Originally posted by Na Horuk Yep, there are many factors to AF.
Subject: - contrasty, well-lit, colourful helps. Some shiny or odd materials might make things more difficult. And subject movement can be problematic as well. Or if there are two good subjects both on the same AF point, but at different distances - the camera might lock onto one you don't want
Light conditions: - good, even light. An f8 lens will work if you have enough light, but even an f1.4 lens might fail if you have none. Also, the light would ideally be full spectrum, not just a narrow wavelength LED
Camera: - I think modern digital cameras have special AF algorithms for the lenses that they recognize, allowing for speedier focus. In this case, it would be limited to modern DA and DFA lenses (and maybe FA 31, 35, 43, 50, 77). This would mean that other lenses don't get this boost. Then there are the camera's AF detection points will be important. How many, where are they, how big are they, how sensitive, are they cross type, etc. And also whether the lens and camera are calibrated together or not. If a lens is miscalibrated, then you might have difficulty getting good focus
Lens: - the lens is between the subject/light and camera. The lens might have a slow aperture, or it might have really low contrast and other problems. Another problem is CiF with ultra wides, since the DoF is so great. I find my Samyang 14mm is not very good for CiF at all. Mirror lenses might be poor for AF, as well (low contrast, high aperture, low resolution)
And this doesn't even take into account user error, like setting the CiF wrong, turning the focus ring too fast, mistaking handshake blur for wrong focus, and so on. Also, some lenses will not work with CiF because their mount doesn't short out the AF contacts. Some Takumar and Helios 44-2 have a narrow mount, which doesn't cover the contacts. So you can only get CiF if you add a flange or buy a flanged adapter. Some mounts are plastic or covered with non-conductive paint. CiF also cannot work with lenses that are always in AF mode (like DA 35mm or DA L 18-55mm), because CiF needs lens to be MF and camera AF.
That's all I can say. If you need further help, you would have to do some tests (2sec timer, in natural sunlight, using tripod and lens hood, with an obvious subject at appropriate distance) and upload a couple photos (resized or 100% crops, not full size 16MP photos; also they should contain exif data)
Thanks for your great reply!
Concerning the Samyang 500mm, I have checked the contacts and they are shorted (the T2 ring was anodized but a scraped the coating. No I also have proper metering). Even under bright sunlight (ISO 100, f/6.3 and 1/300s, for example) and on contrasted subject, CiF does not work.
Concerning the camera, only the center focusing point works for CiF on the K10D.
The DOF of the 500mm is very narrow, even at f/6.3
that's why CiF would be great! On wider lenses, I often use the hyperfocal and don't even focus the lens 😆.