[delboy65]

Been reading some reviews of, mainly 3rd party lenses, but see comments like "the lens decided to focus on the person further away"


Surely it is the function of the camera to decide what point to focus on?


Or am I misunderstanding how AF DSLRs operate?

[JohnX]

It's you who decides, via the camera. The lens just does what it's told!

Here's a good source for more info. http://www.cambridgeincolour.com/tutorials/camera-autofocus.htm

[TER-OR]

Of course, if the lens isn't calibrated to the camera correctly (or vice-versa) then it gets tricky. But yes, the sensors are all in the camera.

[rawr]

Surely it is the function of the camera to decide what point to focus on?

There are 4 elements to AF:
- lens;
- camera;
- operator, and
- features of the scene (including light levels).

Each can play a part in getting good results with AF.

But ultimately it's the camera operator who is responsible for getting decent results.

I keep this image handy [from the Nikon D4 manual] as a reminder of some of the issues to note:

[Na Horuk]

Surely it is the function of the camera to decide what point to focus on?

Yes, but..

Of course, if the lens isn't calibrated to the camera correctly (or vice-versa) then it gets tricky. But yes, the sensors are all in the camera.

there! Some lenses are optimized for AF on modern DSLRs and are good at it. Some lenses are made for previous generations, so they are not quite as fast, even on a modern camera (has to do with focus throw, or motor strength). A lens can also have a low contrast, or super shallow DoF, all of which increases the chances of misfocusing.
So, while AF is a camera function, the lens is a part of it, and some lenses will consistently focus well, while some will consistently misfocus (can be fixed with AF adjustment), and some might misfocus randomly (usually lenses with very long focus throw or third party lenses). Lenses with very low f-number require more light to AF correctly. Lenses with very low contrast will also have a harder time with AF. The lens is an important factor. There are also problems with focus distance changing upon stopping down aperture, which is a problem with some lenses, but not all. Sometimes a specific lens will not work best with your specific camera. For example, two DA 50mm lenses might focus with different accuracy on your specific camera. This is because each lens and camera is built to some tolerances, and when tolerances are opposite, there may be problems. With photography, even 1mm difference can cause havoc

I also have this theory that Pentax lenses have special, optimized AF algorithm profiles in the camera, that allow Pentax cameras to do slightly faster AF with them. Third party lenses rely on universal AF algorithms ,which might not be as fast. I cannot prove this, but its a theory. The things mentioned earlier are less theoretical.


Edit: Basically, AF is not something to rely on. Its a function to learn. Once you learn how AF works with your specific cameras and lenses, then you can use it to its optimum. Just like metering and other things.

What the person quoted in OP is saying is that they don't have as many AF problems with other lenses. This can be specific to their own camera and lens, or to a lens model.

[fb_penpho]

Delboy, AF is a closed loop feedback system. That is the AF sensor in the camera generates an error signal, this causes the camera/lens motor to turn the lens focus in such a direction as to lessen the error. As the image focused on the AF error sensor gets better the error signal gets lower. When the error gets down to an acceptable level, the AF system stops turning the motor. The camera is focused. The AF error system in the camera controls the AF. The lens is merely a component in the feedback loop.

In fact manual focusing is exactly the same process. The Mk 1 eyeball passes the viewfinder image to the brain. The brain determines if the image is acceptable, if not instructs the hand to move the focus to a point where the brain says, yes good enough. The hand movement taking the place of the motor. So a form of AF has been around for a long time.

[RMabo]

There are lenses that are better or worse than others on AF. Some Sigma lenses has had problems. This may suggest there's more to it than just the AF control system in the camera. The AF relies on information from the lens, the lens sends turning pulses and distance info to the camera.

Contrast detect AF is constantly monitoring focus, but phase detect is that the camera makes an assumption and then moves the lens to the calculated position and then makes smaller adjustment with live monitoring, The electronics in the lens is important.

And then of course, as has been stated, lens and camera needs proper calibration otherwise it will always be off in one direction or the other - with phase detect. Contrast detect AF works in a different way. Contrast detect AF is actually better than phase detect in terms of precision, but phase detect is faster.

[theEight]

There are lenses that are better or worse than others on AF. Some Sigma lenses has had problems. This may suggest there's more to it than just the AF control system in the camera. The AF relies on information from the lens, the lens sends turning pulses and distance info to the camera.

Contrast detect AF is constantly monitoring focus, but phase detect is that the camera makes an assumption and then moves the lens to the calculated position and then makes smaller adjustment with live monitoring, The electronics in the lens is important.

And then of course, as has been stated, lens and camera needs proper calibration otherwise it will always be off in one direction or the other - with phase detect. Contrast detect AF works in a different way. Contrast detect AF is actually better than phase detect in terms of precision, but phase detect is faster.

I am a bit confused with that question as well. Pentax lens given the fact that the sensor is in the camera, relies on the camera to say when the image is in focus. the lens simply moves when the body says move. The lens has no way of knowing when the subject is in focus. I however can buy the lens contrast argument. I do have one lens , a Tamron 70-200 2,8 , excellent lens, which tends to back-focus so I have to program in an adjustment in the camera. Still confused.

[kh1234567890]

I am a bit confused with that question as well. Pentax lens given the fact that the sensor is in the camera, relies on the camera to say when the image is in focus. the lens simply moves when the body says move. The lens has no way of knowing when the subject is in focus

The lens communicates to the camera, amongst other parameters, the focus distance zone and the set focal length (variable with zooms) as well as the phase detect AF offset required for those settings.

The advantage of the phase detect method is that the correlation algorithm gives a good indication of by how much and in which direction (near/far) the lens is mis-focused, so that the camera can correct this with a minimal overshoot. The contrast detect method has to overshoot the best focus point to detect the maximum contrast point.