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Flash and NEX-5, puzzling

  • Thread starter MarcoVenturiniAutieri
  • Start date
M

MarcoVenturiniAutieri

Guest
Hello,

I am again asking a puzzling question regarding my Sony NEX-5.

I have the tiny SONY flash and anoher (Cullman) old flashgun, that I have mounted on a photocell that triggers the flash when it sees another flash.

The photocell seems to work: when the Sony fires the flash, the photocell responds and fires my Cullman, in a process that to my naked eye looks instantaneous.

The puzzling thing is that I don't manage to have the light of the Cullman in my shots! Somehow the timing must be wrong but I cannot understand how!

This is what I do: I set the flash sync to REAR and I set a long shutter speed (2 s) in order to be sure to keep the shutter open long enough to catch the Cullman light! For some weird reasons, with such a long shutter speed and with the flash set to REAR, my Sony fires its tiny flash at the beginning of the exposure as well as at the end (two fires!). For obvious reasons, my Cullman reacts to the first one only (because it is not recharged yet at the second) but... it should be okay, firing at the beginning, should it not? Well, no, because I cannot believe I cannot see the light in the frame - even though after the flash fired the shutter seems to stay open for a long time!

So I do this: I shoot the Sony without its flash, 2 s exposure, and in between I manually fire my Cullman: in this case I do see the Cullman light in the frame - as expected!

So, what does it happen when I let the Sony trigger the flash? Why can't I see the triggered flash in the image? :mad:

Thanks!!

Marco
 
M

MarcoVenturiniAutieri

Guest
Of course, even when I use a "normal" sync (not REAR) the problem of the Cullman flash not visible persists - the procedure described before is only the troubleshooting procedure.
 
J

jcoffin

Guest
This is a fairly common problem. The Sony on-camera flash is doing a pre-flash, which will trigger the slave. A fair number of current slave units have setting to ignore a preflash (or more than one) like this.

Since the pre-flash is comparatively weak, I've heard of people putting some paper (or whatever) over the slave sensor to reduce the sensitivity enough that it ignores the pre-flash. That's not a particularly great solution (e.g., moving the flash relative to the camera can make it quit working), but if you're someplace remote without the right equipment, it can sometimes work well enough to save the day.

I don't know for sure about the NEX-5, but you may also be able to turn off the pre-flash. On the A700 (for example) there's a "pre-flash TTL" setting that (obviously enough) uses a pre-flash, but also a manual mode that doesn't. Given the low guide number of the on-camera flash, the fact that this always shoots it at full power doesn't usually make a lot of difference (other than slowing the flash recycle time).
 
M

MarcoVenturiniAutieri

Guest
If there is a pre-flash as you say, the problem is easily explained.
BUT:
1. I really don't see it! It looks like one flash to me. I do see two flashes if I REAR sync, though.
2. what is the purpose of this pre-flash? Certainly cannot be for red-eye reduction, being timed so weirdly!
Thanks!
Marco
 
pegelli

pegelli

Well-known member
You're not seeing it because your eye is too slow (mine too by the way ;) ).
The purpose is to do an exposure measurement to determine the power of the main flash during exposure.
 
douglasf13

douglasf13

New member
Yep, you can't turn off the pre flash on NEX. You've gotta find a trigger that has an ignore pre flash setting.
 
J

jcoffin

Guest
Yes, the preflash(es) normally happen(s) about 90 milliseconds before the main flash, and they're extremely short, so I'm not at all sure anybody can really see them as separate flashes. Warning: if you don't like technical "stuff", it's probably better to skip the rest of this post and just move onto something else. The remainder does contain pictures, but they have little technical and no artistic merit.

Just FWIW, here are oscilloscope traces of the flashes from an A700:


ADI mode


Pre-flash TTL mode


Manual mode


Wireless mode

For what it's worth, the "10 ms" in the upper, right-hand corner is saying that there's 10 milliseconds between each of the black vertical lines (and 1 millisecond between each pair of the black dots).

To give an idea of the pre-flashes themselves, here's a faster trace showing only a couple of the pre-flashes:


Pre-flashes

I've placed a couple of cursors (the brighter dots) and the readout at the bottom shows the time between them -- 224 microseconds, so the preflash goes to its full intensity very quickly, but dies down to less than half intensity in less than 1/4000th of a second.

I don't have a NEX 5 (or any NEX) to test, but assuming it uses/supports the same wireless flashes, it must use essentially the same preflash sequences for whatever modes it does support.
 
M

MarcoVenturiniAutieri

Guest
If I could introduce a delay in the photocell, I'd be alright.
But I cannot :-( Especially, I cannot do it easily :-|
At least you have several modes with your Sony. It seems I am stuck with one...
Marco
 
M

MarcoVenturiniAutieri

Guest
I did not know these triggers...
Apparently I am not the only one with the problem.
One thing makes me suspicious though: if it is true that "programmed to ignore a specified number of pre-flashes", then why "EXCLUDING Canon EX and EZ models"? What do these EX and EZ have so... strange?
Thanks for the infos!
Marco
 
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