How do focusing defects arise?

[stu_the_flat]

People talk of third party lenses having focusing defects I can understand how this happens in a range finder camera where the optics are not quite where the camera thinks they are.

But I don’t understand how this happens in a contrast based focusing system where the optics scans back and forth to find the sharpest contrast. So that will be the sharpest point even if the optics are not perfect.

What have I misunderstood?

Cheers Stuart

 

[Arkady]

Because the software and electronics don't quite match with the mechanical assembly that actually moves the lens elements inside the lens. Only has to be a fraction of a turn 'out' to be way off focus.

 

[trencheel303]

Because the software and electronics don't quite match with the mechanical assembly that actually moves the lens elements inside the lens. Only has to be a fraction of a turn 'out' to be way off focus.

Does this mean then that you can tell a faulty lens by using live view to focus, and if the focus is always out it's a duffer? i've thought I had a soft lens before but I realised it was just my AF system being a bitch, as live view always nailed it.

 

[stu_the_flat]

Because the software and electronics don't quite match with the mechanical assembly that actually moves the lens elements inside the lens. Only has to be a fraction of a turn 'out' to be way off focus.

I still don't understand because would that not feed back to the camera and it would detect that the image was still not in foucs?

Cheers Stuart

 

[petersmart]

Does this mean then that you can tell a faulty lens by using live view to focus, and if the focus is always out it's a duffer? i've thought I had a soft lens before but I realised it was just my AF system being a bitch, as live view always nailed it.

In any mechanical system there are always tolerances which in the case of digital cameras have to be incredibly small to get reliable focus.

The focus only has to be a fraction out to produce a slightly unsharp picture.

When you consider the tolerances involved it's a miracle we get pictures as good as they are.

 

[Jamie H]

Well using live view and MF you can achieve perfect focus on the sensor (where it counts, obviously).

The thing is that a DSLR doesn't focus on the image sensor- it uses a separate focus sensor- so if the two don't correspond (and there are quite a few variables in the AF sensor loop alone) then there is plenty of scope for error.

 

[DonnaM]

The thing is that a DSLR doesn't focus on the image sensor- it uses a separate focus sensor- so if the two don't correspond (and there are quite a few variables in the AF sensor loop alone) then there is plenty of scope for error.

Hmm yes but if that were the case then all of your lenses would be out. In the real world SOME lenses focus spot on and SOME dont and I am at a loss to know why this is in a feedback system.

 

[Arkady]

Does this mean then that you can tell a faulty lens by using live view to focus, and if the focus is always out it's a duffer? i've thought I had a soft lens before but I realised it was just my AF system being a bitch, as live view always nailed it.

I don't think so, because the camera is using the same algorithms to determine distance to subject in both instances - I think, maybe?
Plus I've yet to actually use Live View for anything...it's just not part of my repertoire due to the way I shoot...

*edit* this is explained more fully below in the next post by petersmart...

Lens focus realignment is something that we can tweak in-camera with the D700 and D3 series...It's just a calibration adjustment...
In manual-focus days it was never an issue as your eye and brain did the work...

I also have to addmit that most (if not all) of my focus-errors come from using AF then moving slightly (or the subject moving slightly) while keeping the shutter button half-depressed - the distance-to-subject has altered slightly, but the camera hasn't compensated as I've effectively 'locked' the focus and the plain focus-screen doesn't always show this, esp at wide apertures...
Older cameras had all manner of focus-aids - fresnels, split prisms etc. which we no longer employ.

 

[petersmart]

Hmm yes but if that were the case then all of your lenses would be out. In the real world SOME lenses focus spot on and SOME dont and I am at a loss to know why this is in a feedback system.

Not really.

Toerances in the system can be either positive ,zero or negative when viewed as deviations from what is spot on.

For instance the position of the sensor could be slightly out - it could sit a fraction behind where it should be, a fraction in front, or spot on.

And a particular lens could also be slightly out - either back focusing slightly, front focusing or spot on.

Now if everything else in the feedback loop is spot on then if a zero deviation lens meets a zero deviation camera the focus will also be spot on.

And if a slightly back focusing lens meets a camera where the sensor is also sightly back from where it should be the result MAY also be spot on - depending on the deviation.

Similarly with a front focussing lens and a sensor slightly forward of the zero position.

BUT if a back focusing lens meets a front positioned sensor then the deviations will not cancel out, they will add up and produce an OOF image.

Live view fixes this because the feedback is now directly from the sensor and corrects for these deviations.

Which is why Canon introduced micro adjustment on the latest cameras - to allow lenses to be adjusted to a particular camera.

Of course Canon could check every single lens and camera for zero tolerance and discard all that did not meet that requirement - but the costs of such excellence would probably put most DSLRs into the luxury category.

 

[Arkady]

I still don't understand because would that not feed back to the camera and it would detect that the image was still not in foucs?

Cheers Stuart

No, because the software would 'think' it was in focus...it can't actually see, it just responds to electronic signals...if the signals say yes, then yes it is...

 

[Jamie H]

The reason some focus spot on and others don't is that the errors in the lens/camera are sometimes in favour of one another and sometimes they accumulate in a manner which increases the error.

Using the Live View and focussing manually means that the AF system (and any error between the viewfinder and sensor calibration) are completely bypassed - if it is sharp on the screen it MUST be sharp on the output image.

 

[trencheel303]

Not really.

Toerances in the system can be either positive ,zero or negative when viewed as deviations from what is spot on.

For instance the position of the sensor could be slightly out - it could sit a fraction behind where it should be, a fraction in front, or spot on.

And a particular lens could also be slightly out - either back focusing slightly, front focusing or spot on.

Now if everything else in the feedback loop is spot on then if a zero deviation lens meets a zero deviation camera the focus will also be spot on.

And if a slightly back focusing lens meets a camera where the sensor is also sightly back from where it should be the result MAY also be spot on - depending on the deviation.

Similarly with a front focussing lens and a sensor slightly forward of the zero position.

BUT if a back focusing lens meets a front positioned sensor then the deviations will not cancel out, they will add up and produce an OOF image.

Live view fixes this because the feedback is now directly from the sensor and corrects for these deviations.

Which is why Canon introduced micro adjustment on the latest cameras - to allow lenses to be adjusted to a particular camera.

Of course Canon could check every single lens and camera for zero tolerance and discard all that did not meet that requirement - but the costs of such excellence would probably put most DSLRs into the luxury category.

Interesting, thanks.