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AF Fine Tuning - does you camera have it?

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Pete
Pete Site Moderator 1318443 forum postsPete vcard ePz Advertiser England96 Constructive Critique Points
13 Sep 2012 - 2:40 PM

We've just added another type of focusing to our database. AF-Fine Tuning - also known as AF-Micro Adjust. This is the custom function that lets you fine tune focusing for back focusing etc. We've tried to spot every camera with this mode but may have missed one.
If your camera has it and it's not on this list can you let me know please. many thanks

Canon
EOS 1D Mk3
EOS 1D Mk4
EOS 1Ds Mk3
EOS 1Dx
EOS 5D Mk2
EOS 5D Mk3
EOS 50D
EOS 650D
EOS 7D


Nikon
D3
D3x
D3s
D300
D300s
D4
D600
D700
D7000
D800
D800E

Sony
A850
A900
SLT-A77

Olympus
E-30
E-5
E-620

Pentax
K20D
K200D
K2000
K-5
K-7

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13 Sep 2012 - 2:40 PM

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MikeRC
MikeRC e2 Member 93501 forum postsMikeRC vcard United Kingdom
13 Sep 2012 - 2:56 PM

Why do some cameras have this facility and others, Nikon D3200 for instance, not.
....and more important perhaps, is it important ?

My D7000 has, should I utilise it ?

Carabosse
Carabosse e2 Member 1139438 forum postsCarabosse vcard England269 Constructive Critique Points
13 Sep 2012 - 2:57 PM

I never used it on the Canon 5D Mk II.

Pete
Pete Site Moderator 1318443 forum postsPete vcard ePz Advertiser England96 Constructive Critique Points
13 Sep 2012 - 3:00 PM

It depends if the camera lens combination is not focusing accurately the fine tuning can make a big difference. I found the Pentax 100mm was always slightly soft and it turned out it was down to back focusing which I adjusted and now it's better. It's usually only a very little out and when stopped down or shooting at distance it's less noticeable.

Ewanneil
Ewanneil  41118 forum posts Scotland2 Constructive Critique Points
13 Sep 2012 - 3:06 PM

The user manual (page 312) for my D300S says:-

"AF tuning is not recommended in most situations and may interfere with normal focus: use only when required."

That always made me a bit apprehensive because I wasn't sure what fine tuning the AF did and when & why I should use it. I would love to hear from more experienced users on what use they make of this function.

Edit:-Sorry Pete, you got your post in first. Thanks for the explanation.

Last Modified By Ewanneil at 13 Sep 2012 - 3:08 PM
macroman
macroman  1115312 forum posts England
13 Sep 2012 - 3:30 PM

The few sites I've looked at regarding micro-adjust, do a test with the camera bolted down or on a sturdy tripod to show the effect, usually on a cropped part of the image.

Fine if you always use a tripod, but I reckon the major cause of poor(?) image quality is camera/subject movement, and no amount of micro-adjust will cure that, IS is probably more effective.

Steppenwolf
13 Sep 2012 - 4:41 PM


Quote: The user manual (page 312) for my D300S says:-

"AF tuning is not recommended in most situations and may interfere with normal focus: use only when required."

That always made me a bit apprehensive because I wasn't sure what fine tuning the AF did and when & why I should use it. I would love to hear from more experienced users on what use they make of this function.


I think camera manufacturers are unwilling to admit that AF doesn't always get it right, especially on DSLRs with phase detection AF and a separate AF sensor. So they provide the feature (occasionally) but don't say much about it and try discourage its use.

If you haven't noticed any problems with the focus accuracy you get (either because it's accurate or because it's within the DOF) then I wouldn't bother with it. But if you do find the focus is wrong it's nice to be able to set up a focus chart and check where your camera/lens is focusing and fine tune it for each lens.

As to why this inaccuracy should occur, there are two reasons. First, assembly error, where the AF sensor is not aligned properly. This will cause all lenses to either back focus or front focus depending on whether the light path to the AF sensor is too short or too long. In theory the manufacturer checks for this problem and keys data into the firmware to allow the camera's AF to compensate for the assembly inaccuracy - "in theory".

The other problem is caused by differential focus where the outer parts of the elements of the lens focus to a different point than the inner part. So when your camera AFs (using the full aperture and preferentially using the light that arrives at the greatest angle - which phase detection does) it focuses for the outer part of your lens. If you've stopped down the photo will be out of focus. This mainly happens with big aperture wide angle lenses - which I don't use - and you'll probably ask what happens if you shoot at wide aperture. I guess your "fine tuning" will have screwed up your focus Grin.

The only real solution is to focus at the image sensor, which is coming (as in new A99), but micro-adjustment is better than nothing.

Ewanneil
Ewanneil  41118 forum posts Scotland2 Constructive Critique Points
13 Sep 2012 - 4:54 PM

Thanks for that Steppenwolf - really interesting stuff.

Ewan

Keith-Mckevitt
13 Sep 2012 - 5:06 PM

Hi everyone, I'm probably going to sound a bit dumb Tongue but I have a Canon Eos 5d mk 2 and mostly use the 24-105 L lens. If I use this method to fine tune this configuration does the zoom change things, ie if it is fine tuned at 24mm is it still fine tuned at 105mm or does it have to be tuned again? Not particularly planning on using it now but just wondered for future reference.
Thanks
Keith

User_Removed
13 Sep 2012 - 5:14 PM

Basically, you will know if you need it.

