Flash Exposure Inconsistent

[JerseyTom]

I'm using 580EX flash units on my Mk4 cameras with a 24-70 f/2.8 or the 17-35 f/2.8, mostly bouncing slightly with a dome on the flash. Fresh high output batteries or a high-power external source is the power.  The exposure is varying from frame to frame, sometimes with in a one-second period of time, so much so that one frame is properly exposed, the next frame under-exposed , or over-exposed!  The subjects are stationary, the background does not change, the focal length, shuuter spped and aperature are all the same. Yet, the camera or the flash, changes the flash output by about 1 to 1 1/2 stops.

I have tried every setting adjustment possible to no avail. I've cleaned every contact on all the flash connections. I thought these flashes had some distance measuring built into the flash output caluculation, but it seems not.

Has anyone else experienced this frustration?

[TCampbell]

Focus distance is only considered if it's on on-camera flash and it is pointed straight ahead. If you tilt the head up to bounce the that feature is disabled.

Make sure you are allowing the flash time to recycle (if the capacitors didn't have time to recharge then the flash can't fire.).

What metering mode are you using?

[JerseyTom]

No problem with flash recycling: using hi-power AA's and a Jack Rabbit.  Flash power output, in most cases, is probably near 1/64 power or less. Plus, an over-exposed second frame after a properly exposed first frame would negate the possible recycling time issue. Camera exposure mode is set to Manual, flash metering mode has been set to both Average and Evaluative - with identical inconsistent results.

I understand about the focus distance part of the equation now that you've mentioned it.  In my example if two frames shot within one second of each other with the same background and staionary subjects, I was using bounce but all things were equal in the two frames except the difference in exposure. All I can speculate on is there's a miscommunication some where in the chain or a software glitch.  All my equipment is well maintained.  Thanks for the input, I doubt there's a simple explanation for this.

[cicopo]

If your only using very low power output (like you say) why not try without the power pack? It may give a more consistent result.

[JerseyTom]

It make no difference, I 've always switched between the internal AA's for regular shooting and then up to the hi-power pack for more demanding, repetitive shooting

[RexGig]

I do not have a complete answer for you, but will echo TCampbell, in that the flash must be on-camera, and pointing straight ahead at the subject, for the distance calculation to be correct. Anything diffusing the signal will probably affect the reliability of the distance measurement, too. If straight-on flash is not desirable, and you are using optical wireless, and your equipment allows it, set your Master to not flash, but to simply control your Remote/Slave flashes. (I do not have a Mark IV, but my 7D and 7D II allow this to be done with the Flash Control menu in the camera.)

Another suggestion is going to cost money: 580EX II or 600EX-RT. I started with a 580EX, then added a second 580EX, and worked with them for some time, before adding a 580EX II. The difference in exposure reliability was dramatic, and I kicked myself for not adding the 580EX II sooner. My 580EX Speedlites have been relegated to Remote/Slave duties. The 600EX-RT seems equally as reliable as the 580EX II; neither better nor worse. The good thing is that if the better Speedlite is on the camera, it "should" be able to "tell" the remotes/slaves what to do, for an acceptable result.

I know that, in theory, it is the camera telling the dumb Speedlites what to do, so a 580EX II "should' be no better than a 580EX, and of course, my experience is anecdotal, rather than data. 

I am not an expert, but do use 580EX, 580EX II, and 600EX-RT Speedlites regularly, at night, for evidentiary/forensic images. I usually use E-TTL, due to changing distances and time limitations. 

[JerseyTom]

Have tried it all: Optical wireless ETTL, Radio Poppers, Master w/no flash output using slaves too. Based on these past replies, I think it's an inherent issue with the 580EX. My conclusion is the solution is to throw money at it with new speedlites....or shoot manual power! Thanks all.

[RexGig]

If the Speedlite in the hot shoe is newer, that may well be enough to send proper exposure instructions to your existing Speedlites off-camera. I kept my 580EX Speedlites, to use as remote/slave flash.

It is possible, though I am not certain, that the humble, inexpensive 90EX may well suffice. It has Master capability, with cameras that can control it through the Flash Control Menu, can be set to control orher flashes, without providing illumination itself, and benefits from much newer technology than the original 580EX. Again, I am NOT certain that it will solve your
problem, as I do not own one, so do the research to satisfy yourself it would be a good option.

[RexGig]

To test the theory of only needing the one new flash, try renting one 580EX II or 600EX-RT. (I have not seen the 90EX being available for rent, but I have not really looked for it.)