JKT wrote:mawyatt wrote:
A stupid question: We all know that cheap flashes tend to vary their output and sometimes not fire at all. Is the problem in the tube or in the electronics? If it is in the latter...
Actually a good question, as I'll bet many of us have experienced this and ruined long stacking session results
Without having the actual flash unit & setup it's hard to pinpoint but suspect this is caused by a few things. Here's a few that come to mind, I'm sure there are a few more.
1) Trigger pulse coming before flash has had time to fully recharge
2) Trigger not issued by controller or camera hot shoe for various reasons
3) With Wireless Trigger, interference or weak signal at flash receiver
4) Poor flash trigger design common with cheap flashes
5) Weak high voltage flash tube ignition ringing waveform common to cheap flashes
6) Weak batteries if Speed-light
7) Variations in recharge flash tube voltage thus effecting output power.
Note that the indicator LED and/or audio beep is just a lower level voltage indicator of the energy storage capacitors voltage which is still charging towards the peak voltage
after the indicator annunciation. Poor designs on cheap flashes have the indicator level set lower so the user
thinks it's recharging quicker, thus a better flash. To add insult to this the capacitor storage energy, which determines (along with PW) the optical output, is proportional to voltage squared, so a 20% lower voltage causes a 40% optical variation
This is why it's always a good idea to allow many seconds after the annunciation (especially with Speedlights) before issuing another trigger, to allow the capacitor voltage to reach final value. While trying to achieve quicker times between flashes for S&S work I've thought of designing a sense circuit to monitor this voltage and indicate when it's within maybe 1%, but for now I'll just monitor this voltage with an oscilloscope to "see" the charging waveform and timing. If you decide to do this be very careful, the capacitor voltage are usually around 400V and with many strobes the internal ground reference (capacitor negative) is
NOT something you can rely on since it's part of a voltage doubler and actually moves with the AC input waveform, so not a common ground. Connecting this to earth ground will likely smoke your strobe, scope and other AC powered test equipment. You need to use an isolation transformer for your AC powered test equipment, or operate the strobes from batteries if they are compatible. Quality lab DVMs don't have this problem, they have floating negative (black) inputs, and of course handheld DVMs are battery powered so not as issue.
Anyway, I've had better luck with Strobes than Speedlights, but that's just my experience.
Best,