why micro focus adjustment ?

Started Feb 25, 2013 | Questions
Chimere
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why micro focus adjustment ?
Feb 25, 2013

Can someone explain it ? Since the AF works via the light path over the mirror, the camera should find the proper focus even if the lens has a slight deviation from its nominal f value. I would think that a correction would be needed if the mirror path distance and the sensor distance do not agree. But then, the camera would need the adjustment and not the lens.

Chimere

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Shiuming Lai
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Re: why micro focus adjustment ?
In reply to Chimere, Feb 25, 2013

Chimere wrote:

Can someone explain it ? Since the AF works via the light path over the mirror, the camera should find the proper focus even if the lens has a slight deviation from its nominal f value. I would think that a correction would be needed if the mirror path distance and the sensor distance do not agree. But then, the camera would need the adjustment and not the lens.

Chimere

Tolerance errors will always exist between the separate AF module and image sensor, and lenses themselves are never perfect. Come to think of it, the mirror itself can have alignment errors. The only way to eliminate these errors is to acquire focus via the imaging plane itself.

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Chimere
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why micro focus adjustment not in the camera ?
In reply to Shiuming Lai, Feb 25, 2013

It appears to me that the point is missed in your response. I do not question the need for adjustment, but is it the lens to be adjusted and not the camera ?

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Bart7D
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Re: why micro focus adjustment not in the camera ?
In reply to Chimere, Feb 25, 2013

Chimere wrote:

It appears to me that the point is missed in your response. I do not question the need for adjustment, but is it the lens to be adjusted and not the camera ?

Well, with the change in the title you show that your own title missed the point...

On top of that: the adjustment takes place in the camera with MA.

It's a matter of priority: as was said before, tolerances are possible on both sides. If all my lenses except one can focus correctly with the camera at hand I'm glad the camera can adjust for the only one that can not. Thank Sony for it's MA

. If almost any lens has the same type of misfocus on the camera at hand you will probably send it in for correction.  Since this is pretty expensive I thank Sony again for MA

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Tom2572
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Re: why micro focus adjustment not in the camera ?
In reply to Chimere, Feb 25, 2013

Chimere wrote:

It appears to me that the point is missed in your response. I do not question the need for adjustment, but is it the lens to be adjusted and not the camera ?

Lenses are dumb and most have no software in them. Cameras aren't smart, but they do have software in them so it's easier to make the adjustment there regardless of whether it's the lens' fault or the camera's.

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EinsteinsGhost
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Re: why micro focus adjustment not in the camera ?
In reply to Chimere, Feb 25, 2013

Among six A-mount AF lenses I could use on NEX via EA2, only one has a noticeable back focus issue. In this case, the LA-EA2 simply stores the information for that lens and compensates. If I were to get the LA-EA2 adjusted for this lens (so no MA is needed), then the others will require to be compensated for. So, technically, in this case, it is the specific lens.

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OldClicker
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Re: why micro focus adjustment ?
In reply to Chimere, Feb 25, 2013

I have never really read a good answer to this question.

You are right in that a for/back focus adjustment is to compensate for the difference in the length of the light path between the main sensor and the AF sensor.  This is entirerly in the camera body.  So why would the lens change this???

The best I can figure out is:  There are actually two light paths used in PDAF; hitting the AF sensor in two places.  It is the difference seen in these two that allows the PDAF to work.  If you were to slightly tilt the lens (or in some other way misalign the light exiting the lens), one of the light paths would be affected more than the other.  The micro focus adjustment can compensate for these lens specific errors.

Make sense?

TF

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theswede
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Re: why micro focus adjustment ?
In reply to OldClicker, Feb 25, 2013

That is one reason for discrepancy in focus. Since the focus sensor relies on light coming from two pathways any alignment error will cause issues, and these issues may (will!) vary between lenses.

In addition the lense projection plane is not perfect. In the simplest lenses it is part of a sphere, and in more exotic zooms it can have very strange shapes, including a "wavy" pattern. I've never seen a lense with a flat projection plane though some teles come very close.

Any of these shapes can cause the apparent focus plane of the whole plane to differ from the focus plane as measured through two points - simply put, if the camera were to place focus where it appears to be it would be wrong - and this is compensated by the lense focus adjustment value (which has different names with different manufacturers). When a lense without mechanical issues has a focus problem this value is usually wrong, and it can be adjusted by a service centre.

