Canon 5D Mark 3 / 600 RT II flash E TTL underexposure

[Jshemnitz]

Hi All,

I could use some help with a flash issue.

I just did a firmware update today for my Canon 5d Mark III. I am using it with a 600 RT II flash and a 24-105 F4 L Mark 1 lens.

For some reason, my camera is underexposing by several stops when using the lens at 105mm and the flash on ETTL (camera in both program and Manual modes)

At 24mm, the exposure is a lot better. I threw on my 85mm lens and the exposure is perfect.

What would all the sudden cause the dramatic change in exposure at different focal lengths with everything else being the same?

[p4pictures]

If the flash exposure is poor with the EF 24-105mm f/4L IS USM, but fine with your 85mm lens, then there are two possibilities.

First the flash power is not enough for the distance to the subject, the ISO, aperture value and shutter speed you have selected, but assuming the 85mm has a faster aperture then it might be enough to allow a correct exposure.

Secondly the aperture in the EF 24-105mm is at fault.

Try this test, shoot with the camera in manual exposure mode, use the same shutter, aperture and ISO settings for both 85mm and 24-105mm. eg 1/125th, f/5.6 ISO 400. If the two lenses show different results then get your 24-105mm checked out.

[Jshemnitz]

Morning!

For your 1st thought, I was shooting an object about 5 feet in front of me, so I highly doubt there wasn't enough power from the flash to cover that especially since the flash head zooms at the same time making the power equivalent even stronger

For your 2nd idea, when shooting on manual flash mode, the exposure was fine..

The issue only happened in ttl at 105mm which is why I'm so confused. And only happened after the firmware update. I also updated the firmware on my flashes too, so both are on the most up to date firmware edition 

[Jshemnitz]

I also just tested the 24 to 105 on a different canon body with the same flash on ettl and the exposure was fine at both 24 and 105.. so it only appears to be only 105mm on that particular 5d body..so I'm even more confused 

[Hazel_T]

Hi Jshemnitz,

I'm sorry to hear you're having exposure issues when using your EOS 5D Mark III, EF 24-105mm f/4L IS USM lens, and Speedlite 600EX II-RT together. The most recent firmware for the EOS 5D Mark III didn't have any changes related to flash usage. The 1.3.6 update was just to update how the camera communicates with computers. Doing a firmware update would reset all of the settings on the camera, so it would be good to double check some settings.

Both things to check would be in the camera menu External Speedlite Control section. To access that menu the speedlite will need to be in the camera's hotshoe.

The first setting to check would be to see what the E-TTL II meter option is set to. If it is set to Average try using Evaluative or vice versa. The right one to use can vary from situation to situation, so with an exposure issue like this it can be good to double check how the flash metering is set.

The other thing to check would be to make sure the Flash Exposure Compensation is set to 0. If it is set lower it would cause issues like what you are describing.

If you want to reference the manual for those settings they are in the flash section of the camera manual on pages 193-198. If you need a copy of the manual it is available HERE. Once you are on the web page click on the Manuals button and the one to reference is named EOS 5D Mark III Instruction Manual.

[Jshemnitz]

Hi..thanks for both of the suggestions.

I already check both of those potential areas, and both were set correctly.  The odd thing is its only happening at 105mm on my 24 to 105 lens.. normal at 24mm.. and normal on my 85mm prime lens.