Photography Asked by user95069 on August 26, 2021
While trying to work out fill-in flash semantics, it struck me that for the purpose of "overpowering the sun", a short flash duration is rather important since it allows you to open aperture up while decreasing exposure time, keeping the overall brightness while increasing the effect of the flash.
It turns out that my ancient (from the 80s I think) Regula Variant flash specifies 1/1000s flash duration at full power (for which it has guide number 40 at its native f=35mm light angle and up to guide number 70 at f=200mm when using a separate "tele lens" attachment with a fresnel lens).
Going through a list of Metz flashes of similar age and power (including wand flashes with about double the power output), the full power flash duration pretty uniformly ends up as 1/200s. Requiring 5 times the exposure time makes shadow lifting at a distance quite more tricky. Particularly with fast leaf or electronic shutters, it significantly impacts the efficiency of a flash to deal with competing-light situations.
A current-time Godox TT685 has a guide number of 60m at f=200mm (if we consider the Regula specs a bit optimistic, that may be comparable) and a specified duration of 1/300s.
So what gives with regard to the large difference in specification? It cannot be the switch from thyristor technology to IGBT since the older Metz flashes still use thyristors as well. Is the flash bulb different possibly (the size factor seems the same as comparable cobra head flashes today) or driven outside of its comfortable specs?
In analog camera times, the utility in "overpowering light" situations would have been more limited due to larger flash sync speeds, so the main utility of such specs would seem to have been motion freeze in the dark. With modern flashes (and modern superzooms), large reach at short flash time would seem at least as important as it had been at old times.
Why were those kind of specs generally given up on for consumer-level on-camera flashes?
There could be a difference in which duration is being reported, T.1 vs T.5; T.1 is typically ~ 2-3x longer... typically T.5 time is reported. It's also likely that the differences are due to bulb/electronic design differences.
But I think there may be some issues with the premise of your question.
Thyristor controlled flashes have their fastest duration at full power; but it is typically still rather slow. The fastest durations come from LED based flashes, but at very low power. And with modern IGBT flashes higher duration speeds also occur at lower output power; none of those really help you to overpower the sun by using faster SS's.
Most modern IGBT speedlights have a T.1 time around 1/250 and a T.5 time of ~ 1/1000. For instance the SB700 has a T.5 of 1/1042 and a T.1 of 1/305, so I think your information is suspect.
https://gock.net/blog/2012/flash-durations-small-strobes/ https://improvephotography.com/46974/flash-duration-speedlights-long-pop-flash/ https://www.konsul-instruments.com/en/flash-duration-gallery/
And the flash duration doesn't have much to do with the shutter speed you can use; that is controlled by shutter travel time. And most focal plane shutters (mechanical, electronic/rolling) are relatively slow at around 1/250 or longer.
There are two exceptions to where flash duration does relate to usable shutter speed. As you mentioned, aperture (Leaf) type shutters is one; but that is not found in modern DSLRs to my knowledge... it is found in some small sensor/compact cameras. The other is using tail sync which is a dumb (no communication) unsync'd technique which has other drawbacks; such as needing a manual timing offset to correct the sync, and fading across the image. A third potential exception could be a global electronic shutter; if they ever implement those.
But those exceptions are rare at best, or something of a work-around trick; they are not commonly available/used to overpower the sun so there isn't a lot of reason to design for it.
A common method to overpower the sun is to use ND filters which allows you to use slower SS's and full flash power. And with modern IGBT flashes high speed sync is common, which causes the flash to act as a constant light source (at reduced power) allowing the use of higher SS's.
Answered by Steven Kersting on August 26, 2021
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