Speedlite 580EX II problem in E-TTL mode

[Solo]

Hi,

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, 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.

[amfoto1 · Accepted solution]

It doesn't sound like a fault. It sounds as if the flash is working properly.

You do realize, don't you, that the flash in ETTL operates differently in Manual exposure mode, than it does in any of the Auto Exposure modes?

When you have the flash set to ETTL and the camera to Tv or Av or P, the camera will expose by the ambient light as if the flash isn't there, and the flash is treated as a FILL light. The flash's output is reduced by something like 1.5 or 1.7 stops from full power, unless you overrided it with + or - Flash Exposure Compensation.

When you have the flash in ETTL and the set to Manual, the flash is FULL and treated by the camera as if it is the only light source. Ambient light is ignored (but depending upon settings can still be a factor in your images). Once again, you can override with FEC, if you wish. 

If I read your description correctly, you are seeing the difference between Manual/FULL flash and AE/FILL flash. As long as you keep these in mind, you can set up the camera and flash in ETTL mode pretty easily for many different situations.

You'll notice the flash often will recycle much faster when it's acting as a FILL, since it's firing at lower output. I often use fill outdoors in the daytime and can get two or three quick "pops" of the flash before it slows to recycle.

If you want the 580EX II to recycle faster, use quality rechargeable batteries instead of alkalines. For even faster recycling, a CP-E4 battery pack is a nice addition. It adds 8 more AA sized batteries that the flash can draw upon.

I never use built-in flashes, so I really don't know how they compare.

***********
Alan Myers

San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & PRINTROOM 

[Solo]

Many thanks for your very helpful and comprehensive reply. My comments were based on the fact that the on-board flash gave better performance as in order to get good exposure in the speedlite with the same Tv or Av settings it was necessary to increase FEC by one or two stops. For longer subject distances (> 20 feet) I am experimenting setting both the camera and the speedlite to manual. I don't have a flash meter but seem to get decent exposure over a range of distances (sometimes I might reduce the output on the speedlite by one or two stops). However, I find it easier and quicker to adjust settings on the camera rather than on the speedlite. Am I right in thinking that FEC operates so as to adjust flash output (rather than adjust shutter/aperture)? If so, I would set FEC in the camera and just leave the speedlite set to full power.

I live in Jersey in the Channel Islands (just off the coast of France). Like California, the varied coast and countryside in Jersey offer many fantastic opportunities for fabulous photos. There are many on Flickr and other sites.