1.4 TC use with 100 2.8 macro

[max_robinson]

Hi, just got a new 1.4 TC and slapped it behind an ef12 ext tube and my 100mm

macro. It looks clear enough etc. But there's a problem with AF. The AF hunts

back and forth real fast just in front and behind the focus point without ever

stopping. I know this lens isnt a real tc compatible lens, but I have no other

big prime lens to test it on at the moment. Is this a common problem with AF

and this lens, or is it possible i have a lemon?

Thanks.

[marrio]

Yes, <b>manual focus</b> is better for macro photography. <p>Plus I'd try the EF

25mm

extension tube coupled with the 12mm for even greater magnification.

[marrio]

Oh, and remember to set the <b>focus limiter</b> on your lens to the closer distance

range.

[tommyinca]

Nothing wrong with the lens, TC nor the tube. This is "normal" for macro focus distant. As the lens focus, your head/camera may also focus (moves), the lens group inside the bellow moves and with most macro lens the focal length changes a bit too. At normal distant, all these parameters are insignificant and the AF can handle it. Not so, as you get near the subject. The AF system is saying heck, and went nuts. Less light for the AF (3+ stops) is also true. At the closer 2/3 magnification of your setup, the AF is seeing f5.6-f8 effectively. Not enough light to do a good job. This is why AF off in most cases is better at macro distant.

[beauh44]

Hi Theo, When you put a 1.4x TC on your camera you cut out 1 stop of light. This is the same light needed for your AF system to lock on to its target. It's also important to understand that your AF sensor needs an area of high contrast in order to lock focus. If you just point your camera at a clear blue sky, it'll never lock autofocus on anything. Also, at least in Canon cameras, some have better AF systems than others; for example the 1 series cameras tend to do better than their "prosumer" DSLRs. Not all of your AF points are created equal. Usually the center AF point is the most sensitive. And lastly, for most people doing critical macro work, they tend to choose manual focus anyway because your Depth Of Field can be razor thin. Good luck!

[robin_sibson1]

AF often fails to lock with Extender/tube combinations, especially with lenses not designed for the Extender, or where the tube is between the Extender and the lens (so the lens can no longer recognise that it needs to slow its AF down). Similarly, the Life Size Converter can be used with a variety of lenses, in some cases to quite good effect, but AF often fails, whereas it works fine with the 50/2.5. This is not an indication of any problem with your lens.

[lester_wareham]

You should work manual focus for macro work anyway. You can also use the EF 2X in this way with the 100mm macro.

 

Be aware that the camera is not signalled the 1 or 2 stop f-stop loss due to the addition of the TC so you need to make sure you don't stop the lens down too much or you will loose sharpness due to diffraction.

[robin_sibson1]

I don't think the issue here is whether AF is useful for close-up/macro work, but why it is not operating correctly. All my macro lenses (50/2.5, 50/2.5+LSC, EF-S 60/2.8, 100/2.8USM) will AF effectively pretty much down to their close-up limit, and even in quite poor light. That's because they are being used in the way they are designed to be used. The problem in the present case has nothing to do with low light levels. It is caused by creating a major change to the optical context, by adding both Extender and tube and with the lens having no way of knowing that it should adjust its AF behaviour in order to achieve focus. When you put an Extender on a lens designed to work with it, the lens recognises the presence of the Extender (and which one it is) and changes its AF behaviour. People often complain about AF being slower with a lens+Extender combination; that's to enable it to work at all.

[mark_chappell]

What Robin says. It isn't a light loss issue, it's an electrical communication issue between

camera, lens, converter, and tube. You get the identical focus problem if you put the tube

between lens and converter on a 500/4, where the effect of the extension on light levels is

miniscule.

[max_robinson]

Thanks for all the answers. Its true manual is the way to go anyway for macro, but just wanted to make sure extender was not faulting. Thanks again.