How to make a f4 lens AF with a 2X extender on non-1D body

Started Oct 12, 2010 | Discussions
40d_dane Senior Member • Posts: 1,034
How to make a f4 lens AF with a 2X extender on non-1D body

Did you ever wonder: Is there a "taped pins" trick that will make a f4 lens AF with a 2X extender ? There is ! It probably has all the same down-sides of the "normal" "taped pins" trick. I haven't tested it in the field (yet). The extenders have 3 extra electrical pins facing the lens compared to a normal EF body bayonet mount and extender compatible lenses have matching pins. The "taped pins" trick is simply to put non-conductive tape over these three pins to break the contact. This makes the lens unaware of the extender and the max aperture (and AF parameters) of the bare lens is reported to the camera. The trick (may) work with a 1.4X but not with a 2X extender because the AF parameters are way of and the AF diverges as soon as you hit the shutter button (or AF on). What the mk I and mk II extenders does with these pins is very simple: The 2X extender has a jumper between the 1st and 3rd of the extra pins and the 1.4X extender has a jumper between all three extra pins. So the "taped pins" trick for a 2X extender on a f4 lens is to use conductive tape (e.g. copper tape or alu foil held in place with non-conductive tape - the result should be that at least the two outer extra pins is short-circuited ). This will make the lens believe that the extender is a 1.4X extender and report accordingly. The AF parameters for a f4 lens + 2X extender is probably off by as much as a f5.6 lens + 1.4X extender.

I have verified that my my 7D will AF with my old 300mm f4 L + mk II 2X extender with the pins "taped" with alu foil. The max aperture is reported as f5.6.

IMPORTANT DISCLAIMER: IF YOU TRY THIS IT'S ON YOUR OWN RISK! I ASSUME NO RESPONSIBILITY WHAT SO EVER FOR ANY HARM THAT MAY COME TO YOUR GEAR FROM TESTING THIS TRICK

OP 40d_dane Senior Member • Posts: 1,034
IMPORTANT DISCLAIMER: If you try this it's on your own risk !

Sorry, forgot to put in the disclaimer in my original post !

pingu_hk New Member • Posts: 1
Re: How to make a f4 lens AF with a 2X extender on non-1D body

Thanks!

My EF300mm/F4 (non IS) can now AF with extender 2X and 7D using the AF sensors on either side (but not the central sensors).

Entropius Veteran Member • Posts: 4,396
Surprised that this is an issue

I'm over here as a visitor -- normally I shoot another model of camera, but am looking some stuff up for a family member that uses Canon.

I've heard about this "f/5.6 maximum aperture for AF to work on all but 1D bodies" issue on Canon, and I'm a little surprised by it. My own camera (not saying what brand since I don't want to start flamewars) is a cheapie consumer model, and it will AF just fine at f/7 (f/3.5 lens with 2x) and will AF accurately if a little slowly at f/11 (f/5.6 with 2x).

What's actually going on here? And do all those superzooms that are f/6.3 at the long end still AF on Canon? (I've heard that Bigma at least will lie about its maximum aperture to the camera body sometimes... is that related?)

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Karl Gnter Wnsch Forum Pro • Posts: 11,408
Re: Surprised that this is an issue

Entropius wrote:

What's actually going on here?

It's a tradeoff between focusing precision, AF sensor placement and reliability. Canon has decided to rather turn off the AF beyond the design limit of the AF sensors because the results become gradually more unreliable. f/5.6 is the design limit for most AF sensors because then the size of the aperture (which determines how far apart the light paths can be which are compared in the phase detection) allows for a decent phase difference and thus quite a good precision. If you'd design the AF sensors to work at even smaller apertures the precision and speed will suffer even for faster lenses...

And do all those superzooms that are f/6.3 at the long end still AF on Canon?

They will AF (because they lie to the camera about their true aperture) but will be less reliable than lenses which are not crippled by their aperture size...

So to sum it up: An AF that is designed to work at f/8 is far less desirable unless you use much tighter tolerances for the beam splitters and a tighter packed AF sensor array...
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Scales USA Veteran Member • Posts: 3,121
Re: How to make a f4 lens AF with a 2X extender on non-1D body

You can merely use a non-reporting TC, no need to fool around with taping pins. I've tried this with non reporting TC's, and found that different camera bodies respond differently even if using the same lens.

for example, my 40D would AF with a Canon 400mm/5.6 with a 1.4 TC and even with a 2X TC where my XTi would not AF with a TC at all on this lens.

Canon uses 5.6 because of the variance between cameras and camera models to limit AF to something that will always work. In reality, a camera will often autofocus fine in good light on a high contrast subject at f/8 or maybe even f/11. Focus will be slow though.

I can AF using my 70-200mm f/4 IS on my 5D MK II with 1.4 and 2X TC's stacked, but need to prefocus to the approximate correct position, then, it focuses instantly and is right on. My 1D MK III doesn't do quite as good with the same configuration. My 5D2 also autofocuses in very low light just a tad better than my 1D3, so thats consistent.

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Entropius Veteran Member • Posts: 4,396
Re: Surprised that this is an issue

Karl Gnter Wnsch wrote:

Entropius wrote:

What's actually going on here?

It's a tradeoff between focusing precision, AF sensor placement and reliability. Canon has decided to rather turn off the AF beyond the design limit of the AF sensors because the results become gradually more unreliable. f/5.6 is the design limit for most AF sensors because then the size of the aperture (which determines how far apart the light paths can be which are compared in the phase detection) allows for a decent phase difference and thus quite a good precision. If you'd design the AF sensors to work at even smaller apertures the precision and speed will suffer even for faster lenses...

Of course, because of the rangefinder design.