If you don't, then you won't.

I have had the facility on my last three Nikons - D300, D3s and D800 and have never needed to use it. But if I did discover that one of my lenses was not auto-focussing perfectly, then I would be glad to have the adjustment available.

mikehit
mikehit  56454 forum posts United Kingdom9 Constructive Critique Points
13 Sep 2012 - 5:25 PM


Quote: I think camera manufacturers are unwilling to admit that AF doesn't always get it right,

Really? I can't say I have seen manufacturers palming off comments on focus accuracy.


Quote:
In theory the manufacturer checks for this problem and keys data into the firmware to allow the camera's AF to compensate for the assembly inaccuracy - "in theory".

It's called 'manufacturing tolerance'. If you want a production line where all lenses focus perfectly with all bodies then you have better be prepared to pay for it!



Quote: if it is fine tuned at 24mm is it still fine tuned at 105mm or does it have to be tuned again?

For a particular lens you can only AF-tune at a particular focal length so you cannot guarantee focus at other focal lengths. I have read many posts where people have done the fine-tuning then realise that the 'factory' setting was the best compromise.

One thing to remember is that all bodies and lenses are made within a tolerance so if your lens is at one end of the tolerance range and the body is at the other end you will get a noticable mismatch. But from everything I have found on this, it remains very unusual, and even where it happens it is often pixel-peepers who blow up images way beyond anything they would use in real life. It is noteworthy that 'focussing problems' rocketed with digital purely because people could do this.
My view is that there usually things that will affect your picture more than the need (or otherwise) to AF-tune you lens/camera, so unless you have sat there thinking 'this is out of focus', I woudl say don't sweat about it..

strawman
strawman  1022006 forum posts United Kingdom16 Constructive Critique Points
13 Sep 2012 - 5:48 PM


Quote: The only real solution is to focus at the image sensor, which is coming (as in new A99), but micro-adjustment is better than nothing.

Sadly this is not the full solution as you can see when you notice the A99 comes with micro adjust so indicating the problem is more complex. it is not just the error caused by the AF sensor not being on the image sensor, or even issues to do with mirrors, the lenses themselves are part of the problem.

For phase detect autofocus the camera in one measurement calculates how far our of focus the lens is and how much to move the lens. In order to do this it makes the AF sensor measurement and it takes some information from the lens relating to its position and how to rotate it. So to my knowledge accuracy of the AF depends upon the information that is measured about the lens (both stored parameters and actual measurement), the accuracy of alignment of the AF sensor to image sensor, and the resolution accuracy of the AF sensor. In most PDA systems it does not re-sample the focus when the lens has moved so it is a fast system but essentially open loop.

Contrast AF (often used by m4/3 for example or compacts) tends to be slower but as it works on the data read real time off the sensor it tends to get the best AF accuracy for a static subject. These days they are making it very fast.

That the fact the A99 still has the adjustment feature and the adjustments are made on a per lens basis indicates that the lens itself is part of the issue. Of course if you use contrast focus as a final trim you get spot accuracy and can take out the lens parameters, but you sacrifice focusing speed. The latter reason is why people get interested in hybrid AF that mixes phase and contrast AF.

strawman
strawman  1022006 forum posts United Kingdom16 Constructive Critique Points
13 Sep 2012 - 5:58 PM

I do like the thought of the AF range limiter Sony have thrown in to the A99 I can see that being handy.

Steppenwolf
13 Sep 2012 - 6:11 PM


Quote:
It's called 'manufacturing tolerance'. If you want a production line where all lenses focus perfectly with all bodies then you have better be prepared to pay for it!


It's actually not called "manufacturing tolerance" in that manufacturers don't tolerate it. The mass manufacture of cheap DSLRs doesn't allow them to ensure the required accuracy so they measure the light path error and enter the data into the firmware - so I'm told by people who have inside knowledge anyway. This data is then used to correct focus before the camera takes the photograph. In order to make the correction the camera needs to know various data about the lens (either provided by an internal table if the lens is recognised or by the ROM on the lens if it isn't). This is one area where after-market lenses can cause bad focus, either because they "spoof" a lens identity that doesn't match (wrong focus gearing, for example) or because they've simply got the interface data wrong.



Quote:
One thing to remember is that all bodies and lenses are made within a tolerance so if your lens is at one end of the tolerance range and the body is at the other end you will get a noticable mismatch.

Very interesting. Care to explain that statement because I have no idea what you're talking about. In particular what "tolerance" range is a lens made within?

Steppenwolf
13 Sep 2012 - 6:21 PM


Quote: it is not just the error caused by the AF sensor not being on the image sensor, or even issues to do with mirrors, the lenses themselves are part of the problem.



Like I said, it's called "differential focus" - unless you know of another way a lens can fool a camera into focusing incorrectly? In any case it won't fool a sensor that's embedded in the chip, that's for sure. The A99 has micro-adjustment because it has several modes of focus. I don't see any option of micro-adjustment for true "mirrorless" cameras that either use contrast focus or phase detection in the sensor (Nikon 1). And I've never heard of anyone complaining of FF/BF on these cameras either, yet they have interchangeable lenses..

Last Modified By Steppenwolf at 13 Sep 2012 - 6:24 PM

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