Use Lightmeter with Flash (Speedlight)

Discussion in 'Lighting Equipment' started by bellwoods, Dec 3, 2012.

  1. Hello all
    I want to use a Sekonic L-358 light meter to measure the exposure settings for a three light setup comprised of: Three Canon 430EXII’s (in slave mode) controlled via one 580EXII (master mode) on a 5DMkII. In this setup the 580 master is set to not flash i.e. it is disabled.
    The L358 is in the “cordless flash” mode.
    My plan so far was to operate both the Speedlights and the camera in manual mode, test fire using the camera shutter button, use the L358 to measure the resulting exposure values and then readjust camera and / or lights to get the correct exposure i.e. set camera for t=125 f=11 and then change the light output until meter reads same.
    However, when I try this I find that the L358 reads f-2.5 unless I manually trigger the flash with pilot button, in which case it reads f=32.
    I think I have pretty well figured out what’s happening is: meter is reading the “pre-flash” by which the 580EXII “master” is communicating with the 430EXII slaves. Once the pre-flash goes the L358, captures the reading, locks for some time period (90 secs?) and misses the main flash that follows.
    I understand that this whole situation could be avoided by either using radio triggers or mains powered mono-block studio flashes – but I’d like to keep my Speedlights…
    I also suspect that if the test firing is done by activating the flash using the Pilot button of the 580 EX II, there is no pre-flash and a valid light meter reading is obtained. But this is not as convenient as controlling everything from the camera back LCD.
    My questions are:
    1) Is there a workaround – for this?
    2) Are there “smarter” light meters that know how to ignore the pre-flash?
    Thanks much.
     
  2. Consider finding a material you can put over the master flash, allowing only infra-red output. If the Canon slaves are like Nikon's, it's actually the IR part of the spectrum that does the Master-Slave communicating. Some people have success using fully exposed/processed film negs. They're "blacked out," but still pass plenty of IR with the master unit fires.

    Nikon actually makes a hot-shoe mounted device that is an IR-pass shield for use when you're using one of their cameras' pop-up flashes as a commander for slaves, to block the visible spectrum. Essentially, it's a piece of dark red plastic. A little experimenting might find you a material that will keep the meter happy while still triggering the slaves. Of course, radio triggers really are a lot more reliable, and you have many to choose from these days.
     
  3. 1) Is there a workaround – for this?​
    Yes. You have to use the flashes in manual power, NOT TTL. There is only a pre-flash in TTL. And, I am not aware of any light meter that reads TTL flash and TTL (through-the-lens) is a function of your camera's metering system and won't be compatible with a 3rd party system. Not to mention there won't be any consistency with TTL, so no sense in measuring it. Meaning, you might get a full power pop one time and the next you might get 2/3rds under full depending on how the camera meters the whole thing from frame to frame. This in turn answers your second question.
     
  4. Additional Comments / Questions:
    1) Can anyone confirm if EOS Speedlights do their Master-Slave communication by infrared vs visible light? i.e. will the work around suggested by Matt for his Nikon Speedlites work with Canon Speedlights?
    2) Re John Deerfield: I have tried my setup with all components set in manual mode. Anytime the 580 is working as a master - I think its going to send out a pre-flash. I think your suggested approach would require NO communication at all between any of the Speedlights ie the power output of each one is set independently on each flash and the flashes are interconected to the camera by a sync cord.
     
  5. re Suppressing the 580 Pre-Flash:
    Just found this re IR vs. visible light pre-flash communication of Canon Speedlights:
    From Wikipedia: http://en.wikipedia.org/wiki/Canon_EOS_flash_system#E-TTL_II
    The Speedlite 580EX and Speedlite 580EX II flashes use modulation of the pre flash from the main flash tube for communicating with slave flashes. The ST-E2 uses also uses a modulated flash tube for this purpose, but it is fitted with a near infra-red filter (it serves no other purpose). It has a quoted range of 10-15m indoors, and 8-10m outdoors.
     
  6. If you fire a slave flash in addition to the Canon speedlights (by partly blocking the IR preflashes), I am pretty sure underexposure of the photo would be the result. The slave is not communicating with the speedlights, and the flash exposure would be based on the Canon speedlights and not the slave. There is the TTL meetering in addition to the flash meetering, but I doubt that the TTL meetering would be able to meeter another flash output in addition to the ambient light and give you the correct exposure. You can try it, but I doubt it will work.
     
  7. Just ignore my answer, I didn't read your message properly. I REALLY wish there would be a longer time limit for editing the messages.
     
  8. I am going to conduct two experiments on this:
    1) I have ordered a piece of IR filter material from Lee and am going to try surpessing the main (white) tube of the 580
    2) Rent / borrow an ST-E2 and see how the L358 responds to it.
    Further comments / input welcome. I am surprised that I cannot find anything else on the web about this. Surely it must vex others...
     