I do wonder about what's going on on my system, though; it will AF (accurately but slowly) at f/11, AF accurately and quicklyl at f/7, and still autofocuses precisely enough to get it right at f/2.8. (That's the fastest lens I have.)

So to sum it up: An AF that is designed to work at f/8 is far less desirable unless you use much tighter tolerances for the beam splitters and a tighter packed AF sensor array...

Perhaps the engineers have just been persnickety about the AF tolerances, then. I suppose I could take my body apart and do the geometry for the AF system, considering that it's falling apart anyway (because I've given it a lot of abuse and it's old).

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OP 40d_dane Senior Member • Posts: 1,034
Re: How to make a f4 lens AF with a 2X extender on non-1D body

Maybe you should state what TC's you are using. It can't be Canons TC's with or without taping. I tried with my 300mm f4 + Canon 2X TC mk II with taped pins on my 40D. It didn't work - even with pre-focus. First AF step taken by the camera blew the lens so far out of focus that it was instantly game over ! By the way 300mm f4 + (Canon) 1.4x TC with taped pins iterates AF just like a f5.6 lens + 1.4X TC with taped pins - the extra TC-pins informs the lens to send the correct AF parameters the the camera body. A non-reporting TC could probably correct the AF parameters before sending them to the body but it probably wouldn't be as accurate as letting the lens know that the TC was introduced in the optical path and let it select the correct parameter set to use.

Lemming51
Lemming51 Forum Pro • Posts: 15,261
What brand TC are you using?

Entropius wrote:

... I do wonder about what's going on on my system, though; it will AF (accurately but slowly) at f/11, AF accurately and quicklyl at f/7, and still autofocuses precisely enough to get it right at f/2.8. (That's the fastest lens I have.) ...

I'm guessing by your profile that you use Olympus DSLR. Oly's own EC-20 2x TC has the following note in its instructions:

"AF: Autofocus can be used if the maximum open aperture of the lens is f2.8 or less. With a lens with an open aperture of f3.5, autofocus is possible using the center AF target (Except for the 35mm f3.5 Macro and the ED 8mm f3.5 Fisheye lenses). With other lenses use manual focus (MF) mode."

This is virtually the same limitation as Canon, and for the same reason. However, if one uses certain third party TCs that simply pass through the lens's aperture value instead of truing it up (e.g. the standard model Kenko and Tamron), then the camera will attempt AF anyway and - depending on the target and lighting - may still work.

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OP 40d_dane Senior Member • Posts: 1,034
Re: What brand TC are you using?

Lemming51 wrote:

However, if one uses certain third party TCs that simply pass through the lens's aperture value instead of truing it up (e.g. the standard model Kenko and Tamron), then the camera will attempt AF anyway and - depending on the target and lighting - may still work.

Actually you got this upside down. The Canon TC's pass the lens info untouched ! They inform the lens of their presence and the lens sends the applicable parameter parameters to the body (through the pass-through wires in the TC). The Canon TC's doesn't contain any active electronics (at least that was the way of the Canon TC's prior the the mkIII's). If a non-reporting TC has AF working without iteration it must actively attempt to correct the lens parameters before they are send to the camera.

OP 40d_dane Senior Member • Posts: 1,034
Re: How to make a f4 lens AF with a 2X extender on non-1D body

pingu_hk wrote:

Thanks!

My EF300mm/F4 (non IS) can now AF with extender 2X and 7D using the AF sensors on either side (but not the central sensors).

I only tried it with the center sensor and it worked fine. This probably goes to show that AF'ing an f8 lens is outside camera specs for the 7D. Surprise, surprise

Does your 300mm f4 (non-IS) rattle ?

http://forums.dpreview.com/forums/read.asp?forum=1029&message=36607027

Entropius Veteran Member • Posts: 4,396
Re: What brand TC are you using?

Lemming51 wrote:

Entropius wrote:

... I do wonder about what's going on on my system, though; it will AF (accurately but slowly) at f/11, AF accurately and quicklyl at f/7, and still autofocuses precisely enough to get it right at f/2.8. (That's the fastest lens I have.) ...

I'm guessing by your profile that you use Olympus DSLR. Oly's own EC-20 2x TC has the following note in its instructions:

"AF: Autofocus can be used if the maximum open aperture of the lens is f2.8 or less. With a lens with an open aperture of f3.5, autofocus is possible using the center AF target (Except for the 35mm f3.5 Macro and the ED 8mm f3.5 Fisheye lenses). With other lenses use manual focus (MF) mode."

It's a standard Olympus EC-20. It's very common among Oly users to use it with the 50-200 f/2.8-3.5 lens, since Olympus doesn't make a mid-range supertele, and nobody has autofocus problems with the combination.

The combination will AF for me using side AF targets, too.

The fact that so many people have to use a teleconverter on a 50-200 to shoot birds is sort of sad; my only real complaint about the Four Thirds system is that Olympus has never gotten around to making a 300 f/4 or 400 f/5.6.

The Olympus folks would be happy as clams if we had a lens like the Canon 400/5.6; it would be a fantastic birding lens with the extra Four Thirds crop factor and in-body IS.

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GtoJon
GtoJon Veteran Member • Posts: 3,553
Tamron 2X C-AF1 BBAR MC7 works perfect on my 5DII

I just tried my 5DII with the Tamron 2X converter & it works great with the 24-105L lens. Not scientific, but low light handheld autofocus wide open. The focus was fast to aquire at 24mm & 105mm. Below are samples that have only had whit balance adjustment.

2x @ 105mm(210), f4.0, ISO 3200, 1/60s

2x @ 105mm(210), f5.6, ISO 3200, 1/25s

Hope this helps somebody, Cheers
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