Before the rush

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by evan-pak

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I understand that my Canon 450D's (XSi/Kiss X2) passive autofocus system knows a spot in the view is in-focus when its nearby contrast is adequate. Then the camera blinks AF confirm lights to inform me which parts are in-focus (or if the chosen spot is in-focus).

I have few older all-manual lenses, which I use with an adapter. Why the AF confirm lights won't blink when the spot is in-focus while using an all-manual lens with an adapter without any connectors?

I do have an adapter which was marketed as "AF-confirm" and that it has a "chip". With it the camera does blink the AF confirm lights. What I fail to understand is why the connectors are needed? What data the camera needs from the lens? Camera does record some EXIF information when using the "chipped" adapter, but for example the "Lens" field is "1-65536" so it really couldn't be much of an use.

Added some specifics after Staale S's answer: Can I DIY-hack this around firmware/hardware/MacGyver wise so that the camera thinks a lens (any) is attached when triggering the AF?

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up vote 11 down vote accepted

The AF gizmos won't do their thing unless they are told that there is a lens on the camera. (Remember that the EOS/EF lineup was designed from scratch to be the latest and greatest all-electronic autofocus camera system back in the late eighties. The legacy Canon lenses would not fit physically so there was no need to accommodate them at all.)

When using the non-chipped adapter, the camera has no idea that there is a lens there, and the AF is not activated. The chip in the chipped adapter just tells the camera that there is something there, and - I think - that it has a certain focal length; this is enough to get the camera to play nice and activate the AF sensors. This may seem a bit pedantic of the camera, I agree, but let's be charitable and remember that they didn't have this scenario in mind when they designed the system :)

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Oh, and by the way: The chip in the adapter is the workaround hack for this. – Staale S Jan 25 '11 at 12:19
(comment) though not as DIY as I hoped for. – koiyu Jan 25 '11 at 12:24
Maybe I should file a feature-request for Canon firmware to add an option "Always assume a lens is attached when the AF is activated" e.g. to the C.Fn menu. – koiyu Jan 25 '11 at 12:26
Ay. They will probably add that one after they give us a mirror lock-up button. Which we have been waiting for for a decade or so:) – Staale S Jan 25 '11 at 12:36
Every Canon DSLR I've owned has a mirror lockup button: It's called the shutter button. It just has to be set to work that way. – Michael Clark Nov 27 '15 at 6:21

For whatever reason, the body needs to know information about the lens such as its focal length and aperture before it will activate the AF confirm.

It does this by communicating with a chip in the lens, which has to contain appropriate information.

Various makers sell lens adapters that contain a so-called AF confirm chip. Actually that chip has been reverse engineered simply to act like some random digital lens, except that it reports bogus information, for example claiming to be at 50mm f/2.8 regardless of lens attached.

If you have such an adapter (and I don't) check out the EXIF information and see if you can see what it says about your lens - maybe you'll find this bogus info.

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