All of this is complicated by focus plane offsets shifting when a lense is zoomed (or even stopped down, though that is seldom an issue to to DOF increase) and different focus points "looking" at different parts of the focus plane. It's a miracle and a testament to the engineering divisions of the camera companies focusing works as well as it does.

Jesper

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Chimere
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why micro focus adjustment not in the camera ?
In reply to Bart7D, Feb 25, 2013

Granted, I rushed things a bit, I apologize, my fault - and this issue is a bit confusing: however, the point I think you missed is that the AF mechanism in the body should bypass all these tolerances, as it measures only the light coming in from the lens.

And: if there is a problem with the distances the two light beams (to the ST mirror and the APC-snsensor) cover insdie the body, the camera body must be adjusted. However, people in this forum talk about the lens getting calibrated. That would not eliminate a mirror / sensor distance problem.

Chimere

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Allan Olesen
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Re: why micro focus adjustment ?
In reply to theswede, Feb 25, 2013

theswede wrote:

All of this is complicated by focus plane offsets shifting when a lense is zoomed (or even stopped down, though that is seldom an issue to to DOF increase)

Actually, when you use a lens fully open, it is stopped up compared to what the AF sensors saw. And when it is stopped up, DOF decreases. So I think that this is a very common phenomena, and as far as I know, all AF lenses have built-in correction tables which tell the camera how much to correct for the aperture dependent focus shift.

Remember that most cameras have PDAF sensors which see light from light paths around f/5.6-f/6.3.

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Chimere
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Re: why micro focus adjustment ?
In reply to theswede, Feb 25, 2013

Thanks to you and Old Clicker, I think to have understood the essence of the issue. Quite complex it is, specially when one considers that the AF can be instructed to measure on so many different spots, with the focal distance changing in a zoom, with the sensor field and focus plane not fully identical, and the later changing forms when changing focal distance.

Chimere

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Allan Olesen
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Re: why micro focus adjustment not in the camera ?
In reply to Chimere, Feb 25, 2013

Chimere wrote:

However, people in this forum talk about the lens getting calibrated.

There are some lens specific probable causes for focus error.

First there is focus shift when changing aperture. There is a discussion elsewhere in this thread about that topic.

Then there is chromatic aberration. If the red, green and blue light is not focused in the same plane, and the PDAF sensors are more sensitive to one wavelength, there may be a focus error on lenses with heavy CA. This is purely my own theory, and it may be entirely wrong. But it is well known that the necessary micro adjust tends to change with the color temperature of the scene, so that somewhat supports my theory.

And then there is a third possible cause which I have been told about and don't know if I should believe. Apparently, most focusing with AF is one try only: The camera PDAF sensors measures the exact focus error in the image, calculates how much the focus mechanism should rotate, rotates the mechanism exactly this amount. And then it takes the photo, no matter if best focus was achieved. So if the prediction of necessary turning of the mechanism was wrong, you will end up with a focus error.  The relationship between the focus error measured by the PDAF sensors and the necessary rotation of the focus mechanism is of course lens specific, and this relationship is described in data tables which are stored electronically in the lens. So if the prediction was wrong, you will have to change the contents of these tables.

Regarding the last explanation, I think there are several weak points. For example, one should expect that if a lens has front focus when it is first focused far away and then focused on the subject, then it should have back focus if it was first focused near and then focused on the subject. And the idea that the focusing is not at all an iterative process also amazes me.

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Chimere
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when is micro focus adjustment of value ?
In reply to Allan Olesen, Feb 25, 2013

Remains only one question with me right now: how serious is this all ? is it a problem only with a very shallow DOF ? e.g. at f=1.4, macro shots, or with long tele lenses ? I am in the process of upgrading, and wish to know if the MA is an essential feature to consider or not. Are there any global values for at which point MA is of noticeable value ? perhaps anybody has sample pictures of "before" and "after" ?

Chimere

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Allan Olesen
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Re: when is micro focus adjustment of value ?
In reply to Chimere, Feb 26, 2013

On my Minolta 7D the problem was so severe that several holiday photos taken with the Minolta 50/1.7 were ruined. They were not useable for printing or for viewing on a HDTV.

I solved that by mechanically adjusting the AF sensor's position. All my lenses got better after that. The 50/1.7 was the one which was most affected but it turned out that all the other lenses had had a small error in the same direction.

