Sigma 18-35mm infinity?

[george013]

Thank you for this worthy experiment. When we calibrate a lens we are really calibrating lans & Camera and the most significant variable must be sensor placement. A tiny bit back or forth, I presume, is a significant factor or is it variables within the focusing sensor? Inquiring minds want to know...

It's not the eventual sensor misplacement. It's the extra way the light has to go when you use the optical viewfinder. Misplacement occur there. One of the reasons if you want to check your lens to use liveview, that's light direct to the sensor. AF fine tuning is correcting an eventual fault in the extra way when using the optical viewfinder.

Also keep in mind CA is a metric value. On a sensor with a higher pixel density it will be more visible.

Graham,
Nice test.

George

[Stagecoach]

It's not the eventual sensor misplacement. It's the extra way the light has to go when you use the optical viewfinder. Misplacement occur there.

Not quite right George, calibration could be necessary due to film sensor positioning inaccuracy.

Calibration corrects for a number of errors that can occur in mechanical positioning accuracy of components such as sensor, mirror, AF sensor, lens positions and flange as well as electrical control signal errors.

[george013]

Not quite right George, calibration could be necessary due to film sensor positioning inaccuracy.

Calibration corrects for a number of errors that can occur in mechanical positioning accuracy of components such as sensor, mirror, AF sensor, lens positions and flange as well as electrical control signal errors.

That's all part of the way the light travel when viewing through the optical viewer, mirror down. When the sensor is misplaced, it's not perpendicular on the optical ax. Finetuning won't be of a help.

George

[Stagecoach]

When the sensor is misplaced, it's not perpendicular on the optical ax. Finetuning won't be of a help.

George

Not quite correct again George.

The sensor can be misplaced and it could also still be perpendicular to the optical axis.

You need to consider all possibilities where errors can occur.

[xpatUSA]

Not quite correct again George.

The sensor can be misplaced and it could also still be perpendicular to the optical axis.

You need to consider all possibilities where errors can occur.

I side with George on this one Grahame.

Unless you can come up with credible references to horribly mis-aligned sensors, of course.

[Stagecoach]

I side with George on this one Grahame.

Unless you can come up with credible references to horribly mis-aligned sensors, of course.

Equally Ted, I could ask you if you have any credible references to the absolute accuracy of the sensor positioning with respect to distance from camera/lens flanges to sensor face?

[george013]

If the sensor is miss positioned, then you should buy yourself a new camera.
1)
The lens is in the focus plane but twisted around the optical z ax. Result: the horizon is not straight on the picture using the viewfinder or the camera is not straight using live view. Fine tuning won't help.
2)
The sensor is twisted around the x or y ax. The dof on the left and/or right side and on the top and/or bottom sight is different. Fine tuning won't help.
3)
The lens is moved perpendicular to the supposed focus plane. Every lens should have to be fine tuned.

I still don't know what happens when calibrating the Sigma lens.

George

[Stagecoach]

Not sure who you are responding to George, but

If the sensor is miss positioned, then you should buy yourself a new camera.

But if your sensor was simply a few thou out of tolerance along the perpendicular axis you could overcome it by simple fine tuning. Likewise, as fine tuning could compensate for such inaccuracies in flange thickness tolerances.

1)
The lens is in the focus plane but twisted around the optical z ax. Result: the horizon is not straight on the picture using the viewfinder or the camera is not straight using live view. Fine tuning won't help.

I can't imagine anyone would ever think it could or where it's applicable to this thread.

2)
The sensor is twisted around the x or y ax. The dof on the left and/or right side and on the top and/or bottom sight is different. Fine tuning won't help.

I can't imagine anyone would ever think it could or where it's applicable to this thread.

3)
The lens is moved perpendicular to the supposed focus plane. Every lens should have to be fine tuned.

Easy way to compensate for that on certain Nikons would be to use the 'Default' fine tune function.

I still don't know what happens when calibrating the Sigma lens.

Perhaps that's because you keep going off on a tangent

[george013]

Just a summary what wrong positions are possible with the sensor: rotating around 3 axes and a shift perpendicular to the wanted focal plane. And two other more.
With the default, I didn't know it existed, you're calibrating the camera, with fine-tuning you're correcting lens faults.
I think, not sure, that after the production process the camera is placed on a test bank and the AF is adjusted for that specific camera. Something like using the default you mentioned.


George

[Stagecoach]

Just a summary what wrong positions are possible with the sensor: rotating around 3 axes and a shift perpendicular to the wanted focal plane. And two other more.

As raised by Ed in post 19 and covered in my previous comments there is a 'possibility' of the film Sensor (I refer to film sensor as opposed to AF Sensor) being mounted slightly out of tolerance along the perpendicular axis.

Yes it could also be incorrect in other axis, but I fail to see why you keep bringing this up and wanting to complicate something which is pretty basic.

As already mentioned this is just one of numerous possibilities of different component tolerances/values within the camera/lens combination that could affect focus accuracy.

With the default, I didn't know it existed, you're calibrating the camera, with fine-tuning you're correcting lens faults.

With the adjustment available to the user for 'tuning' the PDAF (and I refer to the D800) when using AF Fine-Tune 'Saved Value' OR 'Default' tune options you are doing the same thing which is to compensate/bias the 'demand' signal output from the PDAF system. It is this signal that determines where the lens optics will mechanically position themselves.

'Default' applies the compensation to every lens fitted other than those you have applied a 'Saved Value' to.

You are correcting not only for lens faults/errors but any errors that may be there throughout the entire 'system'.

These can be, mirror angles, flange thicknesses, AF sensor position, film sensor position, voltages out of spec, incompatibility errors of different lens manufacturers. The user will never know what he is actually compensating for, only whether he achieves a good or bad result.

