Speedlite 580EX II underexposure problem

Question

My speedlite has recently started to produce underexposed photos in E-TTL mode. Compared with previous photos the difference is about 2/3 - 1 full stop.
I have conducted a number of tests using my EOS 40D and a 24 - 105 EF f4/L IS USM lens.
I have used fully-charged batteries in both the camera and the speedlite (both alkaline and NiMH in the flash). I have checked the exposure compensation and flash exposure compensation settings as well as all other camera menu settings and the custom functions in both the camera and speedlite. All is in order and there is no conflict between the camera and the speedlite.
I shoot in RAW and as a test I have taken a number of photos in manual, Tv and Av modes, both without flash and using the camera's on-board flash. All were well-exposed (with no exposure or flash exposure compensation).
I tested the speedlite's recycling time on full power several times - about 5 seconds. So, it seems that the batteries were working well.
I then conducted a number of tests in manual, Tv and Av modes at various shutter, aperture and ISO settings to compare the on-board flash with the speedlite's performance in E-TTL mode. Each of the photos taken with the on-board flash was well-exposed with a good histogram. Each comparitive photo with the speedlite was under-exposed, with a poor histogram and smaller file size. With some experimentation, it was necessary to set a flash exposure compensation of +2/3 to + 1 stop to obtain results broadly comparable to the on-board flash.
However, this workaround did not produce satisfactory results for subject distances greater than 20 feet (speedlite only), even at high ISO and slow shutter speeds. The output of the speeedlite was far too low. These tests were all carried out indoors.
I then conducted tests with the speedlite in manual mode with the output set to full. At a range of shutter speeds, apertures and ISO settings the photos were completely blown out. The same result was obtained with the speedlite in manual and with high-speed sync selected if the shutter speed was at or below 1/250 second (the max shutter speed setting available for the on-board flash).
As a workaround I set the speedlite to manual with high-speed sync and set shutter speeds faster than 1/250 second and
tried exposures at various subject distances. Broadly, the high-speed sync kicked in. I have not yet checked performance at subject distances greater than 30 feet but full exposure was obtained in all the tests so far,(mostly better with flash exposure compensation of -1/3). However, depending on ambient light, autofocus was sometimes impossible.
From these tests, it seems that the camera works well, that the speedlite is able to generate full output and that in manual/high-sync mode it will produce correct exposures with high shutter settings.
Whilst both workarounds - E-TTL mode with flash exposure compensation and manual/high-speed sync - do work, some experimentation is required and both have their drawbacks. There are times when I will wish to shoot with the speedlite in manual but, for the most part, I have been satisfied with the results in E-TTL mode and I wish  to be able to restore my speedlite to full working order.
I wonder if anyone can throw light on the fault (sensor/transistor???) and the remedy.
I am sorry to have provided so much detail but I wanted to avoid unnecessary questions, or solutions that I have already tried. Any help from members who are aware of this problem would be greatly appreciated.

Answer 1

ebiggs1 wrote:
"I tested the speedlite's recycling time on full power several times - about 5 seconds. So, it seems that the batteries were working well."
The problem with using rechargeable on any equipment that isn't working properly, is you never know if it is the device or the batteries. It is the same even when it is the camera that is suspect. The first thing to replace is the batteries with known good ones. 
As a amateur or hobbyists I guess rechargeable are fine but as a pro, who has time to monitor 50+ batteries to make sure they are charged and good? Then keep track of them at a shoot? Not for me and I still do not recommend rechargeable to anybody. I am retired now but even now all my equipment gets brand new batteries at each shoot. Too risky.
That's why I'd use "low self discharge" batteries.  Once you charge them, you can generally depend on the fact that they'll be fully charged and ready for use (they still maintain 90% of their charge after sitting for a full year).  Regular rechargeables can lose that much just sitting for a day.
The strobe itself is powered by capacitors in the flash.  The batteries build up the charge in the capacitors.  The better the batteries, the faster the capacitors will recycle -- but regardless of how long it takes, once you get the "ready" indicator, the flash is ready to fire.
Alkalines decrease their power output as they drain.  Many (probably most) rechargeable technologies tend to put out a near-consistent power output until they run out of power and then have a huge drop-off in power.  This means rechargeables tend to give you faster recycle times throughout the day (but it also means they can die pretty much without warning... you know it's time to swap your alkalines when it's taking 10-15 seconds for the flash to recycle.)  The point here is that as long as you have fresh replacements ready to go (and you would regardless of if you're using single-use vs. rechargeables) the rechargable batteries tend to give better performance.
I think you might be surprised at the number of pros who rely on rechargeables.  I started stumbling across guys who would give the "what's in my bag" list and found that many of them will point out that they use rechargeable batteries for their strobes and they'll tell you what brand they use (that's how I found out about Sanyo Eneloop batteries, btw.  I had never heard of them before that.)
I use a battery storage caddy ("Storacell" by a Powerpax) and it's designed to let you put batteries into the holder either +tip up or down.  So I put them in with the postive tip facing up when they are charged.  I pull a fresh set of charged cells (I know they are charged because the tip is "up") and then put the dead batteries in tip "down", so I know those are the cells that need to be recharged when I'm back home.    Incidentally... I use a "smart" charger (La Crosse BC1000).  With the smart charger, I can actually charge just ONE battery at a time if I need to (the battery in the Canon GPS is a single AA but most chargers insist that batteries by charged in "pairs" so this gets me around that problem or having to wait to go through 2 batteries before I can recharge them.)  Also you can set the amount of power applied during the recharge.  Fast chargers are stressful on the battery.  If I'm not in a hurry, I set a fairly slow charge rate so that it takes all night to bring them back up... but I can set a charge rate that will bring them back up pretty quickly if I want.
Tim Campbell
5D II, 5D III, 60Da