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Thread started 16 Feb 2018 (Friday) 05:03

# Micro Focus Adjustment question but not the usual one

Feb 16, 2018 05:03 |  #1

When or how does the camera body apply the determined adjustment to the task of focusing? Does the camera drive the lens to a focus it thinks is right and then move the lens the additional amount, such as a +5 or -3 or whatever you have set the MFA to?

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Feb 16, 2018 05:07 |  #2

Yes, the camera performs AF (algorithms) as if no AFMA function existed, then applies the adjustment at the end (then sends this to the lens).

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Feb 16, 2018 05:32 as a reply to  @ TeamSpeed's post |  #3

Sorry Teamspeed but I disagree, you are implying that it makes two seperate movements.

The software calculates the necessary movement to achieve correct focus then adds the AFMA to the original data before it moves.
There is already a table of data in the camera that the manufacturers setup when the body is calibrated, AFMA is added to that, or subtracted in the case of negative values.

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Feb 16, 2018 05:35 |  #4

I would think that the calculation is done before the camera moves the lens and if you look at the lens while in operation, it looks that way as well. So, if the camera think that the correct focus is at point x, it applies the MFA value (x+MFA) then moves the lens. You can see this by setting the MFA to +/-20 and then focus on something. The lens moves only one time.

Not sure why, but call me JJ.
I used to hate math but then I realised decimals have a point.
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Feb 16, 2018 06:53 |  #5

I am sorry if I implied that, but that was not my intention. My perspective is from the software side, not the mechanical side. I agree with both of you. The AF system operations, adds the offset, then the mechanics happen.

However though, there might be a difference with the closed loop AF that exists with some of Canon's lenses. There are some newer versions where there are some back and forth between the lens and body, and AFMA may operate a bit differently there, ie. "I calculate where to move, then you the lens moves and tell me when you are done and in what position you are, and I might readjust". This might only be in AI servo though, I am not sure, we never received clarification about the improved AF closed-loop communications.

Most lenses however operate under this "I calculate everything, then tell you the lens to do something, and I expect you to comply". The lack of a secondary review of how the image now looks after AF occurs is one of the reasons we have AFMA, the tolerances of the sensor and lens positioning of the focusing lens group can often be off.

This is where Sigma lenses, where you can make 16 different adjustments within the lens firmware, starts to cause some issues. Obviously if you go to that level of AFMA, then you most likely don't use the in-camera version. The lens now sees what the camera sent it, then it runs through its 4x4 grid of AFMA values and decides what to apply to that position, then moves the lens focusing accordingly.

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Feb 16, 2018 07:00 |  #6

TeamSpeed wrote in post #18565002
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However though, there might be a difference with the closed loop AF that exists with some of Canon's lenses. There are some newer versions where there are some back and forth between the lens and body, and AFMA may operate a bit differently there, ie. "I calculate where to move, then you the lens moves and tell me when you are done and in what position you are, and I might readjust". This might only be in AI servo though, I am not sure, we never received clarification about the improved AF closed-loop communications.

Most lenses however operate under this "I calculate everything, then tell you the lens to do something, and I expect you to comply".

Contrast focus I would think does not use MFA at all but relies on the data being fed back by the sensor.

Not sure why, but call me JJ.
I used to hate math but then I realised decimals have a point.
.
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Feb 16, 2018 07:02 |  #7

AFMA is not active during focusing via the image sensor. Only phase detect through the viewfinder... So contrast AF or DPAF are not considered, or shouldn't be, when discussing AFMA.

Those modes are good for helping determine AFMA values however. Canon still refuses to put in an automatic AFMA mode, when other manufacturers are, or are planning to. There is no reason the camera cannot play phase detect AF against contrast or DPAF to determine your offsets for you.

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Feb 16, 2018 09:13 |  #8

Ramon-uk wrote in post #18564956
... AFMA is added to that, or subtracted in the case of negative values.

Actually, if we are nitpicking, this isn't true either.
The adjust offset is simply added. If you subtract a negative value, then you actually do an addition.

Anders

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Feb 17, 2018 05:40 |  #9

Actually, if we are nitpicking, this isn't true either.
The adjust offset is simply added. If you subtract a negative value, then you actually do an addition.

Haha, very true. I added the subtraction bit after I wrote the main text because I was sure someone would say what about negative values

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Feb 17, 2018 08:13 |  #10

Actually, if we are nitpicking, this isn't true either.
The adjust offset is simply added. If you subtract a negative value, then you actually do an addition.

A bit off topic here, but since you brought it up, all arithmetic is just fancy addition.

1+1+1+1 =4

4-1=3, First graders have it right. "4 take away 1 is 3"

4x4=16
Add the first number to itself the second number of times
4+4+4+4 = 16
7x3=21
7+7+7=21.

subtract 4 (denominator) repeatedly until you get to ZERO (or less than 4).
Result is how many times you had to subtract denominator from numerator to get to less than 4.
e.g.: 4/4 = 0: You do the subtraction 1 time to get to less than 4, answer is 1
e.g.: 5/4=1, 5-4=1; you can't subtract 4 again so,
anything left after subtraction is the remainder. 1 is left, so 5/4 = 1 R1.

Same arguments apply to differential calculus, integral calculus, statistics, and linear algebra. They are all just fancy shortcuts for addition. All of them are simply notational methods for making the fancy addition a bit simpler.

You get the same answer for: "What is the integral of: 2x+3, taken from 2 to 4?" with integral notation as you would with addition. The notation of integral calculus makes it easier; it doesn't change the answer; it is just addition done fancy.

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Feb 17, 2018 08:40 |  #11

Take any multiplication problem such a 17x5. Repeatedly half the first number (discard remainders), and double the second number (add it to itself), until the first number is 1. Strike the rows with even numbers at the left. Add the right column to arrive at the answer. Works every time.

17x5 = 80 + 5 = 85.
35 x 3 = 3 + 6 + 96 = 105.

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Feb 17, 2018 08:44 |  #12

Do we have a new all time high in thread derailing here?

Anders

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Feb 17, 2018 08:47 |  #13

Do we have a new all time high in thread derailing here?

Yes, we do. It began when you stated that subtracting a negative number is really addition. I merely pointed out that ALL mathematics is really addition.

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Feb 17, 2018 09:01 |  #14

Mark

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Micro Focus Adjustment question but not the usual one
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