Something interesting about AF on Extenders

[photomarcs]

Hey guys!

Just found out, that a 400mm f5.6L USM, that isn't supposed to AF on a 2x extender, will AF on a 50D on live view, with Live AF mode clicked on. I found it really interesting.. anyone got an idea why it wouldn't AF without that turned on? Thought this might be a post of interest as a good to discuss about, so I do hope to hear the conclusions you guys are fond of =)

[Big V]

Can you check this for me, If you are using centre focus point only, the 5.6 with a 2x will auto focus in bright light, albeit very slowly. When you had live view, were you using the centre focus point? Another trick you can use is to tape the three pins of the converter so that it is not feeding aperture information back to the camera body and this forces the camera to try and use auto focus regardless of the aperture.

[DaveB]

In Live View with Live View AF (instead of Quick AF) it doesn't use the normal phase-detect focus sensors, but uses contrast-detection based on the image on the main sensor.

I can see no reason why this wouldn't work, although I hadn't thought of trying it (I tend not to use that much extension, and when I do I'm typically not using Live View!). I'll have a look with my 5DmkII and 7D in the next day to see if I can reproduce the behaviour. Thanks for the heads-up!

[photomarcs]

Can you check this for me, If you are using centre focus point only, the 5.6 with a 2x will auto focus in bright light, albeit very slowly. When you had live view, were you using the centre focus point? Another trick you can use is to tape the three pins of the converter so that it is not feeding aperture information back to the camera body and this forces the camera to try and use auto focus regardless of the aperture.

It was multiple selection and centre AF that it worked on. I will have to check out the taping trick.... haha not my gear though.. =S but will see if they let me.

In Live View with Live View AF (instead of Quick AF) it doesn't use the normal phase-detect focus sensors, but uses contrast-detection based on the image on the main sensor.

I can see no reason why this wouldn't work, although I hadn't thought of trying it (I tend not to use that much extension, and when I do I'm typically not using Live View!). I'll have a look with my 5DmkII and 7D in the next day to see if I can reproduce the behaviour. Thanks for the heads-up!

I knew it was a different AF form, though losing brownie points for not spotting that out to begin with, How did it end up going on your 5 and 7 ?

[DaveB]

Sorry for the delay.

At first glance, the lack of AF carries through to Live View. But then if I press the "AF" button and scroll the wheel I can get "ONE SHOT" showing on the top LCD, and yes AF then works in Live View on both the the 7D and the 5DmkII!

This was with the 1.4x II and the 100-400mm/4.5-5.6 L IS USM.