  9. Sensors in electronic devices are IR sensitive unless they're specifically coded. Speedlite sensors are not coded, neither are flashmeter sensors - so adding an IR filter to a Speedlite won't disguise the pre-flash from the meter and so it will still miss the exposure flash.
    Even in Manual output eTTL the Master uses preflash signals, so you don't get away with using 'Manual' like a Nikon in SU-4 mode would.
    Slave cells for your remotes and an IR trigger on camera would be the closest solution.
     
  10. Stupid timeout problem.
     
  11. OK, now I am on the same page. Well, ideally you need a 2nd pair of hands. Someone who can simply trip the flash from the flash position thereby skipping the wireless protocol. Other than that, you are going to have to spring for some radio slaves. Keep in mind that the 430 doesn't have a PC (sync) port, so you will either need a radio slave with a hot shoe or some sort of an adapter if going with the radio option.
     
  12. Of course (I expect) all of this is solved by radio triggering - next time I'd just buy the Canon STE-RT-E3 instead of the STE-2 and the 600EX-RT instead of the 580EX II.
    I am intrigued by the comment that the L358 flashmeter sensor is not IR coded - I don't think the L358 would respond to IR.
    The question is will the 430EX's respond to IR only? I think they must since they work with the ST-E2 which is standard flash tube behind a plastic IR filter. I plan on testing the meter's repsonse to the ST-E2 later this week.
     
  13. FINDINGS A ST-E2
    #1 I obtained an ST-E2 and triggered it aimed at the L358 ligthmeter with NO 430 flashes engaged. The meter changed form F=0 to Eu (Exposure Under) indicating that the meter "saw" the presence of the IR output form the ST-E2 but that the intensity of that output was not sufficient to register a reading on the meter.
    #2 Next I engaged the 430 flashes in the "normal way" (ETTL mode, head rotated forward position), before I even triggered the ST-E2, the L358 started to read Eu and continued to read Eu no matter what even after I triggered the ST-E2. No useful reading was ever obtianed form the L358 in this confirguration.
    #3 Finally I repeated #2, but this time I rotated the head of the 430 through 180 degress so that the flash unit was being used "backwards" i.e. the controls and the flash tube were facing the subject and the clear red flashing eye (the AF Assist Beam Emitter) was pointing back towards the camera. This time the L358 renedered a useable reading (T 125, f 9.0) and the resulting image was correctly exposed .
    CONCLUSION: The ST-E2 does not itself interfere with the L358 lightmeter. However the flashing red light emitted from the front of the EX 430 does. I tried setting custom function 8 to disable the AF assist beam but it made no difference. Therefore when using the ST-E2 with the 430 EX the flash head must be rotated backawards so that the flashing red LED is pointed away from the subject (or at least away from the L358 lightmeter). NB: All these tests were done with the flashes operating in ETTL mode.
     
  14. Science! :)
     
  15. Sheesh! All that trouble when you've got the best flashmeter available built right into the camera. What's wrong with taking a test shot or two and simply adjusting the aperture or flash power, either visually or from the histogram?
    Just because you've bought an L358 doesn't mean you have to use it.
     
  16. Mr Rodeo:
    What happens is subject is other than 18% gray?
    What is the subject is a shiny plastic thing with reflective surfaces?
    What if you have 3 or 4 speedlight flashes all on stands inside softboxes?
     
  17. Rodeo Joe technique is the best. It doesn't matter what kind of subject it is. You look at the test shot and you will know what kind of adjustment you need to do. Although I have 2 flashmeters and I shoot film but for a setup like that I would make test shot on a digital before making the film shot. It works well for me.
    When I use the digital camera I never use any kind of meters anymore including the built in one.
     
  18. FINDINGS B - IR FILTERING OF 580 EX II
    I entirely covered the flash tube area of the 580 EX II (ie the clear plastic window) with a Lee type 87C44 polyester infrared filter.
    I found that with this arrangement continued to provide triggering of the 430 EX II "slaves" even though no visible light pulse was emitted from the 580 EX II.
    However, the L358 lightmeter continued to register Eu, indicating that it was either responding to the IR or minute amounts of visible light were still being transmitted by the 580 EX II.
    CONCLUSION:
    1) The ST-E2 will provide optical triggering of Canon speedlights without "upsetting" the L358 lightmeter. However it will only provide ratio control of two groups of speedlights.
    2) The 580 EX II cannot be suppressed enough to avoid having its communication pre-flash upset the L358 lightmeter.
    3) This documents another fundamental weakness in Canon's optically controlled Speedlight technology and confirms their decision to drop that approach in favor of radio control.
    Anyone considering a Speedlight set-up requiring control of multiple groups of Canon Speedlights is advised to avoid the 580EXII / ST-ES / 430EXII altogether and instead purchase either:
    - the new Canon radio controlled Speedlight system (STE RT-E3 and 600 EX-RT)
    - non-Canon radio control system such as the Phottix Odin http://www.phottix.com/en/phottix-odin-ttl.html
     

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