I think, not sure, that after the production process the camera is placed on a test bank and the AF is adjusted for that specific camera.

Yes it is, and each of the AF sensor points are calibrated individually. But, all the PDAF does is send a signal to the lens for a motor to place the optics at exact position 'x' along the axis which may be fine for some lens/sensor distances but not all.

Hence, due to the pixel peepers it made far more sense to give the 'tune' option to the user rather than the camera manufacturer spend time doing it.

[george013]

Thank you for this worthy experiment. When we calibrate a lens we are really calibrating lans & Camera and the most significant variable must be sensor placement. A tiny bit back or forth, I presume, is a significant factor or is it variables within the focusing sensor? Inquiring minds want to know...

No, you're calibrating that specific lens for that specific camera. I must correct my self and others. If there's something wrong with phase focusing then that's for all lenses, it's the camera.

Yes it could also be incorrect in other axis, but I fail to see why you keep bringing this up and wanting to complicate something which is pretty basic.

If I don't, other people will correct me. Most common fault on the sensor is a twist round one of the axes after a drop of the camera.

Still don't know what Sigma is doing when calibrating the lens.

George

[Stagecoach]

Most common fault on the sensor is a twist round one of the axes after a drop of the camera.

Where did you get that 'fact' from George

[george013]

Where did you get that 'fact' from George

Hearing and reading. But there's nothing else left. A change of position parallel to its original is impossible.

Still don't know what Sigma is doing when calibrating.

George

[Stagecoach]

Hearing and reading. But there's nothing else left. A change of position parallel to its original is impossible.

But no one is discussing movement of the film sensor from where it was positioned when the camera was originally manufactured

Still don't know what Sigma is doing when calibrating.

It can only be doing one thing George, and that is, 'modifying' the signal from the camera PDAF within the lens electronics.

Simply, if the Sigma lens electronics receives a signal of let's say 4.2v from the camera PDAF that 'should' make the motor place the optics at position 'x' at a certain FL to be in focus and it's slightly out the Sigma electronics tuning allows you the user to change this within the lens to lets say 4.3v that positions the optics slightly differently.

[george013]

But no one is discussing movement of the film sensor from where it was positioned when the camera was originally manufactured



It can only be doing one thing George, and that is, 'modifying' the signal from the camera PDAF within the lens electronics.

Simply, if the Sigma lens electronics receives a signal of let's say 4.2v from the camera PDAF that 'should' make the motor place the optics at position 'x' at a certain FL to be in focus and it's slightly out the Sigma electronics tuning allows you the user to change this within the lens to lets say 4.3v that positions the optics slightly differently.

If it didn't move, then it was positioned wrong from factory. Send it back. It should have a fault with every lens, not just with one.

I don't think the af motor works like a certain voltage means a certain position. No way. It's more the motor gets a signal and stops when the af-system in the camera is satisfied. There are several ways to obtain that. But I'm not sure. Your example won't work with extension rings.

George

[Stagecoach]

George, it's apparent you are simply arguing for the sake of it.

If it didn't move, then it was positioned wrong from factory. Send it back. It should have a fault with every lens, not just with one.

Read all of my comments, where I clearly repeatedly mention 'tolerances' of many components that possibly can affect the end result.

With regard to your comment 'Send it back' you need to consider the following;

Many people would not even know if a lens was very slightly out in its PDAF system, only those searching for perfection.

It's more the motor gets a signal and stops when the af-system in the camera is satisfied.

That is EXACTLY what I'm saying, BUT, the PDAF system with its 'comparison' and 'feedback' tell the optics you are at the 'RIGHT' position but they have no way of knowing (e.g. have no feedback) if that position is correct/accurate for the lens optics to sensor as with contrast AF that is adjusted by what's actually projected on the film sensor[/QUOTE]

There are several ways to obtain that.

Perhaps you can explain them all

But I'm not sure.

That's obvious.

Your example won't work with extension rings.

Yes it will. Unless you incorrectly assume as you have that I am not taking the PDAF comparison feedback into account.

[george013]

that is exactly what i saying, but, the pdaf system with its 'comparison' and 'feedback' tell the optics you are at the 'right' position but they have no way of knowing (e.g. Have no feedback) if that position is correct/accurate for the lens optics to sensor as with contrast af that is adjusted by what's actually projected on the film sensor

It could be me of course, but I read here something else.

simply, if the sigma lens electronics receives a signal of let's say 4.2v from the camera pdaf that 'should' make the motor place the optics at position 'x' at a certain fl to be in focus and it's slightly out the sigma electronics tuning allows you the user to change this within the lens to lets say 4.3v that positions the optics slightly differently.

Somebody else may come in too.

George

[Stagecoach]

It could be me of course, but I read here something else.


Somebody else may come in too.

George

It would certainly help matters George if you explain what the "something else" is.

One can only assume you have a problem with the two paras of mine you have reproduced

Let's hope you are not querying my very simple explanation representing a signal adjustment

[george013]

It would certainly help matters George if you explain what the "something else" is.

One can only assume you have a problem with the two paras of mine you have reproduced

Let's hope you are not querying my very simple explanation representing a signal adjustment

In the second quote you suggest a relation between power and positioning of the focus motor. There is not something like that. The motor gets a command and will work with a certain power or not. The camera or even the lens are not aware of where it is focusing on. It works also on old lenses with no focus motor. The motor is in the camera. It works with extension rings and with teleconverters with contacts.
Newer, quite a long time already, return a position of the focus motor indicating its position so the flash can make use of it. The D or G lenses of Nikon. And some newer and very expensive lenses have a more sophisticated system, an online chip where the position of the focus motor can be stored and retrieved with a great precision.
There's something else involved when calibrating a lens for that camera.

George

[royent]

Doesn't life get complicated ?
Roy