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.
