility Remote shooting
- Shutter release cable adapter (strap or bracket) as the EOS 500D doesn't normally accept a mechanical shutter release cable
- 2.5 mm to 3.5 mm jack cable (for audio use)
- two Cactus V6 transceivers (I have not tested the V6 II)
- manual flash unit
- USB-mini USB cable to connect 500D to computer
I tested the Yongnuo YN-560 III and decided to convert to for use on a microscope. The set-up also works with the Cactus RV60 flash unit (it has an inbuilt transceiver so you need only one additional V6 unit) but this was tested off the microscope.
The 500D has to be in LiveView and there should be no electrical connection to a camera lens (either use an analogue camera lens or tape off the contacts!).
When setting this up, there is usually some difficulty in getting the camera to accept mechanical triggers while it is also connected to EOS Utility at the same time. I always have to turn the camera off and on and pull cables and reconnect before it finally starts working. I haven't figured out why this is, yet. Once the camera accepts the mechnial triggers, it does so for the remainder of the session.
Even then, the flash frequently misfires (see below). I would describe it as "temperamental" but I figured I'll show it here and maybe get imput for further improvements.
Regards, Ichty
Figure 1: Parts needed

Copyright: ©Canon ©Cactus CC: Xeviro https://commons.wikimedia.org/wiki/File ... elease.jpg
Figure 2: Settings used

©Canon ©Cactus CC: Xeviro https://commons.wikimedia.org/wiki/File ... elease.jpg