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I've discovered consistent focus problems using Rebel XSi + 430EXii + Tamron 28-75. I confirmed this is a common problem which occurs when the Speedlite's AF-Assist is enabled.

Some have suggested that the red color of the AF-Assist light causes the Tamron lens to have problems. So, I am considering removing the translucent red plastic cover that goes over the AF-Assist light. However, I don't want to bother doing this if the actual LEDs are red as well.

Can anyone confirm what color the actual LEDs are on the 430EXii AF-assist?

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Does the lens focus fine in daylight? –  Kaushik Ghose Sep 26 '13 at 10:11
    
In the same conditions, do you have the same focusing problems without the flash? –  BBking Sep 26 '13 at 13:51
    
Did a fair amount of testing to make sure lens focuses properly in other conditions. Lens seems to work fine at all times EXCEPT when the 430exii AF-assist lamp is activated. Tamron works fine with Speedlite as long as AF Assist Lamp is disabled. Also, Canon 50/1.8 works just fine with Speedlite + AF Assist Lamp. –  Desert__Bee Sep 26 '13 at 21:05
    
Discussion of this issue here: (I hope link will work) dpreview.com/forums/thread/2111392 –  Desert__Bee Sep 26 '13 at 21:07

2 Answers 2

up vote 5 down vote accepted

They're red LEDs under the cover. I removed the cover from my 430 EX and whilst it works better (with all lenses, on account of producing a brighter, sharper grid pattern) the light is still red.

Here's what it looks like without the cover:

It's worth noting that you can in theory remove and replace the cover as necessary but I snapped one off the clips whilst removing it for the first time, so I'd have to glue it back!

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Thank you very, very much. Appreciate your help. –  Desert__Bee Sep 26 '13 at 13:41
    
Can you explain why is works better? Is it anything to do with the colour? Maybe the plastic is acting as a filter? –  BBking Sep 26 '13 at 13:53

FWIW, the lens has no intelligence or sensors: it doesn't 'know how to focus'. All the focusing smarts are in the camera, and the camera simply instructs the lens focus motor to move.

There are better and more finely stepped motors in some lenses, such as the Canon L lenses, but the lens doesn't contribute to the focusing. I have seen some accuse a lens of 'hunting' for focus in a dark room, and this is simply a lens having noisy/rough/less fine motors than another lens and the user noticing. Of course, having a lens not as bright (not as wide open) contributes to 'hunting' as well.

There is no reason that the color red would impact a Tamron lens, as there is nothing to impact.

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Yes, thank you. I have seen this discussed on various discussion threads. In any case, there is definitely a problem with some Tamron & Sigma lenses with the 430EXii (when AF assist beam is used). –  Desert__Bee Sep 26 '13 at 13:46
    
Not quite true. There are such things as assist lamps and some cameras even use the flash in low light conditions (this happens the the flash flickers before the photo is taken. No, it's not red-eye reduction). Infact, the lens is the very vital component for focusing. Sure, the body calculates it and controls the lens but the body gets the info from the lens. The lack of assist lamps from Olympus cameras were a complaint. From photographers saying "WHY NOT" and others saying "MY FLASH FLICKERS AND WON"T TAKE A PHOTO!". –  BBking Sep 26 '13 at 13:49
    
BBking- I agree that assist lamps are important, my point is the LENS does not use the assist lamp, the camera body, and it focusing computer use the assist lamp. The info from the lens is simply the position of the lens elements and the aperture setting confirmation. –  cmason Sep 26 '13 at 14:24
    
"There is no reason that the color red would impact a Tamron lens, as there is nothing to impact." Not so. The colour of light can indeed affect autofocus - lenses bring different wavelengths of light into focus at slightly different distances. Normally you'd see strong fringing in an image if red was focussing at a completely different distance, however there may be near infrared light coming from the AF assist beam that is being filtered by the main sensor and not by the AF sensor. Also lenses transmit near IR differently. –  Matt Grum Sep 26 '13 at 14:51

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