how do lenses mis-focus?

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fyonn

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David Haworth
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Hi all,

ok, so I know that some (perhaps all to some extent) autofocus SLR lenses mis-focus, either a bit too close, or a bit far. my question is, why?

my understanding of a camera's AF system is that they work on phase or contrast detection. ie a sensor behind the mirror "sees" the image, looks for edges and then tells the lens to focus close to further away until the edges are sharp, rather than blurry. is that right?

I can understand that both lenses and camera bodies are built within tolerances and that's fair enough. I can understand that if a camera body was a bit out then lenses might not focus as expected, but I'd then expect all lenses to mis-focus, not just some.

lenses however, I don't understand. if the AF system moves the focus forward or backward as required by the phase detect system, then how can a lens focus too far forward or backward than it should? do you see what I mean?

am I missing something here?

David
 
fyonn said:
Hi all,

ok, so I know that some (perhaps all to some extent) autofocus SLR lenses mis-focus, either a bit too close, or a bit far. my question is, why?

my understanding of a camera's AF system is that they work on phase or contrast detection. ie a sensor behind the mirror "sees" the image, looks for edges and then tells the lens to focus close to further away until the edges are sharp, rather than blurry. is that right?

I can understand that both lenses and camera bodies are built within tolerances and that's fair enough. I can understand that if a camera body was a bit out then lenses might not focus as expected, but I'd then expect all lenses to mis-focus, not just some.

lenses however, I don't understand. if the AF system moves the focus forward or backward as required by the phase detect system, then how can a lens focus too far forward or backward than it should? do you see what I mean?

am I missing something here?

David
Click to expand...

I think you are referring to tolerance in the system, of which there is quite a lot. But if everything is working as it should, and assuming that the camera is presented with a clear and unambiguous target (big assumption) then the system will focus accurately - within tolerance.

It doesn't hunt for optimum sharpness, but makes an estimate of where best sharpness will be, within acceptable tolerance, sends the lens there and tells it to stop. If it attempted to hunt, auto focusing would be very much much slower and servo AF wouldn't be possible.

Sometimes the lens doesn't stop in exactly the right place, and lenses have different mechanical tolerances according to focal length and distance. That is in addition to the system's basic accuracy, which varies between models and also according to the maximum aperture of the lens.

I think most focusing errors are a combination of user error and system error when the camera is not presented with a clear and unambiguous target, or it latches on to something else that the photographer didn't intend, eg the sensitive area is often quite a big bigger than the focus point and there is more than one option for it to choose. Or the photographer moves slightly after locking AF, or the subject moves.
 
HoppyUK said:
I think most focusing errors are a combination of user error and system error when the camera is not presented with a clear and unambiguous target, or it latches on to something else that the photographer didn't intend, eg the sensitive area is often quite a big bigger than the focus point and there is more than one option for it to choose. Or the photographer moves slightly after locking AF, or the subject moves.
Click to expand...

I agree with Hoppy. Its really very hard for a lens to mis-focus. The camera and user decides what is or is not in-focus, the lens is actually fairly dumb in this regard.

Now certain things - such as spherical aberrations can cause mis-focusing (uncorrected, the lens can demonstrate focus shift), but really provided a good AF target is used with proper technique, a lens cannot mis-focus without some help from the camera and user.

I could hand a perfect Sigma 30mm f/1.4 (for example) to 10 people - 3 people would have focus problems due to body-depth adjust errors (Nikon parlance), 2 people would have problems because they don't understand how to use a fast lens, and the other 5 would have no problems at all and get well focused sharp images. Probably 5 out of that 10 would blame the lens :)
 
I think it's more that the lens and body combo can misfocus. I borrowed a 300D which was miles out, I mean really bad, with fast glass like 85mm f1.8. Same lens on my 5D thankfully working perfectly :)
 
HoppyUK: I think I see where you're coming from and I guess I see how that can work. However one would think that continuous AF might have some further effect.

I know what you mean about misfocussing more often being the user's fault, but when you're looking at fast lenses then the depth of field can be very fine. even a slight misfocus would show.

I've got a 50mm F1.4 lens I might try some focus adjustment with just to see if it helps or not.

Dave
 
My new 7D and 17-55mm F/2.8 IS lens require +5 micro adjustment to get things spot-on. Things looked ok without any adjustment but with fine tuning per-pixel detail is much sharper. My 70-200mm F/4 IS is just about spot-on (maybe +1 is slightly better but hard to really tell the difference), and my father-in-laws EF 24-70mm F/2,8 seem to need -4.

I guess few bodies and few lenses are absolutely spot-on, or perfectly matched when leaving the factory. Many people would never even notice a slight misfocus.

F/2.8 and brighter lenses benefit more from MFA adjustments than slower lenses because focus are is much thinner. I use a steel rule with a coin on to accurately centre the focus points.
 
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