On my a77 the lenses are all over the place and need adjustment in both directions. So that could not have been solved by a mechanical adjustment of the AF sensors. But to be honest: On the a77, the problem is mostly pixel peeping. If I print the photos or view them in full on a HDTV, I can't see the improvement. I perhaps own one lens out of 10 (the Minolta 28-135 "Secret Handshake") where the effect of the micro adjustment would be visible without pixel peeping.

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JerryCurtis
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You asked, I believe...
In reply to Chimere, Feb 26, 2013

about the type of lenses that benefited from the MA tweaking. I read somewhere (and would have thought it logical) that the WA lenses wouldn't benefit much from this adjustment, due to the deep DOF at almost every aperture. However, the lens that seemed to benefit most from the MA on my A77 was, indeed, my Sigma 10-20. The difference between the edge sharpness at O and -7 was very noticable. Even though I hadn't thought this lens was seriously "off", once I was able to see what the MA adjustment achieved, I was happy I had the feature. The lens that seemed to benefit least from the MA when shooting at long distances was my 70-400G.  This could be because it's quite sharp even when a bit "off", or because it's so difficult to immobilize this lens, that a small amount of improvement from MA is hardly discernable... I don't really know. I know this doesn't sound logical. but it's my experience.

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Allan Olesen
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Re: You asked, I believe...
In reply to JerryCurtis, Feb 26, 2013

JerryCurtis wrote:

I know this doesn't sound logical. but it's my experience.

If the AF sensor is misaligned, it will actually have the largest impact on the focus error when short focal lengths are used. So to me it does sound logical.

You can test this by playing with a calculation tool for macro spacer rings. If you insert a 1 mm spacer ring behind a 50 mm lens, the focus distance changes much more than if you insert a 1 mm spacer ring behind a 200 mm lens.

Of course the shallower DOF of the 200 mm might more easily reveal the focus error, even it is smaller.

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Gordon1965
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Re: Does MA only affect auto focusing - or manual focusing ?
In reply to Chimere, Feb 26, 2013

Forgive possibly a very basic question but as we know the dumbest question is the one you don't ask. I am still on my learning curve for many aspects.

Does MA simply affect the auto focus ?

When I am using manual focus am I seeing WYSIWYG or am I seeing what the camera a"thinks" it is seeing at the time but then the recorded image is different. And thus MA affects both Manual and Auto Focus ? I know of course I am not seeing DOF in the EVF by default.

I adjusted my lenses on the weekend and had a couple of -10 and + 10 and when I was doing the process it seemed that what I saw on the panel EVF was very close to what I saw in the photo.

I also found it a bit confusing trying to adjust my 70-300 Tamron as when zoomed in its at 5.6 so it always had more DOF than the other lenses and thus harder to see the printed image even zoomed in close - maybe I need to print it larger !

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Chimere
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Does MA only affect auto focusing - or manual focusing ?
In reply to Gordon1965, Feb 26, 2013

Things these days never seem to be what they seem to be. However, I venture one opinion: since in the finder you see what the APC-S sensor sees, the sharpness should be what you see in the EFV / OLED.  The AF system - where all the problem and need for MA seems to comes in - is not involved in manual focusing.

Chimere

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Chimere
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You asked, I believe...
In reply to JerryCurtis, Feb 26, 2013

I would expect that WA lenses may have a larger distortion of the focal plane.

Your response is quite disturbing. That means that practically all the Alfas without MA are at risk of noticable lower optical performance, when certain lenses are used, without calibration ?

Chimere

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Marco Cinnirella
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Re: why micro focus adjustment ?
In reply to Chimere, Feb 26, 2013

I do think that the benefits of MA are often over-stated. MA is great for prime lenses but as soon as you start using it with a zoom lens you make a big compromise because the MA setting that is ideal will probably be different for different focal lengths. This is OK if you only use the zoom at one end, but if you use it across its range then you either have to average out the MA settings for different focal lengths and go with that imperfect solution or else just accurately MA it for one preferred focal length. Even then, the correct MA adjustment can differ depending on focus range - the lens can need a different MA setting when focused at infinity for exampe, as opposed to say 10 feet. I know some folks swear by MA and make a big deal about it, but I'd like to know how they cope with the issues I have just described.

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