580EX and Autofocus Assist Beam strange behaviour

[strobemonkey]

Long story short, when AF Assist Beam on my 580EX II is on which is the deafult (C. Fn. 8), I get out of focus results with my Tamron 17-50. When I turn it off, focus is ok. I'm using 40D and new Tamron 17-50.

Edit:

I have done the same test with my Canon EF 50 1.4 with similar result here.

 

[Canon Bob]

The three beams emitted by the 580EXII will be chosen based on which AF points you have selected on the body so it'll help diagnosis if you can look at the RAW files and see the setup used.

The upper assist beams are horizontal and the lower is vertical. If you select the centre AF point or the one above and below it then AF is attempted using the lower assist beam only. If the peripheral AF points are active then the two upper assist lamps operate and AF uses the horizontal beam. (the above is based on my understanding and may be flawed).

A few tests might show a pattern emerging or a faulty assist lamp.

HTH

Bob

 

[strobemonkey]

Thanks for the explanation Bob. I will look at the raw data. I forgot to mention I used centre AF point all the time.

Will do more tests later.

 

[Canon Bob]

Richie,

I've dug this up from a while back.....it's a little crudely put together but maybe it'll help you.
http://bluwiki.com/go/KaiAdin:AF_Assist_Lamp#Other_AF_Points_Selected

Bob

 

[strobemonkey]

Thanks for the link Bob. It certainly helped me a lot.
The assist lamps on my 580EX II all light up in accordance to the AF point selected on the camera.

I have done more tests and found consistent results with what I have described previously in my first post.

Here are some test images. Shot in RAW, center cropped (not 100%) and converted to JPEG. No other post processing done.

Settings:
40D + EF 50 1.4 @ f2.8, Av mode. Centre AF point. Remote shutter. I shot tethered so I can easily check the images on the pc.
Switching to "M" mode does not alter the test result.
1. No flash

2. With 580EX II, AF assist beam OFF

3. With 580EX II, AF assist beam switched ON

40D + Tamron 17-50 2.8 @ f2.8, Av mode.
1. No flash

2. With flash, AF assist beam OFF

3. With 580EX II, AF assist beam switched ON

Here's my set up:

Anyone have an idea of what could be the problem? Or anyone know of a better test?

 

[Chappers]

I wonder if may have something to do with the lens rather than the Flash. Could it be that the lens can't move fast enough to lock on. But I would have thought the camera wouldn't allow you to expose if the focusing is off.

Do you have access to another Canon lens. Also I wonder if moving the target a bit further away may give a different result.

 

[strobemonkey]

I wonder if may have something to do with the lens rather than the Flash. Could it be that the lens can't move fast enough to lock on. But I would have thought the camera wouldn't allow you to expose if the focusing is off.

Do you have access to another Canon lens. Also I wonder if moving the target a bit further away may give a different result.

You're right in saying that the camera wouldn't fire if there is no focus lock unless (I think) the lens is on manual mode.

The only lens I have is the crappy kit lens but will give it a go later on.

I got to help with household chores first as the misus is getting angrier every minute:shake: .

My guess is also the tamron lens but will see what happens later.

 

[strobemonkey]

Here are more tests with 3 lenses. I disabled the flash from firing so its only the AF Assist beam works on the 580EX.

Subject is 5ft. away. Center crop (not sure if 100% as DPP doen't have that feature for cropping)

Tamron 17-50:

18-55 kit lens:

Ef 50:

Looks like Tamron lens is going back to amazon!

 

[Canon Bob]

Well, that looks fairly conclusive.

Bob

 

[strobemonkey]

Edit:

Sent my Tamron 17-50 for calibration. After 3 weeks, it came back spot on.
Sample test pics:

I still need to test with my 580EX, but I'll do it when I have